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COMETS
LONDON : PRINTED BY
8POTTISWOODK AND CO., NEW-STREET SQUAIIE AND PARLIAMENT STREET
PL VI.
Warren DeZa Rue del
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THE GIREAT COMET OF US 61
AS SEEN BY WARREN DE LA RUE. D.C.L.. F. R.S.
^VITH HIS NEWTONIAN EQUATOREAL OF 13 INCHES APERTURE
GELD OF (
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JAMES GLAISHEK, I
EROUS WOODCUT ILLUSTRATIONS and CHI.
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LONP
SAMPSON LOW, MARSTON
CROWN El:
77
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f
THE
WOELD OF COMETS
BY
AMEDEE GUILLEMIN
AOTHOK OF ' THE HEAVElfS '
TRANSLATED AND EDITED BY
JAMES GLAISHEE, F.E.S.
NUMEROUS WOODCUT ILLUSTRATIONS and CHROMOLITHOGRAPHS
LONDON SAMPSON LOW, MAESTON, SEAELE, & EIVINGTON
CROWN BUILDINGS, 188 FLEET STREET 1877
All rights reserved
EDITOB'S PREFACE.
I HAVE great pleasure in introducing to English readers M. GUILLEMIN'S valuable and interesting work on comets. When rapid progress has been made in any branch of science, it is generally very difficult for anyone, who has not been actually concerned in the investigations in question, to obtain accurate, information of the state of our knowledge; and for this reason a book, such as the present, which gives an account of the new results that we owe to very recent researches, really confers a benefit upon many persons who, though taking a strong interest in the subject, have necessarily been quite unable to follow its development in the periodical publications of English and foreio-n scientific societies. There is no work that at all
o
occupies the ground covered by that of M. GUILLEMIN ; and as the subject is one which, always of high interest, has in the last few years acquired great importance in consequence of Schiaparelli's discovery of a connexion between cornets and shooting- stars, I was anxious that it should appear in our language.
Whenever I have thought that additional explanation was desirable, or that the researches of the two years that have elapsed since the publication of the original work threw further
EDITOR'S PREFACE.
light upon the subject, I have added a note of my own, such notes being always enclosed in square parentheses [ ] ; and although I must not be understood to endorse M. GUILLEMIN'S
o
conclusions in every case where I have not added a note, still I may~say that there are very few of his views from which I should feel at all inclined to dissent. Of course T have corrected in the text all errors I have met with, which were evidently purely accidental, and such as always will occur in the first edition of any work. In two cases I have ventured to make more lengthy additions of my own — these relate to Coggia's comet, which had only just left us when M. GUILLEMIN'S work was published, and the connexion of comets and shooting- stars. Some remarks will also be found in the note which follows the catalogue of comets at the end of the book.
In conclusion, I must express my thanks to Dr. WARREN DE LA RUE, F.R.S., for having very kindly placed at my disposal copies of his beautiful drawings of the great comet of 1861, which add greatly to the value of the work.
JAMES GLAISHER.
BLACKHEATH, S.E. : Nov. 15, 1876.
PREFACE.
THE UNIVEKSE is formed of an infinity of worlds similar to our own. The thousands of stars which meet our gaze in the azure vault of the heavens when we contemplate it with the naked eye, and which may be reckoned by hundreds of millions when we explore its depths by the aid of the telescope, are suns. These foci of light, these sources of heat, and incon- testably of life, are not ioolated; they are distributed into groups or clusters ; sometimes by twos or threes, sometimes by hundreds, sometimes by myriads ; the clouds of vaporous light called nebula? are for the most part thus constituted.
Isolated or in groups, the stars seem to us immovable, so prodigious is the distance by which they are separated from the earth and from our sun. They move nevertheless ; and amongst those whose velocities have as yet been measured may be reckoned some which are moving ten times and even fifty times quicker than a cannon-ball when it leaves the cannon. Movement is, therefore, the most universal law of the stars.
In like manner our sun moves through space and compels the earth to follow. He bears along with him, in this voyage through the boundless ether, the globes which form his cortege and gravitate about his enormous mass. During the thousands of years that man has been a witness — an unconscipus witness,
PREFACE.
it is true— of this circumnavigation of the universe, he has seen no change in the aspect of the surrounding worlds; the sidereal shores of the ocean in which this fleet of more than a hundred celestial bodies pursues its way preserves to all appearance its unchanging front. The immensity of the sidereal distances, it is well known, is the sole cause of this apparent immobility.
The solar world is, therefore, separated from all other worlds by unfathomable abysses ; the sun is as it were isolated, lost in a corner of space, far from the millions of stars with which nevertheless it forms a system. Member of an immense asso- ciation, integral molecule of the most vast, to all appearance, of the nebula?, the Milky Way, the central star of our group seems to have no other mode of communicating with its co- associates than by the reciprocal exchange of undulations, that is to say, by the exchange of light and heat. Like disciplined and devoted soldiers, the earth and the planets march in com- pany with the sun, effecting with marvellous regularity their nearly circular revolutions around their common focus, and never deviating from the limits imposed upon them by the law of gravitation.
They remain, therefore, isolated like the sun, separated from other sidereal systems by distances so enormous that the mind is powerless to conceive of them.
A relation, however, exists between our system and these systems, as we have just mentioned: the sun is a star of the Milky Way. But, we may ask, has the solar world no closer and more direct connexion with the rest of the visible universe ?
The movement of translation by which it is animated proves at least that in some quarter of the heavens there is either an unknown celestial body, or a system of celestial bodies, around which gravitation causes the group to describe an orbit of undetermined period. And this movement of the
PREFACE
whole results from the concurrent action of all the stars in the universe. The force of gravitation is, therefore, a common bond of union between our world and all others.
Is it, therefore, steadily advancing to some celestial archi- pelago which it will finally attain in a few millions of years ? Are, then, future generations destined to see other suns, from other points of view ? These are questions whose solution may be considered inaccessible to us.
But, amongst the stars of which the solar system is com- posed, are there not some less immutably attached than the planets and the earth to the focus of their movement? Are there not some which depart to a greater distance from their focus, and which, like messengers detached from the group, carry to neighbouring world news of our own ?
Such a hypothesis is not without foundation.
Astronomers, in fact, have for the last two centuries studied the movement of certain celestial bodies, which come to us and gravitate about the sun, but which, after having, so to speak, saluted on their way the ruler of the planets, return and plunge again to immeasurable distances in the depths of ether. A small number of these stars, retained by the solar power, diverted from their path by the influence of some of the larger planets, have remained tributaries of the group of which they now form an integral part.
These singular stars, long disowned, are COMETS.
I have said long disowned. Comets, indeed, have only been considered during the last two centuries as properly belonging to the family of the stars : before Newton's time they were regarded even by astronomers as transient meteors, whose appearance, disappearance, and movements were subject to no law. For the ancients, and the world in general during the Middle Ages, and even during the Renaissance, they were objects of fear, miraculous apparitions, signs the- precursors
PREFACE.
of terrible calamities, flaming symbols of the Divine anger. For the savants of former times comets were the monsters of
the sky.
Two centuries of scientific progress exhibit these calumniated stars in a very different light. Thanks to Newton's discovery of gravitation, and to the united efforts of mathematicians and observers, their courses have been reduced to the same laws as those which govern the movements of the planets. These vagabonds of the sky have testified, some by their regular motion, others by their return at the predicted dates, their submission to the laws of celestial mechanics. Their very deviations have been recognised as the legitimate consequence of foreign influences. For some years more has been done : an endeavour has been made, and with some success, to penetrate the mystery of their organisation, and to discover the physical and chemical nature of their light; but more especially the part which they perform in the solar system and in the universe itself is beginning to be viewed in its proper light.
Comets, as Laplace had foreseen, are of different origin to the planets. The eccentricity of their orbits, the inclinations of the planes in which they move, their course, sometimes direct, sometimes retrograde, mark a profound distinction between them and the planets. Their interior structure, the nebulous appearance of nearly all, the rapid changes observed both in their nuclei and atmospheres, remove them no less from the permanent and globular figure, either solid or liquid, of the majority of the planets. Some comets move in infinite orbits. They are, therefore, strangers to our world, which they visit on their journey. Those which are periodical have most fre- quently such lengthened orbits that, after voyages the durations of which are measured by thousands of centuries, they are cast adrift far from the sun, far from the directing focus of move- ment. Is it certain that they will rejoin him on their return,
PREFACE.
and that these wanderers will not be, in the end, stars lost to our world?
In any case, they come to us out of the depths of infinite space. And if the views of M. Hoek, a Dutch savant, are well founded, it is not singly but in groups that these nebu- losities— let us say nebulas, since the structure of comets appears analogous to that of the nebulas properly so called — quit the sidereal depths and penetrate to the heart of our system. Here, then, is the material bond, the real connexion establishing a direct and uninterrupted communication between the solar world and the millions, the thousands of millions of stars which constitute the splendour of the heavens.
But the physical nature of these frail messengers of space is such, that they cannot without injury pass through the regions traversed by the planets and the sun. So great is the ' swell ' engendered by the» motions of these massive stars, that comets when navigating in these agitated deeps of the ethereal sea are there subjected to considerable damage ; sometimes they are shattered and broken into fragments ; fre- quently they leave behind them debris which follows in their wake. The interplanetary spaces are in this way strewn with the particles which the planets meet with in their periodical course, and which cause our nights to be illuminated mo- mentarily with brilliant trains of light.
The SHOOTING-STARS are due to these rencontres.
Ten years have hardly elapsed since Schiaparelli, a learned Italian astronomer, by a happy idea connected meteoric with cometary astronomy. If this bold theory, involving, if not the identity, at least the community of origin between meteors and comets, be true, how important do the latter suddenly become in the economy of the universe ! Travelling from world to world, scattering upon their route in the neighbourhood of the permanent stars of each system the dust of the elements of
XI
PREFACE.
which they are composed, may it not be that they modify in the course of time the structure of these stars themselves ? spectral analysis does not err, the matter of comets is chiefly composed of carbon combined with some other element, such as hydrogen. Here, then, are comets, shedding these sub- stances so important to vegetable and animal life, first in the interplanetary spaces, and then, by the fall and combustion of meteors, in the atmospheres of the planets ; thereby, perhaps, maintaining life upon them.
The unformed portions of matter distributed in immense masses in certain unresolvable nebulas, and successively de- tached in separate globules, would continue to describe hyper- bola? or other curves of endless branches, on their passage from star to star, and from world to world : these masses of vapour would be presented to us under the form of comets or long
trains of vapour.
Comets, therefore, which during the reign of ignorance and superstition were looked upon as scourges, are, more probably, not only inoffensive stars but, perhaps, even beneficent regene- rators of life in more advanced worlds.
These views, it is true, are only hypotheses: we know so little of that department of astronomical science which a great writer of our time has called by anticipation Celestial Organism, in opposition to Celestial Mechanics, of which our knowledge is now so far advanced. But they suffice to show what interest, scientific and philosophic, attaches to the subject of this work. Comets up to the present time have furnished only one chapter, and that the briefest, to the science of the heavens. The preceding considerations will have sufficed to show that they perform a part of great importance in the universe, and that their history merits more ample develop- ment than has been accorded to it in most treatises.
Besides, apart from the scientific interest of the subject, an
Xll
PREFACE.
historic interest attaches to it. Considered from this point of view, comets would furnish matter for an interesting work. In the volume before the reader will be found a few chapters devoted to a brief history of what may be called Cometary Astrology. It forms a necessary introduction to the purely astronomical portion ; and, were it omitted, it would be difficult to understand how the world has passed from the most extra- vagant prejudices to the calm and reassuring conceptions of contemporary science.
In ancient Greece, in heroic times, comets, as indeed all ce- lestial phenomena, excited only graceful ideas. Take, for ex- ample Homer: it is Minerva and Apollo, the two brilliant deities of Olympus, who thus manifest themselves to mortals. Later on, they became fatal presages. The Romans, more austere, had already interpreted them as signs of fatal augury, forerun- ners of calamity. In the Middle Ages the ideas connected with the m continued to increase in gloom : comets were then stars only of misfortune, ruin, and death. The terrible and grandiose idea of the end of the world, so universal at that period of darkness, predominated over all and set its seal on all. At last, with the revival of learning, scientific observation slowly dissipated these prejudices. In the eighteenth century the light of a free interpretation of nature resumed its empire : comets were spoken of without awe, and these stars, but lately so formidable, became even a theme for satire.
Contemporary science, more profound, restores to comets their majesty and importance, but it also despoils these appa- ritions for ever of all significance derived from idle superstition and terror.
CONTENTS.
CHAPTER I.
BELIEFS AND SUPERSTITIONS RELATIVE TO COMETS.
SECTION I.
COMETS CONSIDERED AS PRESAGES.
PAGE
Comets have been considered in all times and in all countries as signs, precursors of fatal events — Antiquity and universality of this belief ; its probable origin — Opinion of Seneca ; habitual and regular phenomena fail to attract the attention of the multitude ; meteors and comets, on the contrary, make a profound impression — The moderns in this respect resemble the ancients contemporary with Seneca — The incorruptible heavens of the ancients, in contradistinction to the sublunary or atmospheric regions ; stars and meteors — Inevitable confusion of certain celestial or cosmical phenomena with atmospheric meteors ........ 3
SECTION II.
COMETS IN GREEK AND ROMAN ANTIQUITY.
The apparition of a comet or a bolide is a warning from the gods : the Iliad and the ^Eneid — Supposed physical influences of comets ; Earthquakes in Achaia ; submersion of Helice and Bura ; comet of the year 371 — Comets, presages of happy augury ; Csesar transported to the heavens under the form of a comet ; popular credulity turned to account; opinion of Bayle — Pliny, Virgil, Tacitus, Seneca — The comet of the year 79 and the Emperor Vespasian — Comet of the year 400 and the siege of Constantinople .....
CONTENTS.
SECTION III.
THE COMETS OF THE MIDDLE AGES. ^^
™ters-Halley's comet and the Turks; origin of the Angelas <U M«k- The clet onm and the conquest of England by the Normans ; apostrophe ^ to the comet by a monk of Malmesbury
SECTION IV.
COMETS FROM THE RENAISSANCE TO THE PRESENT DAY. Slow improvement in the beliefs relative to comets-Bayle's remarks upon the comet of 1680-Passage fromMadame de Sevigne's letter referring to this comet and the last hours of Mazarin-In the eighteenth century belief in the super- natural exchanged for belief in the physical influence of comets-Remains oi cometary superstitions in the nineteenth century— The comet of 1812 and the Russian campaign ; Napoleon I. and the comet of 1769 ; the great comet of 1861 in Italy ...•••••'
CHAPTER II.
COMETARY ASTRONOMY TIP TO THE TIME OF NEWTON.
SECTION I.
COMETS AND THE ASTRONOMERS OF EGYPT AND CHALDEA.
Had the Egyptians and Chaldeans any positive knowledge concerning comets ? — Apollonius of Myndus ; the Pythagoreans considered comets to be true stars
According to Aristotle they are transient meteors ; fatal influence of the
authority of this great philosopher upon the development of Cometary As- tronomy .......-•*"*'
SECTION II.
COMETARY ASTRONOMY IN THE TIME OF SENECA.
Book vii. of Seneca's Qu&stiones Ncdurales relates to comets — Seneca defends in it the system of Apollonius of Myndus ; he puts forth just views concerning the nature of comets and their movements — His predictions respecting future discoveries in regard to comets — The astronomers of the future . . 42
CONTENTS.
SECTION III.
COMETS DURING THE RENAISSANCE AND UP TO THE TIME OF
NEWTON AND HALLEY.
PAGK
Apian observes that the tails of comets are invariably directed from the sun — Observations of Tycho Brahe ; his views and hypotheses concerning the nature of comets — Kepler regards them as transient meteors, moving in straight lines through space — Galileo shares the opinion of Kepler — Systems of Cassini and Hevelius . . . . . . .47
SECTION IV.
NEWTON DISCOVERS THE TRUE NATURE OF COMETARY ORBITS.
Newton's Principia and the theory of universal gravitation — Why Kepler did not apply to comets the laws of the planetary movements — Newton discovers the true system of cometary orbits — Halley and the comet of 1682 ; prediction of its return .... 52
CHAPTER III.
THE MOTIONS AND ORBITS OF COMETS.
SECTION I.
COMETS PARTICIPATE IN THE DIURNAL MOTION . . 59
SECTION II.
MOTIONS OF COMETS.
Distinction between comets, nebulse, and temporary stars— Comets, in their motions, are subject to stationary periods and retrogressions — The apparent complications arise, as in the case of the planets, from the simultaneous move- ment of these bodies and the earth . . . . . .61
SECTION III.
IRREGULARITIES IN THE MOTIONS OF COMETS.
Comets appear in all regions of the heavens — Effects of parallax — Apparent motion of a comet, in opposition and in perihelion, moving in a direction opposite to the earth — Hypothetical comet of Lacaille; calculations of Lacaille and Olbers concerning the maximum relative movement of this hypothetical comet and the earth . . . . . .65
xvii a 2
CONTENTS. SECTION IV.
THE ORBITS OF COMETS.
PAGE
Kepler's Laws ; ellipses described around the sun ; the law of areas— Gravitation, or weight, the force that maintains the planets in their orbits— The law of universal gravitation confirmed by the planetary perturbations— Circular, elliptic, and parabolic velocity explained ; the nature of an orbit depends upon this velocity— Parabolic elements of a cometary orbit . . .69
SECTION V.
THE ORBITS OF COMETS COMPARED WITH THE ORBITS OF THE PLANETS. Differences of inclination, eccentricity, and direction of motion . . .83
SECTION VI.
DETERMINATION OF THE PARABOLIC ORBIT OF A COMET.
Three observations are necessary for the calculation of a parabolic orbit — Cometary ephemerides ; what is meant by an ephemeris ; control afforded by the ulterior observations — Elements of an elliptic orbit — Can the apparition or return of a comet be predicted ? — State of the question — Refutation by Arago of a current prejudice ........ 87
CHAPTER IV.
PEEIODICAL COMETS.
SECTION I.
COMETS WHOSE RETURN HAS BEEN OBSERVED.
How to discover the periodicity of an observed comet and predict its return — First method : comparison of the elements of the orbit with those of comets that have been catalogued— Resemblance or identity of these elements : pre- sumed period deduced from it— Second method : direct calculation of elliptic elements — Third method 95
CONTENTS.
SECTION II. HALLEY'S COMET.
PAGE
Discovery of the identity of the comets of 1682, 1607, and 1631 ; Halley announces the next return for the year 1758 — Olairaut undertakes the calcu- lation of the disturbing influence exercised by Jupiter and Saturn upon the comet of 1682 ; collaboration of Lalande and Mdlle. Hortense Lepaute — The return of the comet to its perihelion is fixed for the middle of April 1759 ; the comet returns on the 13th of March — Return of Halley's comet in 1835 ; calculation of the perturbations by Damoiseau and Ponte"coulant ; progress of theory — The comet will return to its perihelion in May 1910 . . 100
SECTION III.
ENCKE'S COMET ; OB, THE SHORT PERIOD COMET.
iDiscovery of the identity and periodicity of the comets of 1818, 1805, 1795, and 1786 ; Arago and Olbers — Encke calculates the ellipse described by the comet — Dates of twenty returns up to 1873— Successive diminution of the period of Encke's comet •••.... 109
SECTION IV.
BIELA'S OR GAMBART'S COMET.
History of its discovery ; its identification with the comet of 1805 — Calculation of its elliptic elements by Gambart — Apparitions previous to 1826 — Pecu- liarities in the apparitions of 1832, 1846, and 1872 . . . .113
SECTION V.
FATE'S COMET.
First comet whose periodicity, without comparison with previous dates, has been determined by calculation and verified by observation — M. Le Verrier demon- strates that it has nothing in common with the comet of Lexell — Slight eccentricity of Faye's comet and great perihelion distance — Dates of its return — Perturbations in the movements of Faye's comet inexplicable by gravitation alone : a problem to be solved . . . . .116
SECTION VI.
BRORSEN'S COMET.
Discovery of the comet of five years and a half period by Brorsen in 1846 — Its supposed identity with the comet of 1532 gives reason to suspect elliptic elements; calculation of these elements — Returns of the comet in 1851, 1868, and 1873 .... .... 119
"
CONTENTS.
SECTION VII.
D'ARREST'S COMET.
PAGE
Discovery of the comet and of its periodicity by D' Arrest— Return predicted by M. Yvon Villarceau for 1867 ; verification to within half a day— Importance of the perturbations caused by Jupiter— Research of MM. Yvon Villarceau and Leveau— Return of the comet in September 1870 . .122
SECTION VIII.
TUTTLE'S COMET.
The period of Tattle's comet is intermediate to that of Halley's comet and those of other periodical comets that have returned — Very elongated orbit of the comet of 13f years period — Previous observation in 1790; five passages not since observed — Next return in September 1885 .... 124
SECTION IX.
WIXNECKE'S PERIODICAL COMET.
Discovery of the periodicity of the third coinet of 1819 ; calculation of its elliptic elements by Encke — Discovery of Winnecke's comet in 1858 ; its identity with the comet discovered by Pons — Return of the star to its perihelion in 1869 ; probable date of its next return in 1875 .... 126
SECTION X.
TEMPEL'S SHORT PKEIOD COMET.
Calculation of the elliptic elements of the second comet of 1867, discovered by Tempel— Perturbations due to Jupiter, and consequent delay in the return of the comet to its perihelion in 1873— Remarkable agreement of observation and calculation ... 128
CONTENTS.
