CHAPTER XIX

COMETS

The reader has, no doubt, been struck by the marked uniformity which exists among those members of the solar system with which we have dealt up to the present. The sun, the planets, and their satellites are all what we call solid bodies. The planets move around the sun, and the satellites around the planets, in orbits which, though strictly speaking, ellipses, are yet not in any instance of a very oval form. Two results naturally follow from these considerations. Firstly, the bodies in question hide the light coming to us from those further off, when they pass in front of them. Secondly, the planets never get so far from the sun that we lose sight of them altogether.

With the objects known as Comets it is, however, quite the contrary. These objects do not conform to our notions of solidity. They are so transparent that they can pass across the smallest star without dimming its light in the slightest degree. Again, they are only visible to us during a portion of their orbits. A comet may be briefly described as an illuminated filmy-looking object, made up usually of three portions--a head, a nucleus, or brighter central portion within this head, and a tail. The heads of comets vary greatly in size; some, indeed, appear quite small, like stars, while others look even as large as the moon. Occasionally the nucleus is wanting, and sometimes the tail also.

These mysterious visitors to our skies come up into view out of the immensities beyond, move towards the sun at a rapidly increasing speed, and, having gone around it, dash away again into the depths of space. As a comet approaches the sun, its body appears to grow smaller and smaller, while, at the same time, it gradually throws out behind it an appendage like a tail. As the comet moves round the central orb this tail is always directed _away_ from the sun; and when it departs again into space the tail goes in advance. As the comet's distance from the sun increases, the tail gradually shrinks away and the head once more grows in size (see Fig. 18). In consequence of these changes, and of the fact that we lose sight of comets comparatively quickly, one is much inclined to wonder what further changes may take place after the bodies have passed beyond our ken.

The orbits of comets are, as we have seen, very elliptic. In some instances this ellipticity is so great as to take the bodies out into space to nearly six times the distance of Neptune from the sun. For a long time, indeed, it was considered that comets were of two kinds, namely, those which actually _belonged_ to the solar system, and those which were merely _visitors_ to it for the first and only time--rushing in from the depths of space, rapidly circuiting the sun, and finally dashing away into space again, never to return. On the contrary, nowadays, astronomers are generally inclined to regard comets as permanent members of the solar system.

The difficulty, however, of deciding absolutely whether the orbits of comets are really always _closed_ curves, that is to say, curves which must sooner or later bring the bodies back again towards the sun, is, indeed, very great. Comets, in the first place, are always so diffuse, that it is impossible to determine their exact position, or, rather, the exact position of that important point within them, known as the centre of gravity. Secondly, that stretch of its orbit along which we can follow a comet, is such a very small portion of the whole path, that the slightest errors of observation which we make will result in considerably altering our estimate of the actual shape of the orbit.

Comets have been described as so transparent that they can pass across the sky without dimming the lustre of the smallest stars, which the thinnest fog or mist would do. This is, indeed, true of every portion of a comet except the nucleus, which is, as its name implies, the densest part. And yet, in contrast to this ghostlike character, is the strange fact that when comets are of a certain brightness they may actually be seen in full daylight.

As might be gathered from their extreme tenuity, comets are so exceedingly small in mass that they do not appear to exert any gravitational attraction upon the other bodies of our system. It is, indeed, a known fact that in the year 1886 a comet passed right amidst the satellites of Jupiter without disturbing them in the slightest degree. The attraction of the planet, on the other hand, so altered the comet's orbit, as to cause it to revolve around the sun in a period of seven years, instead of twenty-seven, as had previously been the case. Also, in 1779, the comet known as Lexell's passed quite close to Jupiter, and its orbit was so changed by that planet's attraction that it has never been seen since. The density of comets must, as a rule, be very much less than the one-thousandth part of that of the air at the surface of our globe; for, if the density of the comet were even so small as this, its mass would _not_ be inappreciable.

If comets are really undoubted members of the solar system, the circumstances in which they were evolved must have been different from those which produced the planets and satellites. The axial rotations of both the latter, and also their revolutions, take place in one certain direction;[24] their orbits, too, are ellipses which do not differ much from circles, and which, furthermore, are situated fairly in the one plane. Comets, on the other hand, do not necessarily travel round the sun in the same fixed direction as the planets. Their orbits, besides, are exceedingly elliptic; and, far from keeping to one plane, or even near it, they approach the sun from all directions.

Broadly speaking, comets may be divided into two distinct classes, or "families." In the first class, the same orbit appears to be shared in common by a series of comets which travel along it, one following the other. The comets which appeared in the years 1668, 1843, 1880, 1882, and 1887 are instances of a number of different bodies pursuing the same path around the sun. The members of a comet family of this kind are observed to have similar characteristics. The idea is that such comets are merely portions of one much larger cometary body, which became broken up by the gravitational action of other bodies in the system, or through violent encounter with the sun's surroundings.

