“When beggars die, there are no comets seen;
The heavens themselves blaze forth the death of princes.”
But, happily, the notions prevalent in former times have been superseded by the more enlightened views naturally resulting from the acquirement and diffusion of knowledge; so that comets, though still surrounded by a good deal of mystery, are now regarded with considerable interest, and welcomed, not only as objects devoid of malevolent character, but as furnishing many useful materials for study. Mere superstition has been put aside as an impediment to real progress, and a more intelligent age has recognized the necessity of dealing only with facts and explaining them according to the laws of nature; for it is on facts, and their just interpretation, that all true searchers after knowledge must rely. Comets are properly regarded as bodies which, though far from being thoroughly understood in all the details of their physical structure and behaviour, have yet a wonderful history, and one which, could it be clearly elucidated, would unfold some new and marvellous facts. Under these circumstances we need evince no surprise that these visitors are invariably hailed with enthusiasm, not only by scientific men, who make them the special subjects of close observation, but by everyone who regards celestial “sights and signs” with occasional attention.
Appearance.—From whatever point of view a large comet is considered, it deserves all the interest manifested in it and all the labour expended in its investigation. Whilst its grand appearance in the firmament arrests the notice of all classes alike, and is the subject of much curious speculation amongst the uninformed, it merits, apart from other considerations, the most assiduous observation on account of the singular features it displays and the striking variations they undergo. Indeed, the visible deportment of a comet during its rapid career near perihelion is so extraordinary as to form a problem, the solution of which continues to defy the most ingenious theories. The remarkable changes in progress, the quickness and apparent irregularity of their development, are the immediate result of a combination of forces, the operations of which can neither be defined nor foreseen. Jets of flame and wreaths of vapour start from the brilliant nucleus; while, streaming away from the latter, in a direction opposite to the Sun, is the fan-shaped tail, often traceable over a large span of the heavens and commingling its extreme fainter limits with the star-dust in the background.
Large number visible.—The orbits of 400 comets have now been computed, and more than 500 others have been observed; so that these bodies are extremely plentiful. Kepler described them to be as numerous as the fishes in the sea, and no doubt the allegory is justly applied. Their vagaries of form, size, and place are equally noteworthy; and those who enter upon the discussion of facts relating to these objects will find an endless store of interesting materials, opening up a wide field for conjecture.
Nature of Apparition.—The apparition of a comet may be either gradual or sudden. Usually the telescope gives us the earliest intimation that one of these bodies is approaching us[47]. It is first seen as a small round nebulosity, with probably a central condensation or stellar nucleus of the 10th or 11th mag. The whole object brightens and expands as its distance grows less, and it assumes an elongated form preparatory to the formation of a tail. The latter varies greatly in different instances: it may either be a narrow ray, as shown in the southern comet of January 1887, or a fan-shaped extension like that of the great comet of 1744. Barnard’s Comet of December 1886 exhibited a duple tail. Occasionally a fine comet bursts upon us suddenly, like that of 1843 or 1861. The former was sufficiently bright to be discovered when only 4° from the Sun, and the latter presented itself quite unexpectedly as a magnificent object even in the strong twilight of a June sky.
Tenuity of Comets.—Comets are noteworthy for the extreme thinness of their material. The smallest stars may be discerned through the denser portions of the head, without suffering any apparent diminution of light. Yet such stars would be quite obscured by the interposition of a minute speck of cloud or by a little fog or any vapour of trifling density. Comets are visible in the form of transparent nebulosities; and their mass must be inconceivably small relatively to the enormous space over which they frequently extend. Sir J. Herschel has described the “all but spiritual texture” of comets; and other authorities have referred to them as feeble wreaths of vapour, which, though obeying the laws of gravitation and suffering much perturbation, are yet themselves incapable of exercising any disturbing influence upon the other bodies near which they pass. It has been asserted that comets would show phases were they rendered luminous by reflected sunlight, and that, such features being absent, these bodies must possess a phosphorescence of their own sufficient to cause the glow observed. This idea, however, is hardly consistent with our present knowledge. Comets are not compact and coherent masses of matter; they more likely represent vast groups of planetary atoms, more or less loosely dispersed and sometimes forming streams. The effect of sunshine upon such assemblages will be that the whole mass becomes illumined according to density, and that no phase will be apparent, inasmuch as the light is enabled to penetrate through its entirety.
Differences of Orbit.—When three trustworthy observations of a comet’s place have been made, its orbit may be computed. This may be either an ellipse, a parabola, or hyperbola. If an ellipse the comet is periodical, and the period depends upon the degree of eccentricity. If a parabola the comet will not be seen again, because this form of orbit does not reunite; it consists of branches equally divergent and uniting at perihelion, but extending outwards indefinitely in nearly parallel lines and without convergence. If a hyperbola, the comet is also not returnable; the branches of the orbit are widely divergent, and show no tendency to parallelism. These several forms of orbit are somewhat different as applied to various comets, but they are the same in effect. Thus Tempel’s Comet of 1867 revolves in an ellipse having an eccentricity of about 0·4630, while that of Halley’s Comet is 0·9674. No doubt some of the parabolic orbits applied to comets really represent very eccentric ellipses; but the parabola is a convenient form of orbit for computation, and unless ellipticity is very decided it indicates the path with sufficient accuracy.
Discoveries of Comets.—In the latter part of the last century Messier, Mechain, and Miss Herschel shared nearly all the cometary discoveries between them. Then Pons entered the field, and he may be said to have monopolized this branch during the period from 1802 to 1827, for he was the first to announce thirty comets. Pons died in 1831, but the search was actively continued by others. In about 1843 a great rise became apparent in the rate of these discoveries; and we find Di Vico, Mauvais, and Brorsen very successful at this period. Later on, the work was sustained with the same prolific results by Klinkerfues, Bruhns, and Donati, and subsequently by Winnecke, Tempel, and Coggia. Swift and Borrelly also assisted materially to swell our knowledge; while during the last few years Barnard and Brooks have exhibited a surprising amount of zeal in this department. Since 1881 no less than twenty-six comets are to be enumerated as the fruits of their endeavours, and they are still engaged in nightly explorations of the sky with similar ends in view. Their diligent pursuit of these fugitive bodies will doubtless result in many further additions during ensuing years.
It is a curious circumstance that Sir W. Herschel, during all his star-gaugings and sweeps for nebulæ, never discovered a comet. He found a nebula on Dec. 18, 1783, near δ Ceti, which he described as “small and cometic.” In Sir J. Herschel’s ‘General Catalogue of Nebulæ,’ 1864, p. 17, this object is presumed to have been a comet, as it could not be identified; but at p. 45 the doubts are cleared up, and Sir W. Herschel’s nebula, the position of which was only roughly given, is shown to be the same as another very near; it is No. 1055 of the new ‘General Catalogue’ published by the Royal Astronomical Society in January 1888. Quite possibly Sir W. Herschel’s lists of nebulæ contain several comets, as some of his objects are missing; but errors of observation in ascribing positions may explain this. Herschel himself, in speaking of a comet visible in the winter of 1807-8, says:—“If I had met the comet in one of my sweeps, as it appeared between Dec. 6 and Feb. 21, I should have put it down as a nebula. Perhaps my lists of nebulæ, then, contain some comets.”