CHAPTER V.
PERIODICAL COMETS.
SECTION I.
COMETS WHOSE RETURN HAS NOT BEEN VERIFIED BY OBSERVATION.
PACK
Periodical comets which have not been seen again; long periods; circumstances unfavourable to observation ; motions possibly disturbed by perturbations — Elliptic orbits determined by calculation — Uncertainty of return under these different hypotheses ........ 131
SECTION II.
INTERIOR COMETS, OR COMETS OF SHORT PERIOD, THAT HAVE NOT YET
RETURNED.
Comets lost or strayed : the comet of 1743 ; the comet of Lexell, or 1770; per- turbations caused by Jupiter ; in 1767 the action of Jupiter shortens the period, and in 1779 produces an opposite effect — Comet of De Vico; short period comets of 1783, 1846, and 1873 . . . . .133
SECTION III.
COMETS OF MEAN PERIOD.
Periodical comets exterior to the solar system ; the type of this class is Halley's comet, which is the only comet of mean period whose return has been verified by observation — Enumeration of comets with periods between 69 and 200 years — Periods ; aphelion and perihelion distances . . . .141
SECTION IV.
COMETS OF LONG PERIOD.
Periodical comets exterior to the known limits of the solar system — Distance to which the comet of longest calculated period recedes from the sun — The so- called comet of Charles V. ; its apparitions in 1264 and 1556; its return predicted for the middle of the nineteenth century, between 1848 and 1860 — Calculation of the perturbations ; another comet lost or strayed — The great comet of 1680 ; the Deluge and the end of the world — Magnificent comets of 1811, 1825, and 1843 144
CONTENTS.
CHAPTER VI.
TBS WORLD OF COMETS AND COMETARY SYSTEMS.
SECTION I.
THE NUMBER OF COMETS.
Arago-Calculation of the probable number of comets from the actual data ; ^ Kepler's remark verified
SECTION II.
COMETS WITH HYPERBOLIC ORBITS.
Do all comets belong to the solar system ?-Orbits which are clearly hyperbolic —Opinion of Laplace with regard to the rarity of hyperbolic comets— Are there any comete which really describe parabolas ?-First glance at the origin
. Io7 of comets
SECTION III.
REMARKS ON THE ORIGIN OF COMETS.
Have all the known comets of the solar world always belonged to it ?— Probable modification of their original orbits through the planetary perturbations- Cause of the gradual diminution of the periods of certain cornets . . 171
SECTION IV.
SYSTEMS OF COMETS.
Comets which have or seem to have a common origin — Double comets — Systems of comets according to M. Hoek — Distribution of aphelia over the celestial vault ; region of the heavens particularly rich in aphelia . . 174
SECTION V.
COMETARY STATISTICS.
Comparison of the elements of cometary orbits — Eccentricities ; numbers of elliptic, parabolic, and hyperbolic comets— Distribution of comets according to their nodes and perihelion distances — Equality of the numbers of direct and retrograde orbits ........ 182
CONTEXTS.
CHAPTER VII.
PHYSICAL AND CHEMICAL CONSTITUTION OF COMETS.
SECTION I.
COMETS PHYSICALLY CONSIDERED.
PAGE
The physical or chemical constitution of a celestial body ; nature of the question
involved •, explained by reference to the earth — A cometary problem . 193
SECTION II.
COMETARY NUCLEI, TAILS, AND COM.S.
Comae and tails — Classification of the ancients according to apparent external form ; the twelve kinds of comets described by Pliny— The ' Guest-star ' of the Chinese — Modern definitions : nucleus, nebulosity or atmosphere ; tails . 196
SECTION III.
COMETS DEVOID OF NUCLEUS AND TAIL.
Gradual condensation of nebulous matter at the centre — Imperceptible transition from comets without apparent tails to the immense luminous trains of great historic comets ........ 201
SECTION IV.
DIRECTION OF THE TAILS OF COMETS.
Direction of the tail opposite to the sun ; discovered by Apian ; the Chinese astronomers were acquainted with this law — Deviations in some comets — Variable aspect of the tail according to the relative positions of the comet, the earth, and the sun ....... 206
SECTION V.
NUMBER OF TAILS.
Double tails of comets; comets of 1823, 1850, and 1851— Tails multiple, fan- shaped, rectilinear, curved — Variable number of tails belonging to the same comet ; comets of Donati, of 1861 and of Ch^seaux . . . 209
PAGE
CONTENTS. SECTION VI.
DIFFERENT FORMS OF TAILS.
Elementary forms of tails-Rectilinear tails, divergent or convergent in fipect of the head of the comet-Curved tails; comets of 1811 and 176£ —Whimsical form of cometary appendages according to ancient o vations •
SECTION VII.
LENGTH OF TAILS.
Apparent and real dimensions of the largest tails on record— Formation and de- velopment of cometary appendages ; their disappearance— Variations of length in the tail of Halley's comet at its different apparitions— Great comet of 1858, or comet of Donati . • 221
SECTION VIII.
FORMATION AND DEVELOPMENT OF TAILS.
Variations of length in the tail of Halley's comet at its different apparitions — Similar phenomena exhibited by Donati's comet in 1858 — Does the maximum development of the tail always coincide with the perihelion passage of the comet? ......... 224
SECTION IX.
BRILLIANCY OF COMETS.
Estimations of the apparent dimensions or brilliancy of comets— Ancient comets said to be brighter than the sun — Comets visible to the naked eye and comets seen at noonday ; great comets of 1744 and 1843 .... 232
SECTION X.
DIMENSIONS OF NUCLEI AND TAILS.
Real dimensions of the nuclei and atmospheres of various comets — Uncertainty of these elements ; variations of the nucleus of Donati's comet — Observations of Ilevelius upon the variations of the comet of 1652 — Do cometary nebulosities diminish in size when their distance from the sun decreases ? — Encke's comet considered in regard to this question at its apparitions in 1828 and 1838 238
xxiv
CONTENTS.
CHAPTER VIII.
PHYSICAL TRANSFORMATIONS OF COMETS.
SECTION I.
AIGRETTES — LUMINOUS SECTORS — NUCLEAL EMISSIONS.
PAGE
Predominance of atmosphere in comets — Luminous sectors; emission of vaporous envelopes from the nucleus in the comets of 1835, 1858, 1860, and 1861 — Formation of envelopes in Donati's comet ; progressive diminution of the velocity of expansion in emissions from the nucleus .... 247
SECTION II.
OSCILLATIONS OF LUMINOUS SECTORS : COMET OF 1862.
M. Chacornac's observations upon the comet of 1862 — Formation of luminous sectors emanating from the nucleus — Oscillation of aigrettes, and flowing back of the nucleal matter ....... 254
SECTION III.
DUPLICATION OF BIELA'S COMET.
First signs of the doubling of Biela's comet, in the month of January 1846 — Observations of the twin comets in America and Europe— Gradual separation and approach of the fragments — The two comets return and are observed in 1852 ; their distances found to have increased — Elements of the orbits of the two comets . . . . . . . . . 258
SECTION IV.
DOUBLE COMETS MENTIONED IN HISTORY.
Is there any example in history of the division of a comet into several parts ? — The comet of B.C. 371 — Ephorus, Seneca and Pingre — Similar observations in Europe and China — The Olinda double comet, observed in Brazil, in 1860, by M. Liais. . 268
CONTENTS.
CHAPTER IX.
MASS AND DENSITY OF COMETS.
SECTION I.
FIRST DETERMINATION OF THE MASSES OF COMETS.
PAOK
Lexell's comet and the calculations of Laplace— The smallness of coraetary masses deduced from the fact that comets exercise no disturbing influence upon the earth, the planets, or their satellites .... 277
SECTION II.
METHOD OF ESTIMATING THE MASSES OF COMETS BY OPTICAL CONSIDERATIONS.
The masses of Encke's comet and the comet of Taurus determined by M.
Bahinet — Objections to this method of determination . . . 281
SECTION III.
THIRD METHOD OF DETERMINING THE MASSES OF COMETS.
Theory of the formation and development of cometary atmospheres under the influence of gravitation and a repulsive force — Calculations of M. Edouard Roche — Masses of the comets of Donati and Encke as determined by this method ..... 286
CHAPTER X.
THE LIGHT OF COMETS.
SECTION I.
INTEREST ATTACHING TO THE PHYSICAL STUDY OP COMETARY
LIGHT .... 291
xxvi
CONTENTS. SECTION II.
TRANSPARENCY OF NUCLEI, ATMOSPHERES, AND TAILS-
PAGE
Visibility of stars through the atmospheres and tails of comets ; ancient and modern observations upon this point — Are the nuclei of comets opaque, or transparent like the atmospheres and tails ? — Reported eclipses of the sun and moon produced by comets ....... 293
SECTION III.
COLOUR OF COMETARY LIGHT.
Different colours of the heads and tails of comets — Examples of colour taken from the observations of the ancients : red, blood -red, and yellow comets — Difference of colour between the nucleus and the nebulosity — Blue comets — The diversity of colour exhibited by comets is doubtless connected with cometary physics, and with the temperature and chemical nature of come- tary matter ......... 299
SECTION IV.
SUDDEN CHANGES OF BRILLIANCY IN THE LIGHT OF COMETARY TAILS.
Rapid undulations occasionally observed in the light of cometary tails ; obser- vations of Kepler, Hevelius, Cysatus, and Pingre ; comets of 1607, 1618, 1 652, 1661, and 1769— Undulations in the tails of the comets of 1843 and 1860 ; do these undulations arise from a cause peculiar to the comet itself, or do they depend upon the state of the atmosphere ? — Objection made by Gibers to the first of these hypotheses; refutation by M. Liais . . . 305
SECTION V.
DO COMETS SHINE BY THEIR OWN OR BY REFLECTED LIGHT?
Do the nuclei of comets exhibit phases ? — Polarisation of cometary light — Ex- periments of Arago and of several contemporary astronomers — The light of nebulosities and atmospheres is partly light reflected from the sun . . 309
SECTION VI.
SPECTRAL ANALYSIS OF THE LIGHT OF COMETS.
Researches of Huggins, Secchi, Wolf, and Rayet — Spectra of different comets : bright bands upon a continuous luminous ground — Analysis of the light of Coggia'a comet in 1874 — Chemical composition of different nuclei and nebu- losities ......... 315
xxvii
CONTENTS,
THE COMET OF 1874, OR COCGIA'S COMET.
PAGE
Of the five comets of 1874 the third, or comet of Coggia, was alone visible to the naked eye— Telescopic aspect and spectrum of the comet during the early part of its apparition, according to Messrs. Wolf and Rayet-Observations of Secchi, Bredichin, Tacchini, and Wright; polarisation of the light of the nucleus and tail— Transformations in the head of the comet between the 10th of June and the 14th of July, according to Messrs. Rayet and Wolf . 328
ON COGGIA'S COMET (III., 1874).
[ADDITION BY THE EDITOR.] . 342
CHAPTER XL
THEORY OF COMETARY PHENOMENA.
SECTION I.
WHAT IS A CO M E T ?
Complexity and extent of the question — The law of gravitation suffices to explain the movements of cornets — Lacunae in the theory ; acceleration of the motion of the comets of Encke and Faye — Origin of comets ; their systems — Questions relative to their physical and chemical constitution — Form of atmospheres ; birth and development of tails . . . 357
SECTION II.
CARDAN'S HYPOTHESIS.
Cometary tails considered as effects of optical refraction — Objections made by Newton and Gregory— New theory of Gergonne ; ideas of Saigey on the subject of planetary tails— Difficulties and lacunce in this theory . . 361
SECTION III.
THEORY OP THE IMPULSION OF THE SOLAR RAYS.
Ideas of Kepler concerning the formation of tails— Galileo, Hooke, and Euler— Hypothesis of Kepler formulated by Laplace— Where does the impulsion come from in the theory of undulations ? 365
xxviii
CONTENTS. SECTION IV.
HYPOTHESIS OF AN APPARENT REPULSION.
PAGE
Views of Newton on the formation of the tails of comets — Action of heat and rarefaction of the cometary matter — The ethereal medium, losing its specific weight, rises opposite the sun, and carries with it the matter of the tail — Objections which have been made to the hypothesis of a resisting and pon- derable medium ........ 369
SECTION V.
THEORY OF OLBERS AND BESSEL.
Hypothesis of an electric or magnetic action in the formation of tails — Repulsive action of the sun upon the cometary matter, and of the nucleus upon the nebulosity — Views of Sir John Herschel and M. Liais — Theory of Bessel — Oscillations of luminous sectors — Magnetic polar force . . . 372
SECTION VI.
THEORY OF COMETARY PHENOMENA.
Researches of M. E. Roche upon the form and equilibrium of the atmospheres of celestial bodies under the combined influence of gravitation, solar heat, and a repulsive force — Figure of equilibrium of a solid mass submitted to gravi- tation and the heat of the sun — Comets should have two opposite tails — Completion of the theory of cometary tides by the admission of a repulsive force, real or apparent — Accordance of the theory so completed with ob- servation ......... 380
SECTION VII.
THK REPULSIVE FORCE A REAL PHYSICAL FORCE.
Theory of M. Faye — Rigorous definition of the repulsion inherent in the solar rays — Its intensity varies with the surfaces of the two bodies ; it decreases inversely as the square of the distance — It is not propagated instantaneously — Discussion and accordance of the facts — Experiments in support of a repulsive force . . . .389
SECTION VIII.
THEORY OF THE ACTINIC ACTION OF THE SOLAR RAYS.
Experiments and hypotheses of Tyndall — Originality of his theory ; objections
and omissions — Is this theory incompatible with that of a repulsive force ? . 399
CONTENTS. SECTION IX.
COMETS AND THE RESISTANCE OF THE ETHER.
PAGE
Accelerated motion of Encke's comet; its periods continually diminish— It describes a spiral, and. will ultimately fall into the sun-Hypothesis of a re- sisting medium; how does the resistance of a medium increase the rapidity of motion ?— The nature of this supposed medium, according to Arago, Encke, and Plana— Objections of M. Faye ; the acceleration of motion explained by the tangential component of the repulsive force . • 406
CHAPTER XII.
COMETS AND SHOOTING STAES.
SECTION I.
WHAT IS A COMET ?
The ancients were unacquainted with the physical nature of comets — False ideas entertained by astronomers of the eighteenth century respecting the physical constitution of comets ; comets regarded by them as globes, nearly similar to the planetary spheroids — Views of Laplace upon comets, compared by him to nebulae — Contemporary astronomers have confirmed these views and rectified the errors of the ancient hypotheses — Desideratum of science ; the rencontre of the earth with a comet or the fragment of a comet . . . 417
SECTION II.
IS THE MATTER OP COMETS DISPERSED IN THE INTERPLANETARY
SPACES? .... 422
SECTION III.
COMETS AND SWARMS OF SHOOTING STARS.
Periodicity of the meteor-swarms ; radiant points ; number of swarms recognised at the present day — Periodical maxima and minima in certain meteoric currents ; thirty years period of the November swarm— Parabolic velocity of shooting stars ; the swarms of shooting stars come from the sidereal depths . 425
CONTENTS. SECTION IV.
COMMON ORIGIN OF SHOOTING STABS AND COMETS.
PAGH
Transformation of a nebula which has entered into the sphere of the sun's attraction ; continuous parabolic rings of nebulous matter — Similarity between the elements of the orbits of meteor streams and cometary orbits — The August stream ; identity of the Leonides and the comet of 1862 — Identity of the Perseids and the comet of 1866 (Tempel) — The shooting stars of April 20 and the comet of 1861 — Biela's comet and the December stream — Did the earth encounter Biela's comet on November 27, 1872 ? 430
ON THE CONNEXION BETWEEN COMETS AND METEORS.
BY THE EDITOR . 436
CHAPTER XIII.
COMETS AND THE EARTH.
SECTION I.
COMETS WHICH HAVE APPROACHED NEAREST TO THE EARTH.
The memoir of Lalande and the panic of the year 1773 — Letter of Voltaire upon the comet — Announcement in the Gazette de France and the Memoirs of Bachaumont — Catalogue given by Lalande of comets which up to that time had approached nearest to our globe ..... 455
SECTION II.
COMETS AND THE END OF THE WORLD.
Prediction of 1816 ; the end of the world announced for July 18 — Article in the Journal des Dtbats — The comet of 1832 ; its rencontre with the orbit of the earth — Notice by Arago in the Annuaire du Bureau des Longitudes — Proba- bility of a rencontre between the comet and the earth — The end of the world in 1857 and the comet of Charles V. . . . . . 460
CONTENTS. SECTION III.
MECHANICAL AND PHYSICAL EFFECTS OF A COLLISION WITH A COMET.
PAGE
Opinions entertained by astronomers of the last century : Gregory, Maupertuis, Lambert— Calculations of Lalande ; comets move too rapidly in the vicinity of the earth for the effects of their attraction to come into play— Opinion of Laplace — The collision of a comet with the earth ; its effect according to the mechanical theory of heat ....... 467
SECTION IV.
CONSEQUENCES OF A COLLISION BETWEEN A COMET AND THE EARTH
ACCORDING TO THE MECHANICAL THEORY OF HEAT . . . 477
SECTION V.
THE COMET OF 1680, THE DELUGE, AND THE END OF THE WORLD.
Ancient apparitions of the comet of 1680, on the hypothesis of a revolution of 575 years — Their coincidence with famous events — Whiston's theory of the earth : our globe is an ancient comet, whose movements and constitution have been modified by comets — The catastrophe of the Deluge caused by the eighth anterior apparition of the comet of 1680 — Final catastrophe : burning of the earth — Future return of our globe to the condition of a comet . 480
SECTION VI.
PASSAGE OF THE EARTH THROUGH THE TAIL OF A COMET IN 1861.
Possibility of our globe passing through the tail of a comet — Has such an event ever taken place ?— The great comet of 1861— Relative positions of the earth and one of the two tails of that comet— Memoir of M. Liais and the observa- vations of Mr. Hind . . 486
xxxu
CONTENTS.
CHAPTER XIV.
PHYSICAL INFLUENCES OF COMETS.
SECTION I.
SUPPOSED PHYSICAL INFLUENCES OF COMETS.
PAGB
The great comet of 1811 ; the comet wine — Prejudices and conjectures — Remark- able comets and telescopic comets — Comets are continually traversing the heavens . . . . . . . . . 495
SECTION II.
DO COMETS EXERCISE ANT INFLUENCE UPON THE SEASONS?
Study of the question by Arago — The calorific action of comets upon the earth appears to be inappreciable — Comparison of the meteorological statistics of various years in which comets did and did not appear — The meteorological influence of a comet is not yet proved by any authentic fact . . 499
SECTION III.
PENETRATION OF COMETARY MATTER INTO THE TERRESTRIAL ATMOSPHERE.
Is this penetration physically possible ? — Cometary influences, according to Dr. Forster— Were the dry fogs of 1783, 1831, and 1834, due to the tails of comets ? — Volcanic phenomena and burning turf beds ; their probable coin- cidence with fogs — Probable hypothesis of Franklin — Dry fogs, atmospheric dust, and bolides ........ 603
SECTION IV.
CHEMICAL INFLUENCES OF COMETS.
Introduction of poisonous vapours into the terrestrial atmosphere — The end of the world and the imaginary comet of Edgar Poe ; Conversation of Eiros and Char- mion — Poetry and Science ; impossibilities and contradictions . . 508
xxxm
CONTENTS.
CHAPTER XV.
SOME QUESTIONS ABOUT COMETS.
SECTION I.
ARE COMETS HABITABLE ?
The inhabitants of comets as depicted in the Plurality des Mondes of Fontenelle —Ideas of Lambert respecting the habitability of comets— That comets are the abode of human beings is a hypothesis incompatible with the received facts of astronomy ....••••
SECTION II.
WHAT WOULD BECOME OF THE EARTH IF A COMET WEEE TO MAKE IT ITS SATELLITE ?
Conditions of temperature to which the earth would be subjected if it were com- pelled by a comet to describe the same orbit as the latter — The comets of Halley, and of 1680, examined from this point of view — Extremes of heat and cold: opinion of Arago: impossibility of living beings resisting such
523
SECTION III.
IS THE MOON AN ANCIENT COMET ?
Hypothesis of Maupertuis : the planetary satellites originally comets, which have been retained by the attractions of the planets — The Arcadians and the moon — Refutation of this hypothesis by Dionys du Se'jour . . . 628
TABLE I. Elliptic Elements of the recognised periodical comets of the Solar system . 631
TABLE II.
General catalogue of the orbits of comets ..... 532
Note on the designation of comets, and on the catalogue of comets, by the Editor 545
xxxiv
LIST OF ILLUSTEATIONS.
PLATES.
The July 2 great comet of 1861, by De La Rue . The great comet of 1861, by De La Rue, July 3 The great comet of 1843
Cheseaux's comet, 1744 .... Donati's comet, 1858 . . .
Frontispiece
To face p. 248
„ . 152
„ . 212
220
WHOLE-PAGE WOODCUTS.
Coggia's comet, 1874 Forms of comets according to Pliny Orbits of periodical comets Donati's comet, September 1858 . October 1858
To face p. 328
„ . 198
„ . 128
„ . 226
230
WOODCUTS IN TEXT.
