BOOK IV. COMETS. CHAPTER I. GENERAL REMARKS. Comets long objects of popular interest—and alarm.-Usual phenomena attending the development of a Comet.-Telescopic Comets.-Comets diminish in brilliancy at each return.—Period of Revolution.—Density.— Mass.-Lexell's Comet.—General influence of Planets on Comets-Comets move in 1 of 3 kinds of orbits.-Elements of a Comet's orbit. - For a parabolic orbit, 5 in number.-Direction of motion.Eccentricity of an elliptic orbit.—Early speculations as to the paths in which Comets moved.--Comets visible in the daytime.—Breaking up of a Comet into parts.— Instance of Biela's Comet.-Comets probably self-luminous.—Existence of phases doubtful.-Comets with Planetary discs.—Phenomena connected with the tails of Comets. Usually a prolongation of the radius vector. Vibration sometimes noticed in tails.-Olbers's hypothesis.-Transits of Comets across the Sun's disc.— Variation in the appearance of Comets exemplified in the case of that of 1769. THE HE class of bodies which will now come under our notice is one of the most interesting with which the astronomer has to deal. Appearing suddenly in the nocturnal sky, and often having attached to them tails of immense size and brilliancy, comets were well calculated in the earlier ages of the world to attract the attention of all, and still more to excite the fear of many. It is the unanimous testimony of history, during a period of upwards of 2000 years, that comets were always considered to be peculiarly "ominous of the wrath of Heaven, and as harbingers of wars and famines, of the dethronement of monarchs, and the dissolution of empires." I shall hereafter examine this question at greater length. Suffice it for me here to quote the words of the Poet, who speaks of "The blazing Star, Threat'ning the world with famine, plague, and war; To princes, death; to kingdoms, many curses; To all estates, inevitable losses; To herdsmen, rot; to ploughmen, hapless seasons; To sailors, storms; to cities, civil treasons." However little attention might have been paid by the ancients to the more ordinary phenomena of nature (which, however, were very well looked after), yet certain it is that comets and total eclipses of the Sun were not easily forgotten or lightly passed over; hence the aspects of remarkable comets that have appeared at various times have been handed down to us, often with circumstantial minuteness. A comet usually consists of 3 parts, developed somewhat in the following manner :-A faint luminous speck is discovered by the aid of a good telescope; the size increases gradually; and after some little time a nucleus appears-that is, a part which is more condensed in its light than the rest, sometimes circular, sometimes oval, more rarely presenting a radiated appearance. It is a remark of Arago that this nucleus is generally excentrically placed in the head, lying towards the margin nearest the Sun. Both the size and the brilliancy of the object still progressively increase; the coma, or cloudlike mass around the nucleus, becomes less regular; and a tail begins to form, which becomes fainter as it recedes from the body of the comet. This tail increases in length so as sometimes to spread across a large portion of the heavens; sometimes there are more tails than one, and occasionally the tail is much narrower in some parts than in others. The comet approaches the Sun in a curvilinear path, which frequently hardly differs from a right line. It generally crosses that part of the heavens in which the Sun is situated so near the latter body as to be lost in its rays; but it emerges again on the other side, frequently with increased brilliancy and length of tail. The phenomena of disappearance are then (but in the reverse order) the same as those of its appearance. In magnitude and brightness comets exhibit great diversity: some are so bright as to be visible in the daytime; others, indeed the majority, are quite invisible, except with powerful optical assistance. Such are usually called telescopic comets. The appearance of the same comet at different periods of its return is so varying that we can never identify a given comet with any other by any mere physical peculiarity of size or shape until its elements have been calculated and compared. It is now known that "the same comet may, at successive returns to our system, sometimes appeared tailed, and sometimes without a tail, according to its position with respect to the Earth and the Sun; and there is reason to believe that comets in general, from some unknown cause, decrease in splendour in each successive revolution "." The periods of comets in their revolutions vary greatly, as also do the distances to which they recede from the Sun. Whilst the orbit of Encke's comet is contained within that of Jupiter, the orbit of Halley's extends far beyond that of Neptune. Some comets indeed proceed to a much greater distance than this, whilst others are supposed to