The second class is composed of comets which are supposed to have been seized by the gravitative action of certain planets, and thus forced to revolve in short ellipses around the sun, well within the limits of the solar system. These comets are, in consequence, spoken of as "captures." They move around the sun in the same direction as the planets do. Jupiter has a fairly large comet family of this kind attached to him. As a result of his overpowering gravitation, it is imagined that during the ages he must have attracted a large number of these bodies on his own account, and, perhaps, have robbed other planets of their captures. His family at present numbers about thirty. Of the other planets, so far as we know, Saturn possesses a comet family of two, Uranus three, and Neptune six. There are, indeed, a few comets which appear as if under the influence of some force situated outside the known bounds of the solar system, a circumstance which goes to strengthen the idea that other planets may revolve beyond the orbit of Neptune. The terrestrial planets, on the other hand, cannot have comet families; because the enormous gravitative action of the sun in their vicinity entirely overpowers the attractive force which they exert upon those comets which pass close to them. Besides this, a comet, when in the inner regions of the solar system, moves with such rapidity, that the gravitational pull of the planets there situated is not powerful enough to deflect it to any extent. It must not be presumed, however, that a comet once captured should always remain a prisoner. Further disturbing causes might unsettle its newly acquired orbit, and send it out again into the celestial spaces.

With regard to the matter of which comets are composed, the spectroscope shows the presence in them of hydrocarbon compounds (a notable characteristic of these bodies), and at times, also, of sodium and iron. Some of the light which we get from comets is, however, merely reflected sunlight.

The fact that the tails of comets are always directed away from the sun, has given rise to the idea that this is caused by some repelling action emanating from the sun itself, which is continually driving off the smallest particles. Two leading theories have been formulated to account for the tails themselves upon the above assumption. One of these, first suggested by Olbers in 1812, and now associated with the name of the Russian astronomer, the late Professor Bredikhine, who carefully worked it out, presumes an electrical action emanating from the sun; the other, that of Arrhenius, supposes a pressure exerted by the solar light in its radiation outwards into space. It is possible, indeed, that repelling forces of both these kinds may be at work together. Minute particles are probably being continually produced by friction and collisions among the more solid parts in the heads of comets. Supposing that such particles are driven off altogether, one may therefore assume that the so-called captured comets are disintegrating at a comparatively rapid rate. Kepler long ago maintained that "comets die," and this actually appears to be the case. The ordinary periodic ones, such, for instance, as Encke's Comet, are very faint, and becoming fainter at each return. Certain of these comets have, indeed, failed altogether to reappear. It is notable that the members of Jupiter's comet family are not very conspicuous objects. They have small tails, and even in some cases have none at all. The family, too, does not contain many members, and yet one cannot but suppose that Jupiter, on account of his great mass, has had many opportunities for making captures adown the ages.

Of the two theories to which allusion has above been made, that of Bredikhine has been worked out so carefully, and with such a show of plausibility, that it here calls for a detailed description. It appears besides to explain the phenomena of comets' tails so much more satisfactorily than that of Arrhenius, that astronomers are inclined to accept it the more readily of the two. According to Bredikhine's theory the electrical repulsive force, which he assumes for the purposes of his argument, will drive the minutest particles of the comet in a direction away from the sun much more readily than the gravitative action of that body will pull them towards it. This may be compared to the ease with which fine dust may be blown upwards, although the earth's gravitation is acting upon it all the time.

The researches of Bredikhine, which began seriously with his investigation of Coggia's Comet of 1874, led him to classify the tails of comets in _three types_. Presuming that the repulsive force emanating from the sun did not vary, he came to the conclusion that the different forms assumed by cometary tails must be ascribed to the special action of this force upon the various elements which happen to be present in the comet. The tails which he classes as of the first type, are those which are long and straight and point directly away from the sun. Examples of such tails are found in the comets of 1811, 1843, and 1861. Tails of this kind, he thinks, are in all probability formed of _hydrogen_. His second type comprises those which are pointed away from the sun, but at the same time are considerably curved, as was seen in the comets of Donati and Coggia. These tails are formed of _hydrocarbon gas_. The third type of tail is short, brush-like, and strongly bent, and is formed of the _vapour of iron_, mixed with that of sodium and other elements. It should, however, be noted that comets have occasionally been seen which possess several tails of these various types.

We will now touch upon a few of the best known comets of modern times.

The comet of 1680 was the first whose orbit was calculated according to the laws of gravitation. This was accomplished by Newton, and he