FIG. PAGE
1 Phenomena of the Year 1000. Fac-simile of a drawing in the Theatrum
Cometicum of Lubienietzki . . . . . .21
2 Comet of 1528. Fac-simile of a drawing of Celestial Monsters from the
work of Ambrose Pare" . . . . . .' .22
3 Halley's comet on its apparition in 1066. From the Bayeux Tapestry . 24
4 Halley's comet in 684. Fac-simile of a drawing in the Chronicle of
Nuremberg ........ 25
5 Proper motion of a comet ; distinction between a comet and a nebula . 62
XXXV
LIST OF ILLUSTRATIONS.
PAGE
67
TIQ Maximum apparent movement of a comet and the earth .
7 Second Law of Kepler. The areas swept out by the radius vect ^
proportional to the time
8 Relation between the velocities and forms of orbits 74
9 Cometary orbits, elliptic, parabolic, and hyperbolic
10 Confusion of the arcs of orbits of different eccentricities in the neighbour-
hood of the perihelion .
11 12 13, 14 Determination of a cometary orbit: parabolic elements
16 Comparison of the eccentricities of the orbit of Faye's comet with that of the planet Polyhymnia .
16 Halley's comet in 1835. 1. As seen by the naked eye October 24. 2.
As seen in the telescope the same day .
17 Encke's comet, at its passage in 1838, August 13
18 Brorsen's comet, as observed May 14, 1868, from a drawing of Bruhns . 120
19 Great comet of 1264, from Theatrum Cometicum of Lubienietzki . . 146
20 Great comet of 1811 . .... 151
21 Cometary nebulosities ; central condensation ; absence of tail and nucleus. 201
22 Encke's comet according to Mr. Carpenter . 202
23 Encke's comet, December 3, 1871, according to Mr. H. Cooper Key . 203
24 General direction of cometary tails . • . 207
25 Double tail of the comet of 1823 . . . 210
26 Double tail of the comet of 1850 . . ... 210
27 Comet of 1851 ........ 210
28 Sextuple tail of the comet of 1744, according to Che"seaux . . 211
29 Fan-shaped tail of the great comet of 1861, according to the observation
of June 30 and the drawing of Mr. G. Williams . . . 213
30 The two tails of the comet of 1861, according to Secchi, June 30 and
July 2 214
31 Winnecke's comet, June 19, 1868 . . . . . .216
32 Comet of P. Henry, August 26 and 29, 1873 . . . .217
33 The comet of 1264 218
34 Aspect of Donati's comet on December 3, 4, and 6, 1858, according to the
observations of M. Liais ...... 226
35 Variations of length in the principal tail of Donati's comet . . 227
36 Parabolic orbit of Donati's comet. Projection of the earth's orbit upon
the comet's orbit. Relative positions of the two bodies . . 229
37 Projection of the orbit of Donati's comet upon the plane of the ecliptic.
Relative positions of the earth and comet .... 229
38 Encke's comet, according to the observations of Schwabe. 1. October 19,
1838 ; 2. November 5 ; 3. November 10 ; 4. November 12 .242
39 Luminous sectors and aigrettes of Halley's comet, according to Schwabe.
(1) October 7, 1835 ; (2) October 11 ; (3) October 15 ; (4) October 21 ;
(5) October 22 ; (6) October 23 . . . . . .250
xxx vi
LIST OF ILLUSTRATIONS.
FIO. PAflE
40 Formation of luminous sectors and envelopes. Donati's comet, Sept. 8,
1858 . . . . . . . .251
41 Comet of I860, III. June 27, according to Bond. Aigrettes and en-
velopes . . . . . . . . ' 251
42 Luminous envelopes of Donati's comet. September 30, 1858 . . 252
43 The same comet. October 2. From a drawing by Bond . . 252
44 Formation of the luminous envelopes in Donati's comet. October 6 . 253
45 The same. October 8. Both from drawings by Bond . . . 253
46 Biela's comet after the duplication on February 21, 1846. According to
Struve . . . . . . . . 261
47 The twin comets of Biela at their return in 1852. According to Secchi . 263
48 The Olinda double comet on February 27, 1860, according to M. Liais . 270
49 The Olinda double comet on March 10, 1860, according to M. Liais . 271
50 The Olinda double comet on March 11, 1860, according to M. Liais . 272
51 Supposed phases of the comet of 1819, according to Oacciatore: observa-
tions of July 5 and 15 ....... 311
52 Comet of 1868, II. (Winnecke's.) From a drawing made by Mr.
Huggins . . . . . . ' . . .319
53 Spectra of the light of the comets of 1868, I. (Brorsen), and 1868, II.
(Winnecke) from the observations of Mr. Huggins: (1) Solar spectrum: (2) spectrum of carbon spark taken in olive oil ; (3) spectrum of carbon spark taken in olefiant gas; (4) spectrum of comet of 1868, II.; (5) spectrum of Brorsen's comet, 1868, I.; (6) spectrum of an induction spark ..... . . 320
54 Spectrum of the comet 1873, IV. (Henry's) (1) August 26; (2) Au-
gust 29 ......... 321
55 Coggia's comet, June 10, 1874, according to the drawing of M. G. Rayet 322
56 Spectra of the comet of 1874, III. (Coggia's), according to Father Secchi 331
57 Coggia's comet seen in the telescope on June 22, 1874, according to a
drawing by M. G. Rayet ...... 335
58 Coggia's comet on July 1, 1874, according to a drawing by M. G. Rayet 336
59 Coggia's comet on July 13, 1874, according to M. G. Rayet . . 337
60 Coggia's comet on July 14, according to M. G. Rayet . . . 338
61 Comet of 1618, according to Hevelius. Multiple nuclei . . . 341
62 Comet of 1661, according to Hevelius. Multiple nuclei . . . 341
63 M. Roche's theory of cometary phenomena. Limiting atmospheric sur-
face of equilibrium ....... 382
64 Flow of cometary matter beyond the free surface of the atmosphere. No
repulsive force ....... 383
65 Development of cometary tails, on the hypothesis of an intense repulsive
force. M. Roche's theory ...... 386
66 Development of cometary tails on the hypothesis of a feeble repulsive
force. M. Roche's theory . . . . . .386
67 Influence of a resisting medium upon the orbit of a comet . . 409
xxxvii
LIST OF ILLUSTRATIONS.
FIO. PAGE
68 Radial and tangential components of the repulsive force, according to M.
Faye . .... 412
69 Shooting stars of November 13-14, 1866. Convergence of the tracts, ac-
cording to A. S. Herschel and A. MacGregor .... 426
70 Orbits of the Meteoric Streams of November, August, and April, and of
the comets of 1866 and 1861 . . . . . .432
A Tracks of Meteors observed at the Royal Observatory, Greenwich, on the
night of November 13-14, 1866 . . . . .441
B Showing the rate of frequency of Meteors seen per minute at the Royal
Observatory, Greenwich, on the night of November 13-14, 1866 . 442
C Tracks of Meteors recorded by Professor Tachini at Palermo on August 8
to August 12, 1868 . . . . . . .443
D (1) Nucleus of the comet of 1618, observed telescopically by Cysatus.
(2) Comet of 1652, as seen December 27, according to Hevelius . 444
E Positions of Biela's comet relatively to the earth in 1798, 1838, and 1872 446
71 The orbit of the earth and that of Biela's comet in 1832. Relative posi-
tions of the two bodies ....... 462
72 Biela's comet at its node, October 29, 1832. Supposed position of the
comet at its least distance from the earth ... 463
73 Passage of the earth through the tail of the comet of 1861, on June 30 . 489
74 Positions occupied by the earth and the moon in the interior of the second
tail of the comet of 1861 ..... 489
75 Fan-shaped tail of the great comet of 1861 on June 30 . 491
X-xxvlii
CHAPTER I.
BELIEE3 AND SUPERSTITIONS EELATIVE TO COMETS.
i;
SECTION I.
COMETS CONSIDERED AS PRESAGES.
Comets have been considered in all times and in all countries as signs, precursors of fatal events — Antiquity and universality of this belief ; its probable origin — Opinion of Seneca ; habitual and regular phenomena fail to attract the attention of the multitude ; meteors and comets, on the contrary, make a profound im- pression— The moderns in this respect resemble the ancients contemporary with Seneca — The incorruptible heavens of the ancients, in contradistinction to the sublunary or atmospheric regions ; stars and meteors — Inevitable confusion of certain celestial or cosmical phenomena with atmospheric meteors.
IN all countries and in all times the apparition of a comet has been considered as a presage: a presage fortunate or un- fortunate according to the circumstances, the popular state of mind, the prevailing degree of superstition, the imbecility of princes or the calculation of courtiers. Science itself has helped to confirm the formidable and terrible signification most frequently accorded by common belief to the sudden and unexpected arrival of one of these remarkable stars. Not two centuries ago, as we shall shortly see, learned men and as- tronomers of undoubted merit continued to believe in the influence of comets over human events. What wonder, then, if we should find existing in our own time, in the midst of the nineteenth century, numerous vestiges of a superstition as old as the world?
How has this superstition originated ? This is a question we shall not undertake to resolve : we leave it to others more
3 B 2
THE WORLD OF COMETS.
learned and competent than ourselves in similar matters to reply. Let us confine ourselves to a simple and by no means new remark. The things which we see every day, the phenomena which are constantly or regularly reproduced under our eyes do not strike us, and fail to excite either our attention or curiosity. D'Alembert has said : ' It is not without reason that philosophers are astonished to see a stone fall to the ground, and people who laugh at their astonishment will upon the smallest reflection share it themselves.' Yes, it is necessary to be a philosopher, or man of science, as we should say at the present day; it is necessary to reflect in order to diocover the why and the how of facts, of those at least whose production is frequent and regular. The most admirable phenomena remain unperceived. Habit blunts the impression we derive from them and renders us indifferent.
As applied to comets, this idea has been perfectly expressed by Seneca, at the commencement of Book vii. of his Qucestiones Naturales: ' There is,' he observes, 'no mortal so apathetic, so obtuse, so bowed down towards the earth, that he does not erect himself and tend with all the powers of his mind towards divine things, particularly when some new phenomenon makes its appearance in the heavens. Whilst all above follows its daily course, the recurrence of the spectacle robs it of its grandeur. For man is thus constituted: that which he sees every day, however admirable it may be, he passes with indifference, whilst the least important things as soon as they depart from the accustomed order captivate and interest him. The whole choir of heavenly constellations under this immense vault, whose beauty they diversify, fails to attract the attention of the multitude ; but should anything extraordinary appear, all faces are turned towards the heavens. The sun has spectators only when he is eclipsed. The moon is observed only when she undergoes a similar crisis. Cities then raise a cry of alarm,
4
COMETS CONSIDERED AS PRESAGES.
and everyone in panic fear trembles for himself. ... So much is it in our nature to admire the new rather than the great. The same thing takes place in respect to comets. If one of these flaming bodies should appear of rare and unusual form, everyone is anxious to see what it is ; all the rest are forgotten whilst everyone enquires concerning the new arrival: no one knows whether to admire or to tremble; for there are not wanting people who draw from thence grave prognostics and disseminate fears.'
Is it not with us to-day as with the contemporaries of Seneca? Doubtless thoughtful and reflective minds yield themselves readily to a sentiment of contemplative admiration before the majestic spectacle of the heavens. The solemn march of the heavenly bodies, the well-ordered harmony of worlds, are for them the symbol of eternal laws governing the universe; from the unalterability of these laws they derive confidence. But 'the mass of the people ordinarily remains indifferent before impassable and immutable nature. It is reserved for one unusual apparition to rouse all from this indifference, to awaken curiosity in some, fear in others, and, if the phenomenon should be of unwonted proportions, ad- miration in every one.
Moreover, whether it be a. comet, or any other remarkable meteor, bolide, aurora borealis, or stone fallen from heaven, the sentiments of fear inspired by these phenomena are always the same, the superstitious interpretation similar, but closely proportioned in degree to the brilliancy and the more or less whimsical or extraordinary form of the apparition.
Amongst the Greeks and Romans, as we all know, a num- ber of the most ordinary and familiar actions, singular ren- contres, the cries of animals, the flight and the song of birds, were looked upon as omens, as so many means made use of by the gods to communicate with man, to warn him of their
5 <
THE WORLD OF COMETS.
decrees, to signify to him their thoughts and will. But they regarded the importance of the warning as proportional to the grandeur of the sign and the brilliancy of the phenomenon, and it is not difficult to understand that comets amongst these manifestations of the divine will appeared the most significant
and formidable.
A comet, moreover, not being a simple local phenomenon, seen only by some, but exhibiting itself to all, brilliant as a star, and of unusual dimensions, varying from day to day in form, position and size, had all the appearance of a sign fraught with significance to the entire people : this portent addressed itself to those who played an important part in public affairs, and concerned kings, or at least great personages. It had a certain resemblance to the stars, which it sometimes surpassed in the brilliancy of its light, but it differed from them in its erratic course; it had a certain resemblance likewise to atmospheric meteors, by its sudden appearance, and oftentimes as sudden disappearance, and by the rapid changes to which it was ordinarily subjected. The heavens, with their countless hosts, the sun, the moon, the fixed stars and planets, were for the ancients the domain of the incorruptible — cosli incorrupti. Under this name it was the dwelling-place of the gods, the habitation of the immortals. On the contrary, tjie air, the atmosphere, the sublunary space — for the ancients it was all one — was the region of meteors and of things corruptible and fleeting; and in the same manner as the thunder-bolt of Jupiter was the chosen instrument of his vengeance, comets were the selected messengers of fate, sent to announce to mortals, on the part of the gods, events that were inevitable.
In this confusion of certain celestial phenomena with atmospheric meteors lies the chief source of the difficulty experienced by the astronomers of antiquity and the Middle Ages, and even of modern times, in solving the complicated
c
COMETS CONSIDERED AS PRESAGES.
problem of cometary movements. Down to the sixteenth century, we shall see men of undoubted science refusing to comets the quality of stars. They were confirmed in this error alike by the prejudices we have just mentioned and by the superstitious beliefs which are found to be so persistent amongst all people and, as we have already said, in all times: doubtless because these beliefs have the same foundation or common origin, faith in the supernatural intervention of the gods in human affairs.
But let us see what signification was given to the apparition of comets by the ancient Greeks and Romans. It is a curious and instructive side of cometary science; it will aid us further on perhaps in comprehending the ideas entertained by their astronomers concerning the physical nature of these stars.
SECTION II.
COMETS IN GREEK AND EOMAN ANTIQUITY.
The apparition of a comet or a bolide is a warning from the gods: the Iliad and the ^Eneid— Supposed physical influences of comets ; Earthquakes in Achaia ; sub- mersion of Helice and Bura ; comet of the year 371— Comets, presages of happy augury ; C?esar transported to the heavens under the form of a comet ; popular credulity turned to account ; opinion of Bayle— Pliny, Virgil, Tacitus, Seneca— The comet of the year 79 and the Emperor Vespasian— Comet of the year 400 and the siege of Constantinople.
A COMET is thought to have appeared in the last year of the sieo-e of Troy. By Pingre" and Lalande it is considered an apparition of the famous comet of 1680, and whilst the former cites in support of his opinion a passage from Homer, the latter draws attention to certain lines in the ^Eneid probably referring to the same comet.* The following is the passage in the fourth book of the Iliad to which Lalande refers : —
4 Thus having spoken, he urged on Athene already in- clined; she hastening descended the heights of Olympus. As the star which the son of wily Saturn sends, a sign either to mariners or to a wide host of nations, and from which many sparks are emitted: so Pallas Athene hastened to the earth and leaped into the midst [of the army] ; and astonishment seized the horse-breaking Trojans and the well-greaved Greeks
* In the time of Pingr^ and Lalande the period of revolution of the comet of 1680 was believed to be 575 years. Encke has since assigned it the much longer period of 8814 years.
8
COMETS IX GREEK AND ROMAN ANTIQUITY.
looking on. And thus said one to another : " Doubtless evil war and dreadful battle din will take place again, or Zeus, the arbiter of war amongst men, is establishing friendship between both sides." ' *
In our opinion the star to which Homer compares Athene was no comet, but a bolide, the explosion of which is frequently attended with an emission of sparks, and can itself be seen in broad daylight. This remark applies in all respects to the verses of Virgil, who, moreover, mentions the noise of the detonation, oftentimes similar, in explosions of bolides, to the rumbling of thunder. Anchises, in the passage we now quote, has just ceased to invoke Jupiter: —
' Scarcely had my aged sire thus said, when, with a sudden peal, it thundered on the left, and a star, that fell from the skies, drawing a fiery train, shot through the shade with a profusion of light. We could see it, gliding over the high tops of the palace, lose itself in the woods of Mount Ida, full in our view, and marking out the way; then all along its course an indented path shines, and all the place a great way round smokes with sulphureous steam. And now my father, overcome, raises himself to heaven, addresses the gods, and pays adoration to the holy star.' f
The confusion above indicated is of frequent occurrence amongst the ancient writers, with whom the Aurora Borealis, bolides, and comets are evidently phenomena of the same nature, and, so far as their supernatural interpretation is con- cerned, this is not difficult to understand. Even in our day the public is not more exact. Who can have forgotten the impression caused by the splendid Aurora Borealis of Novem- ber 24, 1870, during the Siege of Paris? That reddened glare in the heavens, those shifting paths of light, were they not,
* Iliad, iv. 52-84. t ^Eneid, ii. 674-709.
THE WORLD OF COMETS.
for weak and credulous minds, under the stimulus besides of passing events, certain presages of the blood about to be shed? A comet would have produced a similar effect.
Be this as it may, the meteor, be it either bolide or comet, is in the eyes of all beholders, both in Virgil and Homer, a presage, or warning from Jupiter,-a sign, the pre- cursor of events auspicious or inauspicious, according to the interpretation or circumstances.
Three hundred and seventy-one years before our era a very brilliant comet appeared, which Aristotle has described, and which Diodorus Siculus refers to in the following terms : * In the first year of the hundred and second Olympiad, Alcisthenes being then Archon of Athens, several prodigies announced to the Lacedaemonians their approaching humiliation : a burn- ino- torch of extraordinary size, which was spoken of as a fiery beam, appeared for several nights.' This comet, of which we shall speak again, had more than one claim to notice. According to Ephorus it divided into two ; and about the time of its apparition the earthquakes took place which caused Helice and Bura, two towns of Achaia, to be swallowed up by the sea. Comets, therefore, for the ancients were not only the precursors of fatal events, but they had direct power to occasion them. Such is certainly the opinion of Seneca when he remarks : ' This comet, so anxiously observed by everyone, because of the great catastrophe which it produced as soon as it appeared, the submersion of Bura and Helice.'
Comets not only announced fatal events, disasters, and wars; portents of evil for some, they were naturally presages of happy augury for others. Thus, according to Diodorus Siculus and Plutarch, the comet of the year B.C. 344 was for Timoleon of Corinth a token of the success of the expedition which he directed that year against Sicily. ' The gods by an extra- ordinary prodigy announced his success and future greatness:
10
COMETS IN GREEK AND ROMAN ANTIQUITY
a burning torch appeared in the heavens throughout the night and preceded the fleet of Timoleon until it arrived off the coast of Sicily.'
The births and deaths of princes, especially of those remem- bered in history for the evil they have done, could not fail to be distinguished by the apparition of prodigies, and by comets, the most striking of all prodigies. Thus the comets of B.C. 134 or 137 and B.C. 118 were referred, the former to the birth and the latter to the accession of Mithridates, and the comet of the year B.C. 43 was supposed to be nothing less than the soul of CaBsar transported to the heavens. Bodin ( Universce Naturae Tkeatrum) attributes to Democritus the opinion that such is, in fact, the part performed by some of these stars, and con- fesses that he is not far from sharing the same opinion. ' I reflect,' he says, ' upon the idea of Democritus, and I am led to believe with him that comets are the souls of illustrious per- sons, which, after having lived upon the earth for a long suc- cession of ages, ready at last to perish, are borne along in a sort of triumph, or are called to the starr-y heavens, as brilliant lights. This is why famine, epidemic maladies, and civil wars follow the apparition of comets; cities and nations then find themselves deprived of the help of those excellent leaders who strove to allay their intestine troubles.' We willingly place amongst the beliefs arid superstitions of the ancients this triumphant explanation of the supposed disasters which, ac- cording to the universal opinion, were certain to follow the apparition of a comet. Nor is it in all probability of earlier date than the sixteenth century, for it is very likely that Bodin calumniated Democritus and was himself its true author. Pingre", remarking on the above passage and the apotheosis of Ca3sar, observes with justice: 'The preceding should be added to the number of base and indecent flatteries, rather than be classed among philosophical opinions.'
11
THE WORLD OF COMETS.
Less than a century after Bodin we find Bayle protesting against this superstition, which appeared singular indeed to a man so enlightened as he who has been compared to Montes- quieu. In his Pensees sur la Comete, in which so much good sense is blended with so much irony, Bayle ingeniously shows with what skill popular credulity was turned to account, and how the same comet was made to subserve several ends. ' Augustus,' he says, ' from policy, was well pleased that the people should believe it to be the soul of Caesar ; because it was a great advantage for his party to have it believed that they were pursuing the murderers of a man who was then amongst the gods. For this reason he caused a temple to be built to this comet, and publicly declared that he looked upon it as a very auspicious omen. . . . Those who were still repub- lican at heart said, on the contrary, that the gods testified by it their displeasure that the liberators of their country were not supported.'