pass into curves which do not, like the ellipse, return into themselves. In this case they never come back to the Sun. Such orbits are either parabolic or hyperbolic. The density, and also the mass, of comets is exceedingly small, and their tails consist of matter of such extreme tenuity that the smallest stars are seen through them a fact first recorded by Seneca. That the matter of comets is exceedingly small is sufficiently proved by the fact that they have at times passed very near to some of the planets without disturbing their motions in any appreciable degree. Thus the comet of 1770 (Lexell's) in its advance towards the Sun, got entangled amongst the satellites of Jupiter, and remained near them for 4 months, without in the least affecting them as far as we know. It can therefore be shewn that this comet's mass could not have been so much as that of the Earth. The same comet also came very near the Earth on July 1-its distance from it at 5h on that day being about 1,400,000 miles-so that had its quantity of matter been equal to that of the Earth, it would, by its attraction, have caused our globe to revolve in an orbit so much larger than it does at present that it would have increased the length of the year by 2h 47m, yet no sensible alteration took place. The comet of 837 remained for a period of 4 days within 3,700,000 miles without any untoward consequence. Very little argument, therefore, suffices to shew the absurdity of the idea of any danger happening to our planet from the advent of any of these wandering strangers. Indeed, instead of comets exercising any influence on the motions of planets, there is the most conclusive evidence that the converse is the case-that planets influence comets. This fact is strikingly exemplified in the history of the comet of 1770, just referred to. At its appearance in that year this body was found to have an elliptical orbit, requiring for a complete revolution only 5 years; yet although this comet was a large and bright one, it had never been observed before, and has moreover never been seen since; the reason being that the influence of the planet Jupiter, in a short period, completely changed the character of its path. I cite from Arago the following: "Du Séjour has proved that a comet, whose mass is equal to that of the Earth, which would pass at a distance of 37,500 miles only, would extend the length of the year to 367 16h 5m, and could alter the obliquity of the ecliptic to the extent of 2°. Notwithstanding its enormous mass and the smallness of its distance, such a body would then produce upon our globe only one kind of revolution,-that of the calendar b." A comet may move in either an elliptic, parabolic, or hyperbolic orbit ; but for reasons with which mathematical readers are Pop. Ast., vol. i. p. 642, Eng. ed. acquainted, no comet can be periodical which does not follow an elliptic path. In consequence, however, of the comparative facility with which the parabola can be calculated, astronomers are in the habit of applying that curve to represent the orbit of any newlydiscovered body. Parabolic elements having been obtained, a search is then made through a catalogue of comets, to see whether the new elements bear any resemblance to those of any object that has been previously observed; if so, an elliptic orbit is calculated and a period deduced. The elements of a parabolic orbit are 5 in number. 1. The time of perihelion passage, or the moment when the comet arrives at its least distance from the Sun-denoted by the symbol PP, or 7. 2. The longitude of the perihelion, or the longitude of the comet when it reaches that point.-. 3. The longitude of the ascending node of the comet's orbit, as seen from the Sun.-8. 4. The perihelion distance, or the distance of the comet from the Sun expressed in radii of the Earth's orbit.-q. 5. The inclination of the orbit, or the angle between the plane of the orbit and the ecliptic.-. It is also necessary to know whether the comet moves in the order of the signs, or in the contrary direction: in the former case its movement (u) is said to be direct (+), in the latter retrograde (-). In an elliptic orbit we require to know the eccentricity (e) this is sometimes expressed by the angle 4, of which the previous quantity (e) is the sine. From this, with the perihelion distance, we can ascertain the length of the major axis, and consequently the comet's periodic time. Be it remembered that the eccentricity is not the linear distance of the centre of the ellipse from the focus, but the ratio of that quantity to the semi-axis major. Up to the present time the orbits of nearly 300 comets have been calculated these will be given hereafter. : In an elliptic orbit the corresponding point of extreme distance from the Sun is called the Aphelion. d Gauss's Theoria Motus Corporum Cœlestium is reckoned the standard work on the subject of orbits. See also a paper by Airy, in Memoirs R.A.S., vol. xi. |