In the Natural History of Pliny we find several passages attesting the terrible significance attached to comets by the ancients. ' A comet,' he observes, ' is ordinarily a very fearful star; it announces no small effusion of blood. We have seen an example of this during the civil commotion in the consulate of Octavius.' This refers to the comet which appeared B.C. 86. The following alludes to the comet of B.C. 48, and perhaps no less to the apparition of remarkable bolides and Aurorse Boreales : ' We have, in the war between Caesar and Pompey, an example of the terrible effects which follow the apparition of a comet. Towards the commencement of this war the darkest nights were made light, according to Lucan, by un- known stars; the heavens appeared on fire, burning torches traversed in all directions the depths of space; the comet, that fearful star, which overthrows the powers of the earth, showed its terrible locks.'
12
COMETS IN GREEK AND ROMAN ANTIQUITY.
Virgil, at the eud of the first Georgic, expresses in his harmonious language all the horror caused in the super- stitious minds of the vulgar by the prodigies so skilfully turned to account by politicians arid sceptics. After speaking of the prognostics which may be drawn from the aspect of the setting sun in reference to the weather, he adds : —
4 Who dares to call the sun deceiver? He even forewarns often that hidden tumults are at hand, and that treachery and secret wars are swelling to a head. He also pitied Rome at Caesar's death, when he covered his bright head with murky iron hue, and the impious age feared eternal night; though at that time the earth too, and ocean's plains, ill-omened dogs, and presaging birds, gave ominous signs. How often have we seen ^Etna from its burst furnaces boil over in waves on the lands of the Cyclops and shoot up globes of flame and molten rocks ! Germany heard a clashing of arms over all the sky ; the Alps trembled with unwonted earthquakes. A mighty voice, too, was commonly heard through all the silent groves, and spectres strangely pale were seen under the cloud of night ; and the very cattle (Oh horrible!) spoke; rivers stopped their courses, the earth yawned wide ; the mourning ivory weeps in the temples, and the brazen statues sweat. Eridanus, king of rivers, overflowed, whirling in mad eddy whole woods along, and bore away the herds with their stalls over all the plains. Nor at the same time did either the fibres fail to appear threatening in the baleful entrails, or blood to flow from the wells, and cities to resound aloud with wolves howling by night. Never did more lightnings fall from a serene sky nor direful comets so often blaze.' *
All these prodigies, this mixture of facts natural and true, and the whimsical beliefs of popular credulity, are for the poet
* Georgic, i. 463-488. 13
THE WORLD OF COMETS.
testimonies of the anger and vengeance of the gods, fore- runners of fresh disasters, the precursors of that battle of Philippi in which Roman armies inflamed by civil discord are about to encounter and shed each other's blood. Nature acts in unison with man, and her manifestations are a reflex of his fury. Everything, moreover, concurs to render the divine intervention striking ; earthquakes, volcanic eruptions, and inundations. The comets and bolides with which Virgil con- cludes his enumeration appear to be the supreme signs of this menacing intervention:
Non alias coelo ceciderunt plura sereno Fulgura ; nee diri toties arsere cometae.
Later on, comets were not only presages : they became pretexts for the persecutions of imperial tyranny. Thus, Tacitus says, in regard to the comet of the year 64 : 'At the close of this year people discoursed only of prodigies, the forerunners of approaching calamities ; of thunderbolts more frequent than at any other epoch, and of the apparition of a comet, a kind of presage that Nero always expiated with illustrious blood.' Several comets, in fact, appeared during the reign of this monster, and it is concerning one of them that Seneca had the audacity to say, ' that having appeared in the reign of Nero, it has removed infamy from cornets.' It does not seem, however — and we shall find other proofs of it later on — that the author of the Qacestiones Naturales shared the prevailing prejudices on the subject of comets. He does not deny that they cause disasters, but he manifestly inclines towards a physical explanation of these phenomena. Speaking of the comet of the year 62, he observes : ' The comet which appeared under the consulate of Paterculus and Vopiscus has been attended with the consequences that Aristotle and Theo- phrastus have attributed to this kind of star. Everywhere
14
COMETS IN GREEK AND ROMAN ANTIQUITY.
there have been violent and continual storms: in Achaia and Macedonia several towns have been overthrown by earth- quakes.'
Let us conclude what more we have to say of the super- stitious beliefs of the ancients concerning comets with the mention of two or three famous apparitions; they will suffice to show that from the most remote antiquity down to the Middle Ages, from the erroneous ideas of the pagans to those of Christian nations, during this long dark night of history we pass without interruption or sensible modification.
In the year 69, according to Josephus, several prodigies announced the destruction of Jerusalem. 'Amongst other warnings, a comet, one of the kind called Xiphias, because their tails appear to represent the blade of a sword, was seen above the city for the space of a whole year.'
Pingre* quotes, in reference to the comet of the year 79, this curious passage from Dion Cassius, which proves that there were esprits forts even amongst the Roman emperors : ' Several prodigies preceded the death of Vespasian; a comet was for a long time visible ; the tomb of Augustus opened of itself. When the physicians reproved the Emperor Vespasian for continuing to live as usual and attend to the business of the state, al- though attacked by a serious malady, he replied, " It is fitting that an emperor should die standing." Perceiving some cour- tiers conversing together in a low tone of voice about the comet, " This hairy star," he remarked, " does not concern me; it menaces rather the King of the Parthians, for he is hairy and I am bald." Feeling his end approach, UI think," said he, " that I am becoming a god."
The death of the Emperor Constantine was announced by the comet of the year 336.
In the year 400 the misfortunes with which Gainas menaced Constantinople were so great, say the historians Socrates and
15
THE WORLD OF COMETS.
Sozomenes, that they were announced by the most terrible comet mentioned in history; it shone above the city, and reached from the highest heavens to the earth. The same comet was also regarded as the presage of a plague which broke out about the same time.
Lastly, the invasions of barbarians, at a time when moral disorder and anarchy of ideas were in unison with the disor- ganisation of the Empire, could not fail to be signalised by various prodigies, birds of evil augury, frequent thunderbolts, monstrous hailstones, fires, and likewise apparitions of comets, * that spectacle which the earth has never seen with impunity.' In the Middle Ages, therefore, we shall find that beliefs in the supernatural and the intervention of the gods in human affairs are further strengthened and increased by the mysticism which the ascendency of religious ideas tended to foster in the minds of the people.
16
SECTION III.
THE COMETS OF THE MIDDLE AGES.
Prevalence of popular superstitions — Comets announce wars, plagues, the deaths of sovereigns — Terrors of the year 1000; comets and the end of the world — Gian Galeazzo Visconti and the comet of 1402 — Ambrose Pare" ; celestial monsters — Halley's comet and the Turks; origin of the Anyelus de Midi — The comet of 1066 and the conquest of England by the Normans ; apostrophe to the comet by a monk of Malmesbury.
IF a complete history were desired of all the superstitions which, during the Middle Ages and in modern times, have obtained with respect to comets, it would be necessary to pass in review every apparition of these stars, together with such incidental phenomena as the Aurora Borealis, new and temporary stars, bolides, &c., all of which have been con- verted by popular credulity into as many prodigies. In- teresting in a scientific point of view, this long enumeration derived from the naive chronicles of the time, the only docu- ments available in the absence of a more complete and intelli- gent record, would be but a tedious study of human errors ; a constant and monotonous repetition of the same absurd beliefs. To this state of things savants have themselves contributed, as at the epoch when these voluminous records were compiled cometary influences were still believed in, and the erudite of the day shared the universal prejudice.
I will here limit myself to a few characteristic traits of this
17 C
THE WORLD OF COMETS.
tenacious superstition, in order to exhibit the progress, I was about to say the revolution, of ideas which has taken place under the increasing influence of science, and more especially of astronomy and physics. Wherever the light of science has been able to make its way the phantoms of the supernatural have vanished, and the most extraordinary apparitions, even if they continue unexplained, are no longer regarded as prodigies, presages, or Divine manifestations, but natural phenomena con- cerning which all men of science, without exception, are at one in their endeavour to trace the laws that govern them.
Let us come now to the facts we have to mention.
In ancient times, especially amongst the Greeks, comets, as it has been seen, were not invariably regarded as of evil omen. The darker and more gloomy spirit of the Middle Ages only saw in these apparitions the announcement of terrible events, wars, pestilence, and especially the deaths of sovereigns. The comet of 451 or 453 announced the death of Attila, and the comet of 455 that of the Emperor Yalentinian; comets ap- peared successively to announce the death of Meroveus in 577, of Chilperic in 584, of the Emperor Maurice in 602, of Mahomet in 632, of Louis le De'bonnaire in 837, of the Emperor Louis II. in 875. That the apparition of comets was connected with the death of the great is an idea so widely spread that many chroni- clers appear to have recorded, perhaps in good faith, comets which were never seen ; such, according to Pingre, was the comet of 814, which presaged the death of Charlemagne.
In the year 1024 a comet appeared, an augury, it was sup- posed, of the death of the King of Poland, Boleslas I. ; an eclipse of the sun and a comet marked in 1033 that of Robert, King of France ; comets appeared in 1058, the year of the death of Casimir, King of Poland; in 1060, the year in which died Henry I., King of France, and in the years 1181, 1198, 1223, 1250, 1254, 1264, 1337, 1402, 1476, 1505, 1516, and 1560.
18
THE COMETS OF THE MIDDLE AGES.
Under these respective dates we find the deaths of the follow- ing sovereigns : Pope Alexander III. ; Richard I., King of England; Philip Augustus, King of France; the Emperor Frederick, deposed and excommunicated; Pope Urban IV.; Gian Galeazzo Visconti, Duke of Milan; Charles the Bold; Philip I. of Spain; Ferdinand the Catholic; and Francis II., King of France. This list might be considerably extended. Amongst the chroniclers or historians who relate these coin- cidences we find no shadow of a doubt as to the certainty or signification of the presage. The mention of these signs, forerunners of the deaths of sovereigns, very frequently occurs with a curious naivete", of which we will give two or three examples.
1 At the commencement of July,' says an old French chro- nicle, ' a little before the half, appeared a sign in the heavens called a comet denoting a convulsion of the kingdom ; for Philip, the king, who for a long time had lain ill of a quartan ague, at Mantua, closed his last day on the 14th of July, 1223.' Gian Galeazzo Visconti was sick when the comet of 1402 appeared. As soon as he perceived the fatal star he despaired of life : ' For,' said he, ' our father revealed to us on his death- bed that, according to the testimony of the astrologers, a similar star would appear for eight days at the time of our death.' ' This prince was not deceived,' adds the historian, from whom we borrow this account; 'surprised by an unexpected malady, he died a few days after.' Another chronicler gives us to un- derstand that the comet only appeared when Galeas was already attacked by the malady of which he died. But the faith of the duke in the celestial warning was not less complete. ' At this time a great comet was seen. Galeas was told of it. His friends helped him to leave his bed; he saw the comet, and exclaimed, " I render thanks to God for having decreed that my death should be announced to men by this celestial sign."
19 o 2
THE WORLD OF COMETS.
His malady increasing, he died shortly afterwards, at Marig- nan, on the 3rd of September.'
Pino-re", in quoting the first of these accounts, observes that the unexpected malady of Galeas might well have been occasioned by the chimerical fear of this prince; he might have added, or aggravated. This simple remark of the Canon of St. Ge- nevieve sufficiently marks the difference of the times. Till the eighteenth century the writers who record the coincidences of comets and. great events have implicit faith in them, and naively describe as a self-evident fact the connexion between the comet and the event itself. Pingre, writing in the eighteenth century, less than a century after the labours of Newton, and in quest of dates to enable him to calculate various cometary orbits, esteems it fortunate that in these times of ignorance such absurd beliefs should have existed, as without them history perhaps would never have recorded one of these ap- paritions so valuable to science.
There have been degrees nevertheless, according to the times, in the superstitious terror created by the apparition of a comet; this terror was also proportioned to the degree of bril- liancy of the star, the magnitude of its tail, and the more or less singular form of the coma and luminous appendage. In the year 1000, at that melancholy epoch when the end of the world was so confidently looked for, the most simple pheno- mena, if unexpected, must have assumed terrible proportions. About this time we are told of earthquakes, and a comet was visible for the space of nine days. ' The heavens having opened, a kind of burning torch fell upon the earth, leaving behind it a long train of light similar to a flash of lightning. Such was its light that it frightened not only those who were in the open country but those who were within doors. As this opening in the heavens closed imperceptibly there be- came visible the figure of a dragon, whose feet were blue, and
20
THE COMETS OF THE MIDDLE AGES.
whose head seemed continually to increase.' This evidently relates to the apparition of a bolide, and also perhaps to an Aurora Borealis, but not to the comet, whose apparition lasted nine days.
The drawing of these ' frightful ' meteors, which we here reproduce from the Tlieatrum Cometicum of Lubienietzki, is interesting in various respects. It shows the height to which imagination can attain under the stimulus of terror; it proves also the little value to be attached, scientifically speaking, to the descriptions of the time, whether written or portrayed. This drawing is comparatively modern, probably of much later date than the epoch at which the apparition represented by it took place; but the next which we give (fig. 2) is taken from
Fig. 1. — Phenomena of the Year 1000. Fac-simile of a drawing in the Tfitatrum Cometicum of Lubienietzki.
a work by Ambrose Pare, a contemporary of the apparition. The decapitated heads, the sabres, the arms which accompany the drawing of the hairy star, are only the translation of what the over-excited popular imagination believed itself to have seen in comets or other meteors, signs from heaven.*
* In his admirable work The Universe our late learned naturalist M. F. A. Pouchet with much justice remarks, in a note : ' In Ambrose Pare may be seen to what extent the mightiest minds of these latter centuries allowed themselves to be led astray on the subject of comets. The illustrious surgeon, who was by no means superstitious, gives in his important work the most fan-
21 /
THE WORLD OF COMETS.
Observe in what terms the historian Nicetas describes the comet (or meteor) of the year 1182: ' After the Romans were driven from Constantinople a prognostic was seen of the ex- cesses and crimes to which Andronicus was to abandon him- self. A comet appeared in the heavens similar to a writhing
Fig. 2.— Comet of 1528. Fac-simile of a drawing of Celestial Monsters from the work of
Ambrose Pare.
tastic drawings of some of these bodies. In his chapter entitled Celestial Monsters Ambrose Pare speaks of comets as hairy, bearded, buckler- shaped, lance-shaped, dragon-like, or resembling a battle of the clouds. And he in particular describes and represents in all its details a blood-red comet which appeared in 1528 (the figure above represented (fig. 2). ' This comet,' said he, ' was so horrible, so frightful, and it produced such great terror in the vulgar, that some died of fear,
22
THE COMETS OF THE MIDDLE AGES.
serpent ; sometimes it extended itself, sometimes it drew itself in; sometimes, to the great terror of the spectators, it opened a huge mouth; it seemed that as if, thirsting for human blood, it was upon the point of satiating itself.'
* Comiers,' says Pingre", ' makes a horrible comet appear in the month of October, 1508, very red, representing human heads, dissevered members, instruments of war, and in the midst a sword.' May it not be, with an error of date on the part of one or other of the chroniclers, the comet of which we have spoken, and a fac-simile of which we have reproduced in fig. 2?
Under the heading of periodical comets we shall see that one of the most famous in history is that which is now called Halley's comet, from the name of the astronomer who calcu- lated and first predicted its return. This comet has, in fact, made its appearance twenty-four times within sight of the earth since the year 12 before our era. the most remote date on record of its apparition. Let us here transcribe, according to Babinet, the most remarkable particulars of the events which have been connected with it by popular belief.
' The Mussulmans, with Mahomet at their head, were be- sieging Belgrade, which was defended by Huniades, surnamed the Exterminator -of the Turks. The comet of Halley appeared, and the two armies were alike seized with fear. Pope Calix- tus III., himself struck with the general terror, ordered public prayers to be offered up, and launched a timid anathema against the comet and the enemies of Christianity. He instituted the prayer called the Angelas de Midi, the use of which still con-
and others fell sick. It appeared to be of excessive length, and was of the colour of blood. At the summit of it was seen the figure of a bent arm, holding in its hand a great sword, as if about to strike. At the end of the point there were three stars. On both sides of the rays of this comet were seen a great number of axes, knives, blood-coloured swords, among which were a great number of hideous human faces, with beards and bristling hair.'
23
THE WORLD OF COMETS.
tinues in all Catholic churches. The Franciscans brought 40,000 defenders to Belgrade, besieged by the conqueror of Constantinople, the destroyer of the Empire of the East. The battle took place, and lasted two days without intermission. This conflict of two days caused the loss of more than 40,000 combatants. The Franciscans, without arms, crucifix in hand, appeared in the foremost ranks of the defenders, invoking the exorcism of the Pope against the comet, and turned against the enemy the Divine anger of which no man at this time doubted. What primitive astronomers ! '
Fig. 3.— Halley's Comet on its apparition in 1066. From the Bayeux Tapestry.
But let us go back to an earlier date in the history of this comet. It appeared in the month of April 1066. ' The Normans had at their head their Duke William, since surnamed the Con- queror, and were ready to invade England, the throne of which was at that time usurped by Harold in spite of the faith sworn to William.' That the comet was the precursor of the Conquest no one doubted. A new star, a new sovereign. Nova stetta, novus rex ! Such was the proverb of the time. The chroni- clers say unanimously, ' The Normans, guided by a comet,
24
THE COMETS OF THE MIDDLE AGES.
invaded England.' Fig. 3 reproduces from the celebrated Bayeux tapestry, attributed to Queen Matilda, wife of Wil- liam the Conqueror, the episode in which the apparition of the comet appears.
Halley's comet, by its apparition in 1066, gave rise to the objurgations of the monk of Malmesbury, which have been quoted by Pingre* from an old English chronicle : ' Seeing his country on the point of being attacked on the one side by Harold, King of Norway, on the other side by William, and judging that bloodshed would ensue, "Here art thou, then," said he, apo- strophising the comet, " here art thou, source of the tears of many mothers. Long have I seen thee; but now thou appearest to me more terrible, for thou menacest my country with complete ruin."
Going back further still, we find that Hal- ley's comet is that which announced the death of Louis le Delxmnaire, which came to pass three years later. Lastly, the comet of 684 (fig. 4) is also one of its apparitions.
We will say nothing of the famous comet of 1556, to the influence of which was long attributed the abdication of the Emperor Charles V., because it happens that the celebrated emperor had already descended from the throne when the comet made its appearance. We shall have* occasion to speak further on of the announcement of its return between 1848 and 1860, and of its non-reappearance.
25
Fig. 4. — Halley's Comet in 684. Fac-simile of a drawing in the Chronicle of Nuremberg.
SECTION IV.
COMETS FROM THE RENAISSANCE TO THE PRESENT DAY.
Slow improvement in the beliefs relative to comets— Bayle's remarks upon the comet of 1680 — Passage from Madame de Sevigne"s letter referring to this comet and the last hours of Mazarin— In the eighteenth century belief in the supernatural exchanged for belief in the physical influence of comets — Remains of cometary superstitions in the nineteenth century — The comet of 1812 and the Russian campaign ; Napoleon I. and the comet of 1769 ; the great comet of 1861 in Italy.
WE have just seen that the superstitious ideas of the Middle Ages were yet dominant in the height of the Renaissance, since a man of learning like Ambrose Pare — no astronomer, it is true — could attribute to comets the same malign influences as those ascribed to them in the year 1000, when the end of the world was confidently expected.* Nor could it be other- wise, science not having then assigned to comets, in common with other extraordinary meteors, their true place in the order of nature.
Little by little, however, healthier ideas make their way, and to the supernatural influence of comets we shall now see gradually succeed in the minds of men of science and the
[* Milton has finely expressed the popular superstition with regard to comets in the well-known lines —
' On the other side,
Inwnsed with indignation, Satan stood Unterrified ; and like a comet burned, That fires the length of Ophiuchus huge In the arctic sky, and from his horrid hair Shakes pestilence and wax.'— Paradise Lost, book ii. — ED.]
26
COMETS FROM THE RENAISSANCE TO THE PRESENT DAY.
more enlightened of the people the idea of an influence purely physical, at first under the form of simple hypotheses, and afterwards as a probability deduced from observations and facts. This progress was slow, like that of cometary astronomy, and owed much of its advance to the assistance of men of original thought, who, without being astronomers, were yet conversant with the scientific knowledge of their time.* Such was Bayle. We have already quoted several passages from his Pensees sur la Comete, and we will now complete what still remains to be said in reference to this essay. ^
The Pensees diverses ecrites a un professeur de Sorbonne were composed during the public excitement caused in France and Europe by the apparition of the famous comet of December 1680. From the beginning Bayle adopts the opinion of Seneca, and thus renews the train of rational and sound ideas. ' Comets,' he remarks, ' are bodies subject to the ordinary laws of nature, and not prodigies amenable to no law.' Supposing his corre- spondent to share the current prejudices of the time, he is astonished that so great a doctor should nevertheless suffer himself to be carried along with the stream, and imagine like the rest of the world, in spite of the arguments of the chosen few, that comets are heralds-at-arms sent by God to declare war against the human race,
He then examines the value of the historical testimonies which different writers have applied to the support of the current prejudice on comets.
1 The testimony of historians,' he remarks, ' proves only
* The following anecdote which we borrow from Bayle proves that the wits of the seventeenth century began to treat with ridicule this long-cherished superstition. ' It seems to me,' says M. de Bassompierre, writing to M. de Luynes, in 1621, shortly after the death of Philip III., 'that the comet we laughed at at St. Germain is no laughing matter, as it has buried in two months a pope, a grand duke, and a king of Spain.' A belief which is expressed in these terms may be considered as drawing to its end.
27 '
THE WORLD OF COMETS.
that comets have appeared, and that afterwards there have been many disorders in the world, which is very far from proving that the former are to be looked upon as the cause or the prognostic of the latter, unless we are willing to admit that a woman who never looks out of window in the Rue St. Honore" without seeing carriages pass along the street is to imagine that she is the cause of their passing, or that when she shows herself at the window it is a sign to the whole quarter that carriages will soon pass.'
Bayle next attacks astrology and its pretended princi- ples, as the source of all the extravagant beliefs relative to heavenly phenomena; and, indeed, prejudices in respect to comets form but a portion of the whole, and are contained in a separate chapter, which might well be entitled Cometary Astrology.
' The details of cometary warnings, resting only upon the principles of astrology, cannot fail to be ridiculous, because there never has been anything more impertinent, more chi- merical or more ignominious to human nature, to the eternal shame of which it must be related, that there have been men base enough to deceive others under the pretext of knowing the affairs of heaven, and men foolish enough to believe in them even to the extent of instituting the office of Astrologer, and of not daring to wear a new coat, or plant a tree, without the approbation of that functionary.
' The astrologer will tell you to what people, or to what animals, the cornet has reference, and the kind of evil that may be expected. In Aries it signifies great wars and mortality, the fall of the great and the exaltation of the little, together with fearful droughts in places under the dominion of that sign. In Virgo it signifies dangerous childbirth, imprison- ments, sterility and death amongst women. In Scorpio, in addition to the preceding evils, reptiles and innumerable locusts.
28
COMETS FROM THE RENAISSANCE TO THE PRESENT DAY.
In Pisces disputes concerning points of faith, frightful appari- tions in the air, wars and pestilence among the great, etc. . . .
* It is not in our own time only that astrologers have reasoned upon such extravagances. The same thing pre- vailed in the time of Pliny. "It is," says he, "thought to be a matter not unimportant whether comets dart their beams towards certain quarters, or derive their power from certain stars, or represent certain things, or shine in particular parts of the heavens. If they resemble a flute, the omen relates to music; when they appear in certain parts of a sign, the omen has reference to the immodest; if they are so situated as to form an equilateral triangle or a square with some of the fixed stars, they are addressed to learning and wit. They distribute poison when they appear in the head of either the Northern or the Southern Serpent.' (Pliny, book ii. chap, xxv.)
Bayle cites a remark attributed to Henry IV. which might be applied, at the present day, to many so-called predictions. Speaking of the astrologers who had forewarned him of his death, Henry IV. is said to have exclaimed, ' They will be right some day, and the public will remember the one pre- diction that has come true, better than all the rest that have proved false.'
The letter of the celebrated writer is long; it touches upon very many considerations which, though of interest as regards the history of ideas at the end of the sixteenth century, would appear in the present day far removed from our subject; but the philosophic thought which has inspired him is always true. It may be summed up in these eloquent lines, the last that we shall quote : —
' The more we study man the more does it appear that pride is his ruling passion, and that he affects grandeur even in his saddest misery. Mean and perishable creature that he is, he has been able to persuade himself that he cannot die
29
THE WORLD OF COMETS.
without disturbing the whole of nature and obliging the heavens to put themselves to fresh expense to light his funeral pomp. Foolish and ridiculous vanity ! If we had a just idea of the universe we should soon comprehend that the death or birth of a prince is so insignificant a matter, compared to the whole of nature, that it is not an event to stir the heavens.'
Madame de SevigmS, writing on January 2, 1681, to the Comte de Bussy, mentions the same comet, then in sight, and concludes with a remark which in reality is the same as Bayle's. The following is the passage : —
'We have here a comet — it has the most beautiful tail that could possibly be seen. All the greatest personages are alarmed, and firmly believe that heaven, occupied with their loss, is gi vino- intelligence of it by this comet. It is said that Cardinal
O O O V
Mazarin being despaired of by his physicians, his courtiers con- sidered it necessary to honour his last hours by a prodigy, and to tell him that a great comet had appeared which filled them with alarm for him. He had strength enough to laugh at them, and jestingly replied that the comet did him too much honour. In truth, everyone should say the same, and human pride does itself too much honour in believing that when perforce we die it is a great event amongst the stars.'
At the present day what man of education, what enlight- ened mind would fail to subscribe to the views of the cele- brated author and the spirituelle marquise? Nevertheless false beliefs relative to comets, celestial and even atmospheric meteors, are not entirely destroyed. We might have found traces of them in the last century, but in an epoch so favourable to science, we must seek under another form for the errors of which we have given a rapid sketch from the most ancient down to comparatively modern times ; and in the chapter which we shall devote to the possible influences of comets upon the earth it will be seen that if the popular
30
COMETS FROM THE RENAISSANCE TO THE PRESENT DAY.
fears were then of a different kind they were none the less -vivid. In our nineteenth century these fears have been openly revived; the idea that the end of the world could be brought about by the meeting of the earth and a cornet has found minds disposed to receive it with blind acceptance. Further, the old superstition of the supernatural influence or signi- fication of comets is always rife amongst the ignorant masses of the people, whose minds remain unaffected by the advance of science, because to them science is a dead letter. The following is a fact which occurred in Russia, and hardly more than sixty years ago: —
' It was not by the exchange of diplomatic notes that the inhabitants of Moscow derived a presentiment of some ap- proaching calamity. The famous comet of 1812 first gave them warning of it. Let us see what reflections it inspired in the minds of the Abbess of the Dievitchi Monastir, and the nun Antonina, formerly the slave of the Apraxines. ' One evening, as we were on our way to a commemorative service at the Church of the Decollation de Saint-Jean, I suddenly perceived on the other side of the church what appeared to be a resplen- dent sheaf of flame. I uttered a cry and nearly let fall the lantern. The Lady Abbess came to me and said, " What art thou doing? What ails thee?" Then she stepped three paces forward, perceived the meteor likewise, and paused a long time to contemplate it. " Matouchka" I asked, "what star is that? " She replied, ult is not a star, it is a comet." I then asked again, "But what is a comet? I have never heard that word." The mother then said, " They are signs in the heavens which God sends before misfortunes." Every night the comet blazed in the heavens, and we all asked our- selves, what misfortunes does it bring ? ' — lLa Grande Armee a Moscou dapres les temoignages moscovites? — Revue des Deux Mondes, July 1, 1873.
31 i
THE WORLD OF COMETS.
Can anyone deny that such credulity exists at the present day and elsewhere than in Russia? Are there not persons still who believe that the great comet of 1769, which appeared in the year that Napoleon was born, presaged the era of war which drenched in blood the end of the eighteenth century and the beginning of the nineteenth, and all the disasters which that too famous despot let loose on Europe and at last upon France herself? Have we not seen quite recently, in 1861, when the great comet of that year appeared, how it was cur- rently reported in Italy, and doubtless elsewhere, that the new star was a sign of the speedy return of Francis II. and his restoration to the throne of the Two Sicilies ; and also that it presaged the fall of the temporal power and the death of Pope Pius IX.?
We ought not to be astonished at the persistence of these superstitions, which only the spread of science can annihilate for ever. After seeing, in the following chapter, with what great difficulty true ideas on the subject of comets, suspected centuries ago, have achieved their final victory, we shall not be surprised to find that errors still remain in our own nineteenth century, in the midst of what we regard as enlightened popu- lations, but which will never be truly enlightened until primary instruction shall have given to them more definite notions of physics, natural history, and astronomy.
32
CHAPTEE II.
COMETAEY ASTRONOMY UP TO THE TIME OF NEWTON,
33
, D
SECTION I.
COMETS AND THE ASTRONOMERS OF EGYPT AND CHALDEA.
Had the Egyptians and Chaldeans any positive knowledge concerning comets ? — Apollonius of Myndus ; the Pythagoreans considered comets to be true stars — According to Aristotle they are transient meteors ; fatal influence of the authority of this great philosopher upon the development of Cometary Astronomy.
SUCH is a very brief history of the. errors into which the human mind — we should rather say the human imagination — has fallen with respect to comets. We have now to show how little by little, and by very slow degrees, truth disengaged itself from error, and to supplement the history of superstitions and prejudices by that of science. Both are instructive and throw light upon each other at all stages of their mutual development. Thus, for example, we may readily conceive that the irregular movements of comets, their sudden and unforeseen apparition, to say nothing of the singularity of their aspect, for a long time precluded the idea of their being true stars, subjected to fixed laws, like the planets. Centuries of work, observation, and research were required for the discovery of the true system of the world as far as the sun, the planets, and the earth were concerned ; but a difficulty of another kind stood in the way of the discovery of the true movements and nature of comets, since no trouble was taken to make exact and continuous observations of them. These difficulties, which were so great an impediment to science,
35 ' I) 2
THE WOULD OF COMETS.
gave, on the contrary, singular encouragement to the pre- judices, the superstitions, and the hypotheses which appear so ridiculous in our day. And, in addition, the predominance of mystic ideas contributed to deter astronomers from a study which fell rather within the provinc e of the diviner than the
savant.
It is on this account all the more interesting to see a few just ideas, a few true conceptions, break through the dark night of ignorance and superstition. This happened, it is true, at a time and in countries where philosophy, not yet obscured by scholastic subtleties, was employed in explaining facts according to natural hypotheses ; and where, by a bold and happy intuition, the Pythagorean school guessed without proving the true system of the world.
Are we to attribute to the Chaldeans and to the ancient Egyptians the first true conceptions concerning the nature of comets? That they regarded comets as stars subjected to regular movements, and not as simple meteors, we may believe, if it be true that they were in possession of means for predicting their return. Passages in Diodorus Siculus prove that the Chaldean and Egyptian astronomers hazarded such predictions ; but, so far as our means enable us to judge, there is reason to suppose that these predictions were based upon particular beliefs, more astrological than astronomical. The passage which occurs in Diodorus Siculus relative to the Chaldeans is as follows: —
' The Chaldeans,' says he, ' by a long series of observations have acquired a superior knowledge of the celestial bodies and their movements : a knowledge that enables them to announce future events in the lives of men ; but according to them, five stars, which they call interpreters, and which others call planets, deserve particular consideration; their movement is of singular efficacy. They announce likewise the apparition of
36
COMETS AND THE ASTRONOMERS OF EGYPT AND CHALDEA.
comets, eclipses of the sun and moon, and earthquakes; all changes that take place in the atmosphere, whether salutary or pernicious, both to whole nations and kings and simple individuals.' Diodorus, also, speaking of the astronomical observations of the Egyptians and their knowledge of the movements of the celestial bodies, assures us ' that they often predicted to men what would happen to them in the course of their lives, the event following the prediction.' ' It is not unusual,' he adds, ' to hear of them announcing the maladies which are about to attack men or animals. In short, by means of accumulated observations, they predict earthquakes, inun- dations, the births of comets, and, indeed, all that seems to transcend the limits of the human mind.'
It is clear that, in the opinion of the historian, the pre- dictions relative to comets which he attributes to the Egyptians and Chaldeans have no connexion with astronomy. Comets are confounded with other atmospheric meteors, whose return, according to them, was connected with the course of the stars by rare and mysterious coincidences, with which astrologers had far more to do than astronomers.
Nevertheless, we may suppose that the Chaldeans possessed some just ideas on the subject of comets. From them, indeed, and from the Egyptians * the Greeks derived their first know- ledge of astronomy ; from them, if Seneca is to be trusted, Apollonius of Myndus obtained his ideas concerning these stars. According to Apollonius ' comets are placed by the
* ' Eudoxus first brought with him from Egypt into Greece a knowledge of their movements [the planets]. Nevertheless, he makes no mention of comets. Hence it follows that even the Egyptians, a people more curious than any other in all matters of astronomy, had occupied themselves but little with the study of 'these bodies. At a later period, Conon, a most accurate observer, drew up a catalogue of the various eclipses of the sun recorded by the Egyptians, but he makes no mention of comets, which he would hardly have omitted if he had found any facts respecting them.' — Seneca, Qucestiones Naturales, vii. 3.
37 /
THE WORLD OF COMETS.
Chaldeans amongst the number of wandering stars, and they know their course.' Seneca then explains in detail the opinion of this ancient astronomer. ' A comet is not an assemblage of planets, but many comets are planets. They are not false appearances, nor fires burning on the confines of two stars; they are proper stars, like the sun and moon. Their form is not exactly round, but slender and extended lengthwise. Moreover, their orbits are not visible ; they traverse the highest regions of the heavens, and only become apparent at the lowest part of their course. We are not to suppose that the comet which appeared under Nero, and removed infamy from comets, bore resemblance to the comet which, after the murder of Julius Caesar, during the games of Venus Genetrix, rose above the horizon about the eleventh hour of the day. Comets are in great number and of more than one kind ; their dimensions are unequal, their colours are different ; some are red, without lustre; others are white, and shine with a pure liquid light; others again present a flame neither pure nor fine, but enveloped in much smoky fire. Some are blood -red, sinister presage of the blood soon to be shed. Their light augments and decreases like that of other stars, which throw out more light and appear larger and more luminous in proportion as they descend and come nearer to us, arid are smaller and less luminous when they are returning and increasing their distance from us.' Seneca, as we shall soon see, adopts this system, in which observations and conjectures nearly approaching the truth are mixed with various errors and traces of the reigning super- stitions. The assimilation of comets to the planets as far as concerns their movements is a luminous idea, which is all the more truthful because Apollonius points out at the same time a characteristic difference between the two kinds of celestial bodies — viz., that comets are only visible in a small portion of their orbits.
38
COMETS AND THE ASTRONOMERS OF EGYPT AND CHALDEA.
Amongst the ancient philosophers who believed comets to be stars — stars wandering like the planets — must be mentioned Diogenes, chief of the Ionic school after Anaxagoras (Plu- tarch), Hippocrates of Chios, and several Pythagoreans. A passage in Stobseus, 5th century A.D., proves, as also book vii. of the Qucestiones Naturales of Seneca, that this opinion of the ancients concerning the true nature of comets remained uselessly chronicled in the books which have come down to us through the Middle Ages. Astronomers derived from it no benefit, so general was the superstition and so profoundly was it rooted in all minds. The passage in Stobseus runs : ' The Chaldeans believed comets to be other planets, stars which are hidden for a period, because they are too far distant, and which sometimes appear when they descend towards us, according to the law prescribed for them ; they consider that they are called comets by persons ignorant of their being true stars, which only seem to be annihilated when they return to their own region and plunge into the profound abyss of ether, as fishes plunge to the bottom of the sea.'
What was required to render fruitful these remarkable views? Simply to the observation of comets to apply the rules long known and followed by astronomers for noting with precision all the circumstances of the movements of the planets. How precious would such observations now be to us for cometary theories! We must admit, however, that to have extracted from them all that they could yield, it would have been also requisite to have risen at one bound to the conception of the true system of the world, dimly seen by the Pythagorean school, and allowed to repose in the shade till the days of Copernicus and Galileo.
What were the obstacles which opposed so natural a pro- gress in science ? First, and most powerful of all, the enslave- ment of minds to the belief in the supernatural, and the pre- 39
THE WOBLD OF COMETS.
vailing misconceptions on the subject of comets ; prejudices which increased in strength from the time of the Greek philo- sophers to the Middle Ages, when astrological folly attained its maximum intensity. There was at work also the influence of a powerful genius, who adopted— not very decidedly, it is true— the erroneous theory of the comet-meteors. In those ages, when everyone was always ready to swear per verba magistri, the word of Aristotle sufficed to ensure conviction, and the ideas of Apollonius of Myndus and of Seneca were regarded as tainted with heresy.
Pinore' divides the opinions of the ancients about comets into three principal systems : that which we have just noticed, and which is, as it were, a rough sketch of the true system ; that of Panrctius, who regarded comets as destitute of all reality — a simple optical appearance only ; and, lastly, the system according to which comets are simple atmospheric meteors, transient and sublunary. Amongst the authors of these different systems some, like Heraclides of Pontus and Xenophanes, regarded comets as very elevated clouds illu- minated by the sun, the moon, or stars, or even as burning clouds. Transport these clouds from the atmosphere into the heavens themselves, into the regions where the planets perform their revolutions, and we have nearly the opinion of contemporary astronomers. The same might be said of the notion of Strato of Lampsacus, who regarded comets as lights sunk deep in the midst of clouds of great density, thus com- paring them in some sort to lanterns. Does not the luminous nucleus in the centre of the nebulosity, which the telescope of modern times has revealed, correspond, in fact, to the hypo- thesis of the peripatetic philosopher ?
We now come to the views of Aristotle concerning comets, views absolutely false, though maintained but two centuries ago, but yet important, on account of the great influence they
40
COMETS AND THE ASTRONOMERS OF EGYPT AND CHALDEA.
exercised over the astronomers of the Middle Ages, and even over those of the Renaissance. In the opinion of this great philosopher comets are exhalations rising from the earth, which, having reached the upper regions of the air, adjoining the region of fire,* are drawn along by the movement of the surrounding medium. They at last unite with it, condense, and catch fire ; so long as the combustible matter lasts the fire burns ; when there is no more aliment for its supply the fire becomes extinct and the comet disappears.
It is useless to refute this hypothesis, which is entirely without foundation, or to record the objections which have been made to it by writers even of the time of Seneca. But it is well to devote a few words to this last philosopher. The book of the Qucestiones Naturales in which he relates all that was known in his time of comets, their movements and in- fluence, is of great historic value, and the views of the author himself are certainly worthy of attention on their own account.
* According to Aristotle the air is divided into three regions : that in which animals and plants exist ; this is the lower region, which is immovable, like the earth upon which it rests ; the intermediate region, intensely cold, participates in the immobility of the first ; but the upper region, contiguous to the region of fire or the heavens themselves, is carried along by the diurnal movement of the latter. The exhalations arising from the earth ascend to this higher region, and there, heated by the medium they have entered and by their own movement, they engender igneous meteors to which class comets belong.
41
SECTION II.
COMETARY ASTEONOMY IN THE TIME OF SENECA.
Book vii. of Seneca's Quastmnes Naturales relates to comets — Seneca defends in it the system of Apollonius of Myndus ; he puts forth just views concerning the nature of comets and their movements — His predictions respecting future discoveries in regard to comets — The astronomers of the future.
FROM the beginning of his book Seneca fully appreciates the importance of the question, and the connexion that must neces- sarily exist between the nature of the comets and the system of the universe itself. He is led to ask ' if comets are of the same nature as bodies placed higher than themselves. They have points of resemblance with them, ascension and decimation, and also outward form, if we except the diffusion and the luminous prolongation; they have likewise the same fire, the same light.' Here, then, we have comets assimilated to the planetary bodies as regards their movements, the only points of difference being the nebulosities and tails of the former, Seneca is sensible how important it would be * to discover, if possible, whether the world revolves about the motionless earth, or if the world is fixed and the earth revolves ; whether it is not the heavens but our globe which rises and sets.' ' It would be necessary,' he adds, ' to possess a table of all the comets which have appeared; for their rarity up to the present time has been a hindrance to our understanding the laws
o
which regulate their course, and assuring ourselves if their
42
COMETARY ASTRONOMY IN THE TIME OF SENECA.
course is periodical, and if a constant order brings them back to an appointed day. Now, the observation of these celestial bodies is of recent date, and has only been introduced very lately into Greece.' It does not appear that Seneca himself assisted at all the realisation of this reasonable and intelligent desire. In his time several comets appeared, but he hardly mentions them in his book, and relates no circumstance of the apparitions capable of informing us with any certainty of their apparent course.
After these preliminary considerations, which indicate so just a presentiment of the truth in the mind of the Roman philosopher, he proceeds to the explanation of the principal systems of his time, conceived for the explanation of comets. He applies himself to refute the system of Epigenes. who, like Apollonius of Myndus, had. consulted the astronomers of Chaldea, but with a very different result, the theory of Epi- genes being very nearly the same as that of Aristotle, with the exception of a few details equally false. Seneca, in combating these views, opposes to them objections that are sometimes very just, as, for example, when speaking of come- tary movements : ' There is nothing confused,' he says, ' nor tumultuous in their behaviour; nothing by which it might be inferred that they obey elements of disturbance or inconstant principles. And then, even if whirlwinds should be strong enough to seize upon the humid exhalations of the earth and bear them upwards to such heights, they would not rise above the moon; at the level of the clouds the action would cease. Now we see that comets move in the highest heavens amongst the stars.'
Seneca has carefully noted one of the characteristic dif- ferences between comets and the planets. ' Let us bear in mind,' he observes, ' that comets do not show themselves in one region of the heavens alone, nor exclusively in the circle of
43
THE WORLD OF COMETS.
the zodiac. They appear in the east and also in the west, but most frequently towards the north. The comet has its own region; it completes its course; it is not extinguished; it withdraws from our range of sight. If it were a planet, its path, it will be said, would be in the zodiac. But who can assign an exclusive limit to the stars, and confine and restrict these divine beings? The planets themselves, which alone seem to us to move, describe orbits different from each other. Why should there not be stars following courses of their own far removed from the planets ? Why should any region of the heavens be inaccessible ? '
Further on he explains with sufficient clearness the cause of the retrogressions observed in the movement of the stars and comets, and also of their occasional stationary positions.
' Why,' he says, ' do certain stars seem to turn back upon their journey? It is their meeting with the sun which gives an appearance of slowness to their movements ; it is the nature of their orbits and of circles disposed in such manner that at certain moments there is an optical illusion. Thus, vessels even when in full sail appear to be immovable.' This is in effect the true explanation, arid equally applies to the move- ments of the comets.
Seneca enumerates and describes the varied forms presented by their aspect, and then affirms that all comets have the same origin, an opinion altogether arbitrary, and relating to a matter still undetermined at the present day. Upon many points he has caught glimpses of the truth, sometimes supporting his views by reasons dictated by good sense, sometimes maintain- ing his opinion by explanations which in our day create a smile, borrowed as they are from the ideas of meteorology, astronomy, or physics received at that time, ideas quite without value, and which can only be looked upon as the crude utterances of an infant science.
44
COMETARY ASTRONOMY IN THE TIME OF SENECA.
He quotes the passage of the historian Ephorus concerning the comet of B.C. 371, a passage of extreme value, as it testifies to a phenomenon we have seen repeated in our own day, viz., the division of a comet into two parts. But it is only to treat the narrator as a dupe or an impostor. Let us, however, be just: thirty years ago our astronomers held the same opinion as Seneca, and Pingre' does not fail in this case to applaud his discernment. The doubling of Biela's comet under our own eyes was requisite in order to obtain for the testimony of Ephorus the authority which Seneca and, after him, so many modern astronomers had refused to it.
The analysis given by our philosopher of the opinion of Apollonius of Myndus affords him an opportunity of pro- nouncing in favour of a system of which the cometary theories of modern times are the infinitely extended development. But he is not contented with telling us what seems to him most probable ; he boldly prophesies in the name of the science of the future. These passages from the Qucestiones Naturales do great honour to Seneca, and deserve to be quoted as testimonies of the power and penetration of his intellect.
1 Why,' he observes, ' should we be surprised that comets, phenomena so seldom presented to the world, are for us not yet submitted to fixed laws, and that it is still unknown from whence come and where remain these bodies whose return takes place only at immense intervals? Fifteen centuries have not elapsed since
Greece counted the stars by their names.
How many people, at the present day, know nothing of the heavens except their aspect, and cannot tell why the moon is eclipsed and covered with darkness ! We ourselves in this matter have but lately attained to certainty. An age will come when that which is mysterious for us will have been
45 /
THE WORLD OF COMETS.
made clear by time and by the accumulated studies of cen- turies. For such researches the life of one man would not suffice were it wholly devoted to the examination of the heavens. How then should it be, when we so unequally divide these few years between study and vile pleasures ? The time will come when our descendants will wonder that we were ignorant of things so simple. Some day there will arise a man who will demonstrate in what region of the heavens the comets take their way; why they journey so far apart from other planets, what their size, their nature. Let us, then, be content with what is already known; let posterity also have its share of truth to discover.' *
* [Gibbon makes the following excellent remark (Decline and Fall, ch. xliii.) ' Seneca's seventh book of Natural Questions displays, in the theory of comets, a philosophic mind. Yet should we not too candidly confound a vague prediction, a veniet tempus, <J-c., with the merit of real discoveries.' — ED.]
46
SECTION III.
COMETS DURING THE RENAISSANCE AND UP TO THE TIME OF NEWTON AND HALLEY.
Apian observes that the tails of comets are invariably directed from the sun — - Observations of Tycho Brahe" ; his views and hypotheses concerning the nature of comets — Kepler regards them as transient meteors, moving in straight lines through space — Galileo shares the opinion of Kepler — Systems of Cassini and Hevelius.
SIXTEEN CENTURIES passed away between the prediction of Seneca and its full realisation through the accumulated researches of many astronomers and the publication of the Principia, in which Newton demonstrated the law of cometary movements. There is nothing to tell of the history of comets and of systems during this long and dreary period in which the doctrine of Aristotle prevailed, except that it is entirely filled with astrological predictions. Our first chapter contains a resume of all that the learned have found of interest concerning the apparition of comets and their formidable signification.
Towards the middle of the sixteenth century the move- ment of the Renaissance, so favourable to letters and the arts, extended its beneficent influence to the science of observation, At the end of the fifteenth century, we find Regiomontanus describing with care the movements of comets, Apian observ- ing that cometary tails are always turned in a direction from the sun; Cardan remarking that comets are situated in a
47
THE WORLD OF COMETS.
region far beyond the moon, founding his opinion upon the smallness or absence of parallax. The time had arrived when, instead of proceeding by way of conjecture and hypothesis, astronomers began to multiply observations and to give them that character of exactness and precision which they had hitherto so much needed. Many erroneous hypotheses were yet to be made, but they were subjected to discussion, and the geometrical conclusions to which they led were compared with the facts of observation. Astronomers of high repute like Tycho Brahe, Kepler, Galileo, Hevelius and Cassini were to err as to the true nature of cometary orbits; philosophers like Descartes were to seek to connect them with their bold but false conceptions of the system of the world. But the great principle that was destined to bind in one majestic whole the entire edifice of accumulated astronomical knowledge, the principle of gravitation, was ere long to give Newton a right to regard these bodies as members of the solar system, or at least as bodies subject to the same laws as the planets. From this moment cometary astronomy begins, and rises rapidly to a degree of development comparable to that of other branches of astronomy.
We will first give a rapid sketch of the principal phases of this history up to the time of Newton, and then proceed to the study of comets in connexion with their movements, their physical and chemical constitution, &c.
The apparition of the comet of 1577 may be regarded as the starting-point of the new period. Tycho, who had carefully observed the temporary star of 1572, which had suddenly appeared in Cassiopeia, now applied himself to make numerous observations of the new comet; he determined its parallax, and thus proved beyond a doubt that comets move in regions more remote than the moon, as Cardan had already remarked. Tycho endeavoured to represent the movement of the comet
48
COMETS DURING THE RENAISSANCE.
by making it describe around the sun an orbit external to Venus. With respect to its physical nature he regarded it as a meteor, but not an atmospheric meteor, since he supposed it to have been engendered in the depths of space. This was a first blow to the ideas of Aristotle, which other contemporary astronomers, such as Maestlinus and Rothmann, continued to profess.
The comets of 1607 and 1618 furnished Kepler with an opportunity of explaining their apparent movements, and in- venting an hypothesis which, although false, was ingenious. According to the immortal author of the three great laws of the planetary motions, comets traverse the solar system in rectilinear orbits, and Pingre* justly remarks that the apparent movement of the comets of 1607 and 1618 is more naturally explained by this hypothesis than by that of Tycho, which is equivalent to saying that the paths of the two comets were more nearly straight lines than circles. As to the physical nature of comets, believed by Kepler to be as numerous in the heavens as fishes in the sea, his remarks on the subject taken from the second book of his work upon comets are as follows : ' They are not eternal, as Seneca imagined; they are formed of celestial matter. This matter is not always equally pure; it often collects like a kind of filth, tarnishing the brightness of the sun and stars. It is necessary that the air should be puri- fied and discharge itself of this species of filth, and this is effected by means of an animal or vital faculty inherent in the substance of the ether itself. This gross matter collects under a spherical form; it receives and reflects the light of the sun, and is set in motion like a star. The direct rays of the sun strike upon it, penetrate its substance, draw away with them a portion of this matter, and issue thence to form the track of light which we call the tail of the comet. This action of the solar rays attenuates the particles which compose the body of
49 • , E
THE WORLD OF COMETS.
the comet. It drives them away; it dissipates them. In this manner the comet is consumed by breathing out, so to speak, its own tail.' We see that although, in the opinion of Tycho and Kepler, comets are raised to the rank of heavenly bodies, they continue to regard them as stars of temporary origin, destined to disappear.
Some of the views of Kepler are affected by the singular and mystic conceptions of the great astronomer concerning the heavenly bodies; yet those relating to the formation 'of cometary tails, as we shall see further on, have been perfected and adopted by contemporary astronomers, and form the starting-point of one of the most accredited modern theories of cometary phenomena.
Galileo also believed that comets move in straight lines, but he was unable to rise above the common opinion, according to which they were mere transient meteors, exhalations of the earth.
The remarkable comets which appeared about the middle of the jixleenth century — namely, those of 1664, 1665, and 1680 — attracted the attention of all men of science; the idea that they were veritable stars more and more gained ground, and, after the lapse of fifteen centuries, a definitive return was made to the system of Apollonius of Myndus; but modern astronomy was more exacting than the science of the ancient Greek philosophers. It was necessary to satisfy numerous and precise observations and to pass beyond vague ideas and con- jectures. Henceforth the whole question reduced itself to the investigation of the geometrical form of the orbit described by comets and to the determination of the laws governing their movement.
Cassini attacked this great problem, but he did not arrive at its solution, which is not surprising, when we bear in mind that this illustrious astronomer did not yet dare to abjure the
50
COMETS DURING THE RENAISSANCE.
beliefs that Copernicus and Galileo had overthrown concerning the system of the world. By regarding the earth always as a fixed observatory he could not but confound the apparent motions of comets with their real motions. Cassini rightly supposed them to be stars, old as the world, but he made them describe circular orbits very eccentric to the earth, in order to Account for the slight portion of the orbit that is visible during the brief durations of their apparitions.
Hevelius, a laborious observer, came back very nearly to Kepler's system, that is to say, to rectilinear orbits, or orbits sensibly rectilinear. Comets, in his opinion also, are the products of exhalations rising from the earth, the planets, or the sun. Drawn away at first by an ascensional movement, combined with the rotatory movement of the planet that has given it birth, the mass, after having described a spiral, finally attains the limit of the vortex which surrounds the planet; there it dies or escapes along the tangent to the limiting surface. The resistance opposed to it by the ether modi- fies the form of its orbit, which would otherwise be rec- tilinear, and causes it to take the form of a parabola. The whole of this system is purely imaginary, and must have made great demands upon the imagination of its author ; it rests upon no solid basis of astronomical mechanics. The ideas of Hevelius found but few partisans amongst men of science; the work in which they are developed, valuable for the historic details it contains, and for various observations of comets, more especially those of 1652, 1664, and 1665, is little more than an object of curiosity in the history of science.
Newton, moreover, was about to put an end to all these hypotheses, by connecting the movements of comets with the laws that govern the motions of all the heavenly bodies which move within the sphere of the sun's attraction.
61
SECTION IV.
NEWTON DISCOVERS THE TRUE NATURE OF COMETARY ORBITS-
Newton's Principia and the theory of universal gravitation — Why Kepler did not apply to comets the laws of the planetary movements — Newton discovers the true system of cometary orbits — Halley and the comet of 1682; prediction of its return.
KEPLER, in 1618, had already discovered the three laws upon which his fame rests, and which will render his name im- mortal. These laws govern the movements of bodies which, like the planets and the earth, revolve about the sun in regular periods. In virtue of the first law the orbit described about the sun is an ellipse, of which the sun itself occupies one of the foci ; the second relates to the velocity of the planet, a velocity which is greater the nearer the planet is to the sun, and less in proportion as it is further removed; or more accurately the velocity is such that the areas of the sectors swept out by the radius vector of the planet are equal in equal times; hence it follows that the maximum of speed takes place at the perihelion, and the minimum at the aphelion. The third law expresses the constant relation which connects the duration of each periodic revolution with the longest diameter, or major axis of the orbit.
Why did not Kepler apply the planetary laws to the move- ments of comets? Why did he leave to Newton the merit of an extension which now appears so natural? Because those
52
NEWTON DISCOVERS THE TRUE NATURE OF COMETARY ORBITS.
portions of the cometary orbits visible from the earth are nearly always small fragments only of the immense and elon- gated curve described by comets in their total revolution ; because in Kepler's time no instance was known of a comet having effected its return; and, lastly, because the powerful mind of Kepler himself was, doubtless, enslaved by the general belief that comets were passing, transitory meteors.
Newton, aided by the recent progress of mathematical and physical science, attained to a higher conception of the move- ments of the celestial bodies ; he discovered the reason of those laws which the genius of Kepler had extracted from Tycho Brahe's observations and from his own; he gave them a mechanical interpretation; in short, he deduced from them the celestial movements as so many necessary consequences of a single principle — the mutual gravitation of the masses of these bodies and the earth.
From that time comets no longer eluded the investigations of science. Obeying the law of gravitation, describing orbits like the planets, owning the sun for their common focus, their movements are distinguished from those of the planetary bodies chiefly by two important differences, the first of which arises from the inclination of their orbits to the plane of the earth's motion : instead of being confined within narrow limits this inclination may assume any value whatever. From the earth comets can be seen, and indeed are s'een, in all regions of the heavens, whilst the apparent paths of the planets are confined to the narrow zone called the zodiac. The second difference arises from the fact that a comet generally performs its revolution in a very elongated ellipse; for this reason we see only a very restricted portion of its orbit; beyond this arc of visibility, on either side, the comet is plunged into depths of space so remote from the earth that it is lost to view. And then, again, the duration of a comet's revolution is generally
63
THE WORLD OF COMETS.
so great as to render impossible the recognition of the same comet on two successive apparitions; at any rate, this had been the case up to the time of Newton. Ellipses so elongated if we confine ourselves only to the arc described in the neigh- bourhood of the perihelion, are undistinguishable from parabolas havino- the same focus and the same vertex. Newton, taking
O
advantage of this approximate assimilation, gave the means of determining, by the employment of a small number of observa- tions, the elements of a comet's orbit regarded as a parabola, a problem much more simple than that which has for its object the investigation of the complete ellipse.
It still remains to point out another difference between the motions of comets and the planets. The movements of the latter are always direct, and invariably take place, for an observer situated upon the northern side of the plane of the ecliptic, from left to right, or from west to east. The move- ment of some comets is direct, and that of others retrograde. This circumstance had great weight in securing the adoption of Newton's Primipia in preference to the vortices of Descartes. If the planetary heavens were filled with vortices of matter circulating in the same direction around the sun and around each body belonging to the system, how could we explain the fact that comets are able to traverse these media in a direction opposite to that in which the latter are moving?
All these views, so simple, and at the same time so grand in their entirety, were not, as we know, readily admitted by the philosophers and astronomers of the time of Newton. Still imbued with the spirit of systems and sects, some in- clined to the old doctrines derived from Aristotle, and others to the bold novelties of Cartesianism.
But the actual truth was very shortly to be made clear.
Halley, an illustrious contemporary of Newton, contributed to its triumph in the matter of cometary theories. He under-
54
NEWTON DISCOVERS THE TRUE NATURE OF COMETARY ORBITS.
took the calculation — at that time a very laborious task — of the orbits of twenty-four comets of which the observations appeared to be sufficiently numerous and accurate. He compared them with one another, and thought he recognised the identity of several amongst them. A comet lately observed — that of 1682 — appeared to him similar to the comets of 1607 and 1531. He satisfied himself of this agreement ; he affirmed it to be the same comet, observed on several successive apparitions, and finally predicted its return. Neither Halley nor Newton were able to see the prediction verified by the event. But the year 1759, when the return of the comet of 1682 did actually take place, marks an important date in the history of cometary astronomy, and, from this memorable epoch, there was no longer room for hypotheses — at all events, so far as the motions of comets are concerned.
The time has now come for us to enter upon the scientific portion of our subject.
CHAPTER III.
THE MOTIONS AND ORBITS OF COMETS.
SECTION I.
COMETS PARTICIPATE IN THE DIURNAL MOTION.
COMETS participate in the diurnal motion of the heavens. During the time of their apparition they rise and set like the sun, the moon, the stars, and the planets. In this respect, therefore, they do not differ from other celestial bodies.
Let the observer, when a comet is in sight, note the point in the heavens which it occupies when his attention is first directed to it. This is easily done by referring the nucleus, the brilliant point from which the tail proceeds, to two adja- cent stars. Let a certain time elapse — an hour, for example; at the end of that time the three luminous points, the two stars and the comet, will be found to have changed their position with respect to the horizon, each having described an arc of a circle in the heavens. The common centre of these arcs is the celestial pole, a point situated within a very small distance of the pole-star ; the lengths of these arcs depend upon the interval of time between the observations, and the angular distance of each body from the pole. The direction is that of the general movement of the heavens and the stars ; that is to say, from east to west.
We have here, then, a fact which clearly teaches us that a comet moves in regions beyond the atmosphere of the earth; for the diurnal motion is an apparent motion, foreign to the
60
THE WORLD OF COMETS.
cornet, and belongs in reality to the observer, or, as we may say, to the observatory. It is caused by the rotation of the earth upon its axis. The entire atmosphere of the earth participates in this movement, and a body immersed in it — although it might, of course, have a separate motion of its own — would not participate in the diurnal motion. This is so elementary a fact that there is no need to insist upon it further.
The ancients, and even those amongst the moderns who have regarded comets as meteors of atmospheric origin, have been compelled either to consider the earth as immovable or to admit that comets, after being formed within the atmo- sphere, withdraw from our globe, and, becoming independent, move in the heavens — a theory, as we have already seen, adopted by Hevelius.
60
SECTION II.
MOTIONS OF COMETS.
Distinction between comets, nebulse, and temporary stara — Comets, in their motions, are subject to stationary periods and retrogressions — The apparent complications arise, as in the case of the planets, from the simultaneous movement of these bodies and the earth.
THEKE is nothing in the foregoing section to distinguish comets from the multitude of brilliant stars which nightly illuminate the azure vault of heaven. Comets, it is true, appear in regions where before they had not been visible, and after a time they disappear ; but in this respect they resemble those remarkable stars which have been seen to shine out suddenly in the midst of a constellation, to increase in brilliancy for a time, and afterwards to become faint and disappear; such as the famous temporary stars of 1572 (the Pilgrim), 1604, 1670, and 1866, which appeared and became extinct in the constella- tions of Cassiopeia, Serpens, Vulpecula, and Corona Borealis respectively. These stars, however, have, without exception, been distinguished by this peculiarity, that from the first to the last day of their apparition they continued immovable in the spot where they first appeared; or, more correctly, that their only motion was that due to the diurnal revolution of the heavens. Situated, like the fixed stars, at immense dis- tances from our system, they had no appreciable movement of their own during the whole time of their visibility — in some
61
THE WORLD OF COMETS. ,
instances of considerable duration. The same is true of the nebula, which are distinguished from comets by the fact of their immobility. Hence comet-seekers have only to pursue a method analogous to that which astronomers follow for the discovery of small planets.
Comets, on the contrary, have a motion of their own, a motion oftentimes of great rapidity ; we can see that they perceptibly change their places from day to day, and some- times hour by hour, amongst the constellations. This move- ment they have in common with the planets, and it is due, as we are about to see, to the same causes.
In the first place, to confine ourselves to the real movement of a celestial body and its gradual change of place in space. Let us for a moment suppose the earth at rest. The
Fig. 5.— Proper motion of a Comet ; dis- observer Situated Oil its SUl'faCC
tinetion between a Comet and a Nebula. . ,
•would in that case see the
body in motion gradually overtake and pass the different stars in its course, and describe upon the concave sphere of the heavens a curve whose form, position, and apparent dimen- sions would depend upon the actual path of the body, and its velocity of motion. For example, the moon, which describes an oval-shaped curve or ellipse around the earth, in about a month would appear to describe a great circle in the heavens from west to east. The planets Mercury and Venus, which revolve about the sun, and describe closed orbits differ- ing more or less from a circle, but enclosed by the earth's orbit, would appear to move from one side to the other of the central luminary of our system, oscillating periodically to the
G2
MOTIONS OF COMETS.
east and west of it. The superior planets, Mars, Jupiter, and Saturn, as seen from the earth, would make the tour of the heavens in unequal periods of time, because these planets describe orbits exterior to that of the earth, and the actual time of their revolution depends upon the dimensions of their orbits.
But this simplicity of motion does not exist for an observer situated upon the earth, and for the following reasons.
The real and regular motion of the planets becomes com- bined with the motion of the earth; in the interval of a year our globe itself moves likewise round the sun in a closed curve or orbit differing but slightly from a circle; in fact, our earth moves in an ellipse whose focus is the sun. This displacement of the earth, it will be readily understood, has the effect of complicating the apparent motion of the planets ; that is, their change of position upon the starry vault. Some- times this motion appears accelerated, as will naturally happen when the planet and the earth are describing arcs in opposite directions; the two velocities are then added together, just as to a traveller in a railway train a second train, moving in the contrary direction, appears to pass with a speed equal to the sum of the velocities. But should the two trains be moving in the same direction, they then separate with a speed equal to the difference only of their velocities; and if the velocities are equal, each appears to the other motionless. This is what occurs in the case of the planets as seen from the earth ; for we observe that their velocities sometimes decrease and become nil, in which case the planet is to all appearance stationary among the stars ; and at other times it appears to retrograde.
Thus these effects admit of a very simple explanation. They are merely the result of the combination of the respective movements of the planet and of the earth in their orbits. Whatever may be the true orbit of a comet in the heavens, its
THE WORLD OF COMETS.
apparent path will always be modified by the continual change of position of our earth.
In order, then, to determine the orbit of a comet we must take into account the motion of the earth in its orbit during the time of the comet's apparition. The stationary periods and retrogressions — although, as we have seen, admitting of a most simple explanation — long embarrassed astronomers; but when the true system of the universe was discovered by Copernicus, and more fully developed by Kepler, these ap- parent complications of the celestial movements, which had always been stumbling-blocks in the way of the erroneous systems, became so many striking confirmations of the true theory.
Difficulties analogous in kind, but much more numerous and grave, long prevented astronomers from discovering the true nature of comets and the laws which regulate their move- ments. We shall now see why.
04
SECTION III.
IRREGULARITIES IN THE MOTIONS OF COMETS.
Comets appear in all regions of the heavens — Effects of parallax — Apparent motion of a comet, in opposition and in perihelion, moving in a direction opposite to the earth — Hypothetical comet of Lacaille ; calculations of Lacaille and Olbers concern- ing the maximum relative movement of this hypothetical comet and the earth.
THE orbits which the planets describe about the sun are not circles, but oval curves, termed ellipses ; these ellipses differ but little from circles ; that is to say, their eccentricities are small. Moreover, the planes of the orbits in which they move are inclined at small angles to the plane of the ecliptic. Hence it follows that their apparent paths are confined to a compara- tively narrow zone of the heavens, which zone is called the zodiac. If we imagine these curves pressed down, as it were, up- on the ecliptic they will appear as nearly concentric circles de- scribed about the sun, and so disposed as not to intersect each other. The distances of the earth and of each of the planets vary according to the position occupied by these bodies in their re- spective orbits ; but these variations are confined within very narrow limits, and hence it follows that the velocities of the planets change so slightly that the difference is all but imper- ceptible. The mean diurnal motion of Mercury, which of all the planets moves the most rapidly, amounts to only 4°5'. With comets the case is very different. These bodies, as we
fc5 F
THE WORLD OF COMETS.
have seen, are restricted to no region of the starry vault, and traverse the heavens in all directions, and with very different ve- locities. The third comet of 1739, and the comet of 1472, mentioned by Pingre", described in a single day, the first an arc of 120 degrees— that is to say, the third part of the whole ce- lestial circumference— the second, an arc of 41 degrees and a half in longitude and nearly 4 degrees in latitude. Their real movement was, it is true, in a direction contrary to that of the earth, so that their apparent velocities were in both cases made up of the sum of their own and the earth's velocity combined. Here, then, we have an instance of what is called parallax; that is to say, the apparent movement of the object is affected by the observer's own displacement. We might multiply examples of a similar kind, but the following will suffice. ' The comet of 1729,' says Lalande, ' observed by Cassini during several months, after advancing more than 15 degrees towards the west from the head of Equuleus to the constellation Aquila, suddenly curved round to retrace its path towards the east, thus showing in a very striking manner the effect of the annual parallax.'
These rapid movements are produced by very simple causes, the most important of which are the near proximity of the comet to our globe, and the direction of its movement in relation to that of the earth. The following is a supposititious case, imagined by Lacaille, in which the apparent angular velocity of a comet would be enormous.
This astronomer supposes a comet to be moving in a direction contrary to that of our globe, and in the plane of the ecliptic ; it is in perihelion, or at its least distance from the sun, and consequently at that point of its orbit in which its velocity is at its maximum. At the same time the earth is supposed to be in perihelion, and is also moving in its orbit with its greatest velocity. Lastly, the comet is to be not more distant from the earth than the moon, and it is to be in opposition.. It is, of
(36
IRREGULARITIES IN THE MOTION OF COMETS.
course, extremely improbable that all these hypotheses should be realised in the same comet, but there is nothino- im-
o
possible in them. Under these exceptional conditions the comet, seen from the earth, would describe in the heavens an arc of nearly 39 degrees in longitude during the first hour, and of 32 degrees in the hour following. In three hours the total arc described would amount to 92° 58', and this independently
Fig. 6. — Maximum apparent movement of a Comet and the Earth. '
of the diurnal movement, which would further increase the velocity by 15 degrees per hour. To an observer situated near the tropics the comet would ascend from the horizon to the zenith in less than two hours; it would, however, take a some- what longer time to perform the second half of its journey and pass from the zenith to the horizon.
The calculation of Lacaille (modified by Olbers, on account of an error) is by no means difficult to verify; and there is
67 F 2
THE WORLD OF COMETS.
nothing surprising in the result, if we reflect that the velocity of each of the two bodies, the comet and the earth, is then at its maximum; that our globe in one hour at its perihelion passes over in space a distance nearly equal to nine times its own diameter (or 67,000 miles/; that the cornet has a velocity greater than that of the earth, and passes over 94,000 miles in an hour ; so that, in the direction of their motion, these two bodies are receding from one another at the rate of 161,000 miles per hour. At the end of a day the comet and the earth would be more than 3,800,000 miles apart.
It is, therefore, easy to comprehend to what irregularities of movement comets may be subject. Traversing the heavens in all directions, in orbits the planes of which cut the orbit of the earth at every possible inclination, approaching to and receding from the earth in very short spaces of time, influenced by the diurnal motion and their own proper motion, in combi- nation with the earth's displacement, they sometimes suddenly appear, pursuing a rapid course amongst the stars ; then, to all appearance they relax their speed, and after coming to a momen- tary stop reverse their motion, and continue their journey in an opposite direction, sometimes disappearing at a distance from the sun, sometimes being drowned in his rays.
It was these movements, these singular appearances, which so long baffled astronomers, and which the genius of Newton, guided by a higher conception, finally explained. We will now proceed to define geometrically the movements and orbits of comets.
(58
SECTION IV.
THE CEBITS OF COMETS.
Kepler's Laws : ellipses described around the sun; the law of areas — Gravitation, or weight, the force that maintains the planets in their orbits — The law of universal gravitation confirmed by the planetary perturbations — Circular, elliptic, and parabolic velocity explained ; the nature of an orbit depends upon this velocity- Parabolic elements of a cometary orbit.
WHAT is the nature of a true cometary orbit? In other terms, what is the geometrical form of curve which a comet describes in space — what is its velocity — how does this velocity vary—- and what, in short, are the laws governing the movement of a comet ?
In order to reply to these questions, and to enable them to be clearly understood, we must first call to mind a few notions of simple geometry, and also the principal laws which govern the motions of the planets.
Kepler, as we have already said, discovered the form of the planetary orbits, hitherto supposed to be circles more or less eccentric to the sun. This great man demonstrated that the form of a planetary orbit is actually an ellipse, that the sun is at one of the foci of the curve, and that the planet makes its complete revolutions in equal periods of time, but with variable velocity ; in fact, that in equal intervals the elliptic sectors described by the radius vector * directed from the sun to the planet are of the same area.
* [The straight line joining the sujn to a planet or other body moving under its action is called a radius vector. — ED.]
60
THE WORLD OF COMETS.
Let us take an example. S being the sun, the closed curve APB ... is the ellipse described by a planet. The distance of the planet from the sun is variable, as we see: it attains its minimum value at A, and its maximum value at B, that is to say, at one or other extremity of the greatest diameter of the
orbit.
For this reason A is called the perihelion (from rip/, near, and fcie*, the sun); B is called the aphelion (from a™, from, and fai*g, the sun). For brevity the radius vector AS is called the perihelion distance; the radius vector SB the aphe- lion distance ; and the two united, or the sum of these two distances, forms the major axis AB of the orbit, Lastly, the
IVnlielie. A \~_
pjg. 7. —Second Law of Kepler. The areas swept out by the radius vector are proportional to the time.
mean distance of the planet from the sun is exactly equal to half the major axis.
Let us suppose that the arcs AP, P\P^ and P3B have been described by the planet in equal spaces of time. Accord- ing to the second law of Kepler, mentioned above, the three sectorial areas ASP, PiSP2, and P3SB are equal. If the curve were a circle, of which the sun occupied the centre, it is clear that the equality of these areas would involve the equality of the corresponding arcs; and as the arcs are described by the planet in equal times, it would necessarily follow that the velo- city would be the same throughout the entire orbit. In other words, a circular orbit presupposes an uniform movement.
70
THE ORBITS OF COMETS.
Hut, as a matter of fact, the planets, without exception, describe around the sun ellipses more or less elongated, that is to say, orbits differing more or less from a circle. In every case their velocity is variable ; it is the greatest possible in perihelion ; it then decreases by imperceptible degrees until the aphelion is attained, when the minimum of speed takes place. This is a direct consequence of the second law of Kepler.
A third law, discovered after years of research by this powerful genius, connects the duration of the planetary revolu- tions with the length of the major axis of their orbits. This law we have given elsewhere,* as well as some numerical examples for making it more intelligible to the non- scientific reader. We shall not return to it here, but confine ourselves to the remark that, the time of the revolution of a planet being known, the dimensions of the major axis of the orbit — that is to say, of twice the planet's mean distance from the sun — -can be deduced by a simple calculation.! These laws are not rigorously obeyed by the planets in their movements. The strictly elliptic motion
* Le Ciel, 4th edition, p. 602.
| [Kepler's third law is that the squares of the periodic times are as the cubes of the mean distances, that is to say, that if r and R be the mean distances of two planets from the sun, and t and T be the durations of their revolutions round the sun, then —
t x t : TxT :\ rxrxr '. ExUxR.
For example, taking the mean distance of th3 earth from the sun as unity, the mean distance of Venus is 0'7233 ; and the earth performs its revolution round the sun in 365'26 days, Venus in 224'70 days ; so that, according to Kepler's law,
2247x224-7 : 365'26x365'2G :: 0-7233 x 0-723,3 x 07283 : 1; or, working out the multiplications indicated,
50,490 : 133,415 :: 0-37845 : 1,
and by division it will be found that each ratio of this proportion is ec^ual to 2-642.
As another example, suppose there were two planets whose periods of revo- lution were found to be to one another as 27 to 8, then we should know that their mean distances were as 9 to 4 ; for
27x27:8x8:: 9x9x9:4x4x 4.— ED.] 71
THE WORLD OF COMETS.
supposes ideal conditions that are not present in nature. But, by advancing them at an epoch when observations were so far from accurate, Kepler left it for astronomers and mathema- ticians coming after him to discover the cause of that mechan- ism of which he had only been able to detect the general laws. Huygens, Newton, and later many illustrious mathematicians (foremost among them Euler, D'Alembert, Clairaut, Lagrange, and Laplace), have explained the reasons not only for the general movements of the celestial bodies, but also for all the irregularities and inequalities which their movements undergo in the course of time.
Ultimately the whole matter resolves itself into a question of two causes, or of two forces. One of these forces is none other than weight or gravitation— the tendency that two bodies or two stars have to become united, a tendency which is propor- tional to the product of their masses, and which varies inversely as the square of the distance that they are apart. By their weight bodies fall to the surface of the earth when left to them- selves in the atmosphere. If the force of gravitation alone existed, the moon would fall upon the earth, and both would together fall with ever increasing speed into the sun, and so likewise would the planets arid all the bodies of the solar system.
But, in addition to the central force of gravitation, each planet is animated by another force,* which of itself would cause
* [It is perhaps well to explain that this so-called centrifugal force is not a force in the sense in which gravitation is, i.e., it is not an external force acting upon the body. If a body were projected in space and were not interfered with by any external force, it would continue to move in a straight line. In order, therefore, that it may deviate from a straight line it must be acted upon by some external influence or force, and the resistance this force woiild have to overcome for the body to change its direction of motion is called ' centrifugal force. ' Thus the ' centrifugal force' measures the tendency the body has to continue to move in the direction in which it is moving at thje instant. If then a body describes a curve, some external force must be continually acting upon it, as it is
72
THE ORBITS OF COMETS.
the planet to escape in a straight line in the direction of a tan- gent to the point of its orbit occupied by the planet. By com- bining these two forces, and seeking by geometry and analysis to determine the actual motion resulting from their simultaneous and constant action, Newton demonstrated that the laws of this movement were in conformity with those which Kepler had dis - covered. If one planet alone existed and circulated around the sun, and if its mass were inappreciable in comparison with the enormous mass of that luminary, the elliptic movement would conform rigorously to Kepler's laws. But the planets are more than one in number ; they act and react upon each other ; their dimensions and masses are more or less unequal ; they recede from and approach one another in the course of their revolu- tions, and their mutual action upon one another is an incessant cause of disturbances and perturbations. It is important to notice that these perturbations are not exceptions in the true meaning of the word; far from invalidating the theory, they afford the most striking confirmation of it, since each of these deviations may be calculated beforehand by the theory of uni- versal gravitation.
But let us here terminate this necessary digression and return to the comets.
Newton, as we have seen, by a bold but logical generali- sation, supposed comets to be subject to the same influences as the planets, to be borne along by a primitive force of impulsion, and continually drawn by gravitation towards the sun, the focus of all the movements of our system. Let us endeavour to ex-
continually changing its direction of motion. In the case of a planet or other body describing an ellipse round the sun, the sun is continually pulling it towards itself ; and this continued action is necessary to overcome the centrifugal force, i.e., to balance its tendency to move at every instant in the tangent to its path ; in fact, if the action of the sun suddenly ceased, the planet would immediately move off along the tangent to the ellipse at the point where it was, and with the velocity it had, at the instant. — ED.]
73
plain by some simple examples what must be the orbit of a body acted on by such influences ; to explain, let it be under- stood, not to demonstrate.
Consider, then, a heavy mass, a planet M, gravitating towards the sun, and at the same time moving with a certain velocity due to an impulsion foreign to gravitation; and suppose, for
pig. s. — Relation between the velocities and forms of Orbits,
the sake of greater simplicity, that M is situated at the point where the planet is moving in a direction perpendicular to the radius vector joining the planet and the sun.
The geometrical form of the orbit described by the planet about the sun will depend solely upon the relation between the initial velocity of the planet and the distance of the latter from the sun. For a certain value determined by this relation
74
THE (WHITS OF COMETS.
the curve described becomes a circle of which the sun occupies the centre, and the planet traverses with uniform velocity every part of the circumference. The velocity which for a given distance compels a mass subject to the law of gravitation to describe a circle is known as circular velocity. A less velocity would give rise to an elliptic orbit; in which case, the sun, instead of occupying the centre of the curve, would be situated at one of the foci, namely, that which is the further removed from M\ and the point M would be the aphelion of the planet.
If the velocity be greater than circular velocity the orbit would still be an ellipse, having the sun in the focus; but in this case J/ is the perihelion, and the planet attains its greatest distance from the focus of attraction at the opposite extremity of the diameter MS.
The greater the initial velocity the more elongated will be the orbit, and the greater the eccentricity* of the ellipse. But if the velocity should attain a certain value — viz., should be equal to circular velocity multiplied by the number 1*414 (or by the square root of 2) — at this moment the ellipse, the major axis of which has been continually lengthening, and has at last increased in the most rapid manner, changes into a curve with endless branches, called a parabola. A planet animated by this velocity, or, let us say, by parabolic velocity, at the moment when it is at its least distance from the sun — i.e. when it is at its perihelion — is a body which comes to us out of infinite
* The eccentricity is the distance from the centre of the ellipse to one of its foci, measured in parts of the semi-major axis, which is taken as unity. In an elliptic orbit the eccentricity is always less than unity, and is usually expressed in decimal fractions. Amongst the orbits of the eight principal planets that of Mercury has the greatest eccentricity. 0'2056 ; Neptune and Venus have the smallest, 0'0087 and 0'0068. Both these orbits differ very slightly from a circle. In a parabola the eccentricity is equal to 1. In a hyperbola it is greater than unity.
75
THE WORLD OF COMETS.
space and returns into infinite space; such a body, supposing one to exist, before arriving at -that region of the heavens where the action of the sun preponderates, could form no part of the solar system. After passing its perihelion it would depart to an infinite distance; and unless the form of its orbit should become changed by the disturbing influence of the planets, it would again become alien to the solar group.
Lastly, to omit no case that can possibly occur, we must consider that of a planet moving with a velocity greater than parabolic velocity; the orbit now described will continue to be a curve of endless branches, but it will be an hyperbola, of which the sun is, as before, situated at one of the foci.
These preliminary notions understood, we are in a position to consider the question of the geometrical determination of cometary orbits.
These orbits are, in general, very long ellipses, of con- siderable eccentricity, that is, of eccentricity very nearly equal to unity. And this explains why comets remain visible during comparatively so short a time, as the arc which they describe is only a very limited portion of the entire orbit. During the remainder of their journey they are too far distant from the earth to be perceived either by the naked eye or by the aid of the most powerful telescope.
The orbit of a comet being thus a very long ellipse, and the portion of the arc observed being of very limited extent as compared with the dimensions of the whole orbit, it follows that it is generally very difficult to determine to what ellipse this arc belongs, or even to decide whether it may not form part of a parabola or hyperbola of the same perihelion distance.
These different curves are, so to speak, blended into each other, and only become sensibly distinct at a distance too remote for the comet to be within our range of vision. In these dif-
76
THE ORBITS OF COMETS.
ferent orbits the positions of the comet obtained by calculation would not be distinguishable from the positions obtained by direct observation, or would differ by quantities so small as to be liable to be confounded with the errors made in the obser- vations themselves.
Fig. 9. — Cometary Orbits, elliptic, parabolic, and hyperbolic.
This was recognised by Newton, who at once conceived the idea of simplifying the problem involved in the determination of cometary orbits. He assumed the orbit, in the first instance, whatever might be its real form, to be parabolic, because the elements of a parabola, or the conditions which determine its position in space, its form, dimensions, &c., are less numerous and more simple than the elements of an ellipse.
Let us, then, consider what are the elements of a parabolic orbit. A parabola is a plane curve, that is to say, a curve all
77
THE WORLD OF COMETS.
the points of which are situated in the same plane, which in our case passes through the centre of the sun. The first thin* therefore, is to define the true position which this plane occupies in space. This will be accomplished by deter- mining first the line of intersection in which it cuts the plane of the earth's orbit, or the ecliptic ; and, secondly, the inclination or the angle which the two planes make with one
another.
The comet in its movement necessarily cuts the ecliptic
in two diametrically opposite points, called the two nodes ; the
line which joins these two points and passes through the centre of the sun is called the line of nodes. It is suffi- cient to know one of the nodes — for example, the as- cending node — that is to say, the node which corresponds to the passage of the comet from the region south of the ecliptic to the region north of the ecliptic. Let N (fig. 11) be this point, which can be obtained by calculation from observations of the co- met; its position will be de- termined if we know in de- grees, minutes, and seconds
Fig. 10.— Confusion of the arcs of Orbits of the ValllC of the arc 0& Ol'
different eccentricities in the neighbourhood of /. i i /-\OTVT J
the perihelion. of the angle OhJy measured
from the zero of the eclip- tic, in the direction in which the celestial longitudes are reckoned.
78
THE OK BITS OF COMETS.
This first element is called the longitude of the ascending node, or, more simply, the longitude of the node. But the plane of the orbit remains undetermined, unless we add to it a second element, viz., its inclination.
If, through the centre of the sun, we imagine two straight lines drawn perpendicularly to the line of the nodes, the one in the ecliptic, and the other in the plane of the comet's orbit, these two lines will make between them two angles, the smaller of which measures the angle between the two planes. The angle i is the inclination.
It next becomes necessary to define and fix accurately the actual curve described by the comet in this plane, deter- mined by the longitude of its' node and its inclination. In the first place, we must know the position of the planet at its peri- helion, or least distance from the sun. Let A be this point. SA is then the axis of the parabola, the direction of which will be known, if we determine the longitude of the point A, or of the point TT, obtained by projecting SA upon the ecliptic. If to the longitude of the perihelion we add another element, the length SA, or the perihelion distance — which, like all celestial distances, is measured in parts of the sun's mean distance from the earth — the vertex of the parabola will be completely nxed.
The parabolic curve described by the comet is now entirely defined, both as regards its position in space and its dimen- sions. It remains, however, to find the direction of the comet's movement, and to determine at what epoch the comet will occupy any given position in its orbit. For the purpose of obtaining the direction we will suppose the para- bola laid or, pressed down upon that side of the ecliptic where the inclination is least, or more simply, in the language of geometers, projected upon the plane of the earth's orbit. The direction of movement will be called direct, if, when
79
THE WORLD OF COMETS.
estimated from above the ecliptic or from the north region of the heavens, it takes place in a direction from right to left,
Fig. 11.
Fig. 13. Fig. 14.
Determination of a cometary orbit : parabolic elements.*
or from west to east, as is the case with the earth and all the planets, and retrograde when it takes place in the contrary direction.
* 1. Inclination, 20°. Direction in longitude of the line of Nodes, 35° to 215°.
Fig. 11. — Movement retrograde.
Node Perihelion
35C . 318C
Fig. 12. — Movement direct. Node .215°
Perihelion
. 318C
Node Perihelion
Node Perihelion
Movement direct.
Movement retrograde.
Fig. 13. — Movement retrograde.
Node 215° | Node
Perihelion . 318° | Perihelion
Fig. 14.— Movement direct.
Node 35° Node
Perihelion . . . 318° Perihelion
80
Movement direct.
Movement retrograde.
35C 112C
215° 112C
215C 112C
35C 112C
THE ORBITS OF COMETS.
Lastly, the exact date of the perihelion passage of the comet completes the determination of the orbit both in time and space, so that all other positions are deducible by calcu- lation from the elements we have mentioned. Figs. 11, 12, 13, and 14 show the different cases that may arise, that is to say, the different positions the same parabolic orbit may occupy with respect to the plane of the ecliptic, when the inclination, the line of the nodes and the perihelion distance remaining the same, the direction of movement only is varied. It will be seen that eight distinct paths are open to the comet in space.*
Briefly to recapitulate, we subjoin in the following table these various elements, in the order usually adopted by as- tronomers, taking for examples the two great comets which appeared in 1744 and 1858 : —
T, Epoch of perihelion passage, 1744, March 1. 7h. 55m. 39s. Paris mean time. TT, longitude of perihelion . . 197° 13' 58" ^
Q. longitude of node . . . 45 47 54 ,, . I^^A'
<,,* r, A-, / Mean equinox, 1744*0 i, inclination . . . . 47 7 41 |
q, perihelion distance . . . 0-222209 J
Movement direct, D.
T, Epoch of perihelion passage, 1858, September 29. 23h. 8m. 51s. TT, longitude of perihelion . . 36° 12' 31" Q, longitude of node . . . 165 19 13
i, inclination . . . . 63 1 49
q, perihelion distance . . . 0*57847
Movement retrograde, R.
Such are the elements the determination of which is neces- sary to enable us to find the orbit of a comet supposed to be parabolic. These elements are not determined directly, but
* [There are two planes (N A N) each having the same inclination i, and, the perihelion distance remaining the same, there are therefore four positions of the vertex (A) of the parabolic orbit, viz., two in each plane, one above and the other below the plane of the ecliptic, as shown in the four figs. 11-14. As, also, the direction of motion of the comet in the parabolic orbit may be either direct or retrograde, we have, in all, eight cases. — ED.]
81 G
THE WORLD OF COMETS.
are derived by mathematical calculation from a certain num- ber of observations of the comet, at least three accurately observed positions of the comet being required. Three com- plete observations are strictly indispensable; but in order to deduce from them the true curve of the orbit it is necessary that they should have been made with the utmost precision. One or two positions of the comet would leave the problem indeterminate. If we have more than three, they are of great value for verifying the results given by calculation. Of course all the observed positions should correspond to points lying on the orbit which has been determined, or, in other words, the calculated ephemeris should agree with the apparent path obtained from direct observations of the comet.
But if, all these considerations being fulfilled, the difference between the observations and the calculated results should nevertheless prove too great to be attributed to errors in the observations themselves, it is then proper to conclude that the comet is not describing a parabola, and that the hypothesis of a parabolic orbit must be rejected, in which case there remains no other alternative than that of a hyperbolic or elliptic orbit. The latter are much the more common ; and it is thus that we have been led to recognise the periodicity of certain comets. We are, in this case, concerned with a body which forms a part of the solar system, and Avhose movements are regulated in the .same manner as those of the planets.
82
SECTION Y.
THE ORBITS OF COMETS COMPARED WITH THE ORBITS OF THE
PLANETS.
Differences of inclination, eccentricity, and direction of motion.
IF, then, periodical comets, calculated as such, and known to be periodical by their return, are governed by the same laws as the planets, why is a distinction made between these two kinds of celestial bodies? This is a question of high importance, and one which we cannot completely answer at the present moment. A full reply would necessitate some definite know- ledge concerning the origin of the bodies which compose the solar world. It would be necessary to have studied and com- pared the physical constitution of comets with that of planets. Both in origin and constitution we shall see further on that they appear to be essentially different. Surveying the ques- tion, however, from a single point of view, regarding it as a question of movement only, we can already show differences which separate these two classes of celestial bodies, and justify the double denomination by which they are distinguished.
Comets, as we have already seen, appear in any quarter of the heavens, instead of moving, like the planets, in the narrow zone of the zodiac. This difference arises from the inclinations of their orbits to the plane of the ecliptic. Among the prin- cipal planets Mercury alone has an inclination as great as
83 G 2
THE WORLD OF COMETS.
7 decrees; and among 115 telescopic planets 29 only have an inclination greater than 10 degrees, and very few exceed 30 degrees;* but we see, on the contrary, the planes of come- tary orbits admit of all inclinations. Out of 242 comets which have been catalogued 59 have an inclination included between 0 and 30 degrees, 93 have inclinations between 30 and 60 degrees, and 90 an inclination amounting to between 60 and
90 degrees.
This first characteristic is important. When we add to it . the second distinction, that, whilst the movement of the planets is without exception direct, out of 242 comets 123 have a motion that is retrograde, it is impossible not to recognise a difference of origin in the two classes of bodies. It is never- theless curious to remark, that out of nine comets whose return has been established there are eight whose movement is direct; one alone, the great comet of Halley, which is a comet of long period, moves in a direction contrary to that of the planets ; and one alone, that of Tuttle, a comet of mean period, moves in a plane whose inclination to the ecliptic is considerable (54 degrees). The inclinations of each of the eight others are less than 30 degrees.
Let us proceed to another distinctive feature of cometary and planetary orbits. We have already seen that of the eight principal planets Mercury is that which describes an orbit which differs most from a circle. The distance, however, between its aphelion and perihelion distances does not amount to half its mean distance. Its mean velocity is 29-2 miles per second ; at the aphelion it is not less than 24 '9 miles ; at the perihelion it attains 37 miles per second. The orbits of the other principal planets differ much less from the
* Felicitas has an inclination of 31^°, Pallas of 34°. The very great incli- nations of some of the small planets, belonging to the group comprised between Jupiter and Mars, have obtained for them the appellation of extra-zodiacal planets.
84
THE WORLD OF COMETS.
figure of a circle. But in the group of small planets there are orbits the eccentricity of which markedly exceeds the orbit of Mercury; twenty-six of these ellipses have greater eccentricities ; but one in particular, that of the planet Polyhymnia, has an eccentricity comparable to that of some elliptic cometary orbits. Fig. 15, in which the orbit of Faye's comet and the orbit of the planet Polyhymnia are represented, as regards their forms and relative dimensions, clearly shows how close is sometimes the degree of resemblance in point of eccentricity between cometary and planetary orbits.
The divergence may be of any amount; the eccentricity of the great majority of cometary orbits is so great that it may be considered equal to unity, and this is ex- pressed, let us repeat, by as- similating them to parabolas. Is this assimilation to be considered absolute, or are we to suppose that all comets belong to the solar world? It appears certain that some orbits at least are hyperbolic. Fig 15._Comparison of the eccentricities of the As regards these there can be ^it,of F^e's Comet with that of the Planet
Jrolynymma.
no doubt. But if so, it may
be regarded as not improbable that amongst observed comets there are some which describe true parabolas; so that, after having once arrived within the sphere of the solar gravitation, like those which describe hyperbolic orbits, they take their leave of us for ever.
Amongst the comets whose periodicity has been calculated there are some which describe ellipses of such great eccentricity
85
THE WOULD OF COMETS.
that as far as we or our descendants are concerned, it is almost the same as if they were non-periodic. The great comet of 1769 (eccentricity 0-9992) has a period of about twenty-one centuries ; at its aphelion it will reach a point in space the distance of which from the earth will be 327 times the distance of the earth from the sun. The comets of 1811 and 1680 have periods respec- tively of 3,065 and 8,814 years (eccentricities 0*9951 and 0-9999). The first cornet of 1780 and that of July 1844 will only return to their perihelia after journeys the respective durations of which will be 75,840 years and about a thousand centuries. These comets will penetrate so far into the depths of space that at the time of their aphelion they will be distant from our world about 4,000 times the distance of the sun.
If the calculations upon which these necessarily uncertain values depend are not rigorously exact, they nevertheless show that the comets to which they relate always remain an integral part of our system. Their greatest distance is still fifty times less than that of the nearest fixed star. The action of the sun < «viV /A/./ upon these bodies will, therefore, always preponderate over that of any other body, and their masses will be incessantly drawn towards those regions of the heavens traversed by our earth, unless, indeed, the perturbations which the planets can exer- cise upon them should interfere so as to divert them from their course and modify the elements of their orbits.
SECTION VI.
DETERMINATION OF THE PARABOLIC ORBIT OF A COMET.
Three observations are necessary for the calculation of a parabolic orbit — Oometary ephemerides ; what is meant by an ephemeris ; control afforded by the ulterior observations — Elements of an elliptic orbit — Can the apparition or return of a comet be predicted ? — State of the question — Refutation by Arago of a current prejudice.
THREE observations of a comet — that is to say, three different positions (in right ascension and declination) of the nucleus of a comet, or, in a word, three points of its trajectory or apparent orbit sufficiently distant from each other — are required, as we have said, for the calculation of the parabolic elements of the true orbit.
In the last century this determination was not only a long and laborious operation, but involved much tentative and uncertain work. Before engaging in the difficult calculation of the elements of an orbit, astronomers made trial graphically and even mechanically of different parabolas, and only began £he calculation after satisfying themselves that one of these curves nearly represented the positions furnished by obser- vation. Great improvements were introduced into these methods during the last century by Lalande, Laplace, and Gauss. But the calculation of a cometary orbit is always a sufficiently complex operation, even if it be simply parabolic, and it still takes a skilful computer accustomed to this kind of work, several hours to find approximate values of the different elements. This is not the place for us, of course, to attempt an explanation of the work itself.
87
THE WORLD OF COMETS.
A first orbit having been found, what astronomers call an ephemeris is then deduced from it. This term is applied to the calculated positions which the comet must have occupied or will occupy day by day during the period of its visibility. These calculated positions should agree with the observed positions, that is to say, with the positions obtained by direct observations with instruments. This comparison furnishes a means of control from which it will result either that the elements are correct, or, on the contrary, that the parabola is unfitted to explain the movements of the comet. In the latter case it remains to examine whether this movement might not be better represented by an hyperbolic orbit, or, as most frequently happens, by an ellipse. In this way a certain number of comets have been found to describe ellipses round the sun, and have been accordingly classed amongst periodical comets of the solar system.
We may remark, while speaking of elliptic orbits, that two more elements must be added to the elements of parabolic orbits for the purpose of determining an elliptic orbit : firstly, the eccentricity above defined, and v/hich, in conjunction with the perihelion distance, enables us to calculate the major axis of the orbit ; secondly, the duration of the revolution, a duration connected with the value found for the major axis by the third law of Kepler.*
* Take for example, the following table of the elliptic elements of Tempel's short period comet, 1867. II., for its return in May 1873 ; e is the eccentricity, a the semi-axis major : —
Perihelion Passage, May 9-74218. TT, longitude of perihelion . 238° 2' 34"
£3, longitude of node . . 101 12 50
z, inclination ... 9 12 6
e.
eccentricity ..... 0-5076428 a, semi-axis major .... 3-1721
Movement direct.
Duration of the
revolution, 5 years 97 days.
DETERMINATION OF THE PARABOLIC ORBIT OF A COMET.
This leads us to say a few words on a question which has nearly always been imperfectly understood by the public, notwithstanding the repeated explanations of astronomers : we mean the possibility of predicting the advent of a comet.
Can the apparition of any comet whatever be predicted ?
In these terms the preceding question has been invariably asked. As regards the public which has faith in astronomical science, but very little knowledge of astronomy, an answer in the affirmative is not for a moment doubted ; and, in their opinion, astronomers who allow themselves to be surprised by the apparition of a comet have certainly failed in their duty — in their duty as observers, if the discovery of this new comet rests with an amateur, and in their duty as mathe- maticians, if they have not foretold it.
As a rule these reproaches are unjust. They are founded upon a false idea of the power of astronomy and the nature of cometary orbits. Arago, who never lost an oppor- tunity of endeavouring to destroy popular misconceptions on scientific matters, has given a perfect refutation of this error, which, nevertheless, is still widely spread. The opportunity was furnished by the brilliant comet of 1843, which appeared unexpectedly, its arrival not having been announced by astronomers, and with reason. Let us, therefore, endeavour, following the example of the late well-known Secretary of the Academy of Sciences, to dissipate this generally received error as far as lies in our power,
On referring to the preceding sections of this chapter we perceive that the greater number of comets which have been seen and observed * from the earliest times to the present day
* It should be borne in mind that to see a star and to observe it are two very different things. In the long list of comets mentioned in history, from the earliest times to the eighteenth century, when Pingre lived, the indefatigable author of the Cometographie is unable to find more than sixty-seven comets observed with sufficient accuracy to allow of their orbits being calculated.
89
THE WORLD OF COMETS.
describe parabolas, or, at all events, ellipses so elongated that we may be certain either that these comets have never visited our world before, or that their visits have been made in pre- historic times. For this reason they will never return, or if they should return it will be at an epoch so far distant from our own that it need not for a moment occupy our attention. It is, therefore, evident that a cornet which thus appears for the first time within sight of the earth could not have been announced before it was perceived: no prediction of its appa- rition was possible.
Here, then, is a first point established, which, I repeat, applies not only to the great majority of recorded comets, but also to comets which have been catalogued 5 that is to say, to comets whose orbits have been calculated with more or less precision. Out of the 262 comets in the catalogue that we publish at the end of the present volume nine only are periodical comets whose return has been verified by observa- tion ; sixty others have elliptic orbits, but the greater number of these are so eccentric that for our present purpose they practically fall within the category of comets with infinite orbits.
Arago was, then, perfectly justified in the following remarks in reference to the above question so incessantly repeated by persons who are not astronomers. ' Is it reasonable to hope,' said he, 'that a time will come when we shall be able to predict the arrival within our sphere of vision of comets which have remained for ages as if lost in the furthest regions of space, which no one has ever seen, whose action upon the bodies of the solar system is too small to be appreciable, both in consequence of the excessive rarity of the vaporous matter of which they are composed, and of their prodigious distance? A comet is revealed to man when it becomes visible or pro- duces some perceptible effect. That which has never been
90
DETERMINATION OF THE PARABOLIC ORBIT OF A COMET.
beheld, and has never produced any observed displacement,* is for us as if it had never existed. The announcement of the apparition of a new and totally unknown comet would belong to the domain of sorcery, and not to that of science. Astrology itself never pushed its pretensions so far even in the day of its greatest favour.' — Annuaire de 1844.
* Theoretical astronomy has attained, in fact, to such perfection that the perturbations of unknown bodies have led to the discovery of new planets, as in the case of Neptune. Arago, who wrote the above passage in 1844, ten years before the discovery of the planet Neptune, has thus, as it were, foreshado wed the possibility of such a prediction. •
91
CHAPTER IV. PERIODICAL COMETS.
SECTION I.
COMETS WHOSE RETURN HAS BEEN OBSERVED.
How to discover the periodicity of an observed Comet and predict its return — First method : comparison of the elements of the orbit with those of comets that have been catalogued — Resemblance or identity of these elements ; presumed period deduced from it — Second method: direct calculation of elliptic elements — Third method.
THERE are, however, a certain number of comets of whose re- turn astronomers are certain, and the time of whose apparition they can calculate. The prediction of the probable epoch at which these comets will be situated in regions of the heavens where they will be visible from the earth, and the determina- tion of their perihelion passage, can be effected more or less accurately. These are the comets whose orbits, when calcu- lated from a sufficient number of observations, prove to be neither parabolas nor hyperbolas, but, on the contrary, are closed and elliptic, and such that - the comet thenceforth continues to describe them in regular periods ; in a word, they are periodical comets* Newton treated the orbits of comets as parabolic, merely in order to so represent the arc, always very short, described in the neighbourhood of the perihelion, when the" comparatively small distance of the comet from the sun
* [It may be stated here that the duration of revolution of a body, that is, the time occupied by it in a complete revolution round the sun, is called its ' period.' And, in general, the period of any periodical phenomenon is the in- terval of time between two of its successive returns to the same position. — ED.]
95
THE WORLD OF COMETS.
renders observations possible. In his opinion comets were bodies of regular periods, and which described ellipses, certainly very elongated, but in all respects similar to the planetary orbits. The first certain proof of the periodicity of a comet, the indis- putable return of a comet in the same orbit, was, therefore, a confirmation and a brilliant triumph for the Newtonian theory. Neither Halley, who had the glory of the first prediction, nor Newton, who made it possible, lived long enough to see the event justify the theory. Since then, as we are about to see, facts of the same kind have been multiplied, and the number of comets whose return can be calculated, and which, more- over, have actually reappeared, is already considerable, and is gradually increasing. Side by side with the planetary system, therefore, another system was being founded, and the history of this part of our solar world is sufficiently interesting and instructive to be given with some detail.
But first let us endeavour to explain by what methods astronomers discover the periodicity of a comet.
When a new comet, or one supposed to be new, makes its appearance, can we tell if it has been seen or observed at any previous epoch? The reply to this question serves as a foundation to the first method employed for the resolution of the problem. But the reply is not easy if the apparition or previous apparitions of the comet (supposing it to have appeared to us before) have not been observed with some degree of precision, and if the tradition or record is limited to a vague mention of the size, the brilliancy of the nucleus, the form or the dimensions of the tail. The outward ap- pearance of a comet, its physical aspect, would be in almost all cases insufficient. We shall see as we proceed that these are variable features, that the aspect of a comet changes in the course of a single apparition. But even if it remained the same, the different circumstances of its visibility and distance
96
COMETS WHOSE RETURN HAS BEEN OBSERVED.
from the earth would suffice to prevent the identification of the two comets. A comet formerly of extreme brilliancy might reappear as a feeble nebulosity. It would have been difficult to recognise the same body in the comet of 1607, whose light appeared to Kepler pale and weak; in that of 1682, which Lahire and Picard compared to a star of the second magnitude; in that of 1759, which appeared to Messier like a star of the first magnitude; and, lastly, in the famous comet of 1456, ' which all historians (except two Poles),' says Pingre, ' agree in describing as great, terrible, and of an extraordinary size, drawing after it a long tail which covered two celestial signs, or 60 degrees.' These were, nevertheless, one and the same comet. Astronomers, it is true, mistrust, and justly, the nearly always exaggerated expressions of the ancient chroniclers; but precisely for that reason a resemblance of aspect is not to be relied upon for establishing the identity, and consequently the periodicity, of two comets. We must have more precise elements of comparison. These elements are those of the parabolic orbit, when records have been left of observations — that is to say, of positions and dates sufficient for the calculation of the orbit — when, in a word, the comet instead of having been simply seen has been observed. A catalogue of ancient comets is therefore necessary, and it was whilst consulting the table of twenty-four comets which he had calculated that Halley made the prediction, the history of which we are about to give.
If the longitudes of the ascending node and of the peri- helion, the inclination of the plane of the orbit, the perihelion distance, and the direction of movement, are all the same, or nearly the same, in two cometary orbits, in all probability we have two successive, if not consecutive, apparitions of the same comet. Taking the interval between the apparitions for the period itself, we are enabled by the third law of Kepler to
97 II
THE WORLD OF COMETS.
calculate the dimensions of the major axis of the corresponding elliptic orbit, and to assure ourselves that the new orbit is in accordance with the whole of the known observations. If this be so, we can calculate more or less exactly the comet's next return; that is to say, its perihelion passage, and all the circumstances of its future apparition.
The second method consists in the direct calculation of the elliptic elements. It requires, as a rule, exact observations, especially if the orbit be greatly elongated, since there is then but little difference between the apparent path followed by a comet, whether it be a parabola, a very long ellipse, or an hyperbola slightly flattened. The first attempts by this method — a very legitimate one in theory — prove that it is subject to many difficulties and uncertainties. Euler, on first applying it to the comet of 1744, obtained a hyperbolic orbit from the observations made at Berlin. But afterwards, having received the observations made by Cassini, he found the orbit to be a very long ellipse, with a period of many centuries.
The first example of an elliptic orbit calculated with pre- cision by this second method is, we believe, that of Lexell's comet (or comet of 1770), a comet of short period (five years and a half), and having an orbit of comparatively slight elon- gation, but which, unfortunately — we shall come to its history further on — has undergone enormous perturbations, and has not again been seen. Since then the direct calculation of the elliptic movement, without reference to previous observations, has been employed for various comets, and with success in several instances, as the return of the periodical comets of Faye, Brorsen, d' Arrest, and Winnecke (1819) has been ren- dered certain by numerous and careful observations.
The above two methods both require observed positions of the comet, whose periodicity is to be discovered, and also that these observations should possess a certain degree of accuracy.
COMETS WHOSE RETURN HAS BEEN OBSERVED.
In the absence of these conditions the end may, however, be attained, but the result is, in that case, as conjectural as the method itself. This third method consists in making a com- parison of the different historical comets, in noting the resemblance of their aspect, and in ascertaining if the intervals of their successive apparitions agree with the hypothesis of a certain period, whose duration, in this case, must be neces- sarily contained nearly an exact number of times in these intervals. The elements calculated for one apparition may then suffice to render probable the identity of several comets. In this way M. Laugier is of opinion that he has identified the comets of 1299, 1468, and 1799 by assuming a period of one hundred and sixty-nine years, which is twice included between the two last dates. In the same manner the comets of 1301, 1152, 760, and several others (which we shall mention pre- sently) have been identified as former apparitions of Halley's comet, the true period of which has long been calculated and known.
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SECTION II.
HALLEY'S COMET.
Discovery of the identity of the comets of 1682, 1607, and 1531 ; Halley announces the next return for the year 1758— Olairaut undertakes the calculation of the disturbing influence exercised by Jupiter and Saturn upon the comet of 1682 ; collaboration of Lalande and Mdlle. Hortense Lepaute— The return of the comet to its perihelion is fixed for the middle of April 1759 ; the comet returns on the 13th of March— Eeturn of Halley's comet in 1835 ; calculation of the perturbations by Damoiseau and Pontecoulant ; progress of theory — The comet will return to its perihelion in May 1910.
LET us recal the memorable words of Seneca in his Qucestiones Naturales : ' Why should we be surprised that comets, pheno- mena so seldom presented to the world, are for us not yet submitted to fixed laws, and that it is still unknown from whence come and where remain these bodies, whose return takes place only at immense intervals'? ... An age will come when that which is mysterious for us will have been made clear by time and by the accumulated studies of centuries. ... Some day there will arise a man who will demonstrate in what region of the heavens the comets take their way, why they journey so far apart from other planets, what their size, their nature.' Eighteen centuries have elapsed, and not one man, but the accumulated efforts of many men have raised a corner of the veil spoken of by Seneca. As far as the laws of cometary movement are concerned Newton has realised his prediction ; whilst that which relates to the return of comets and their calculated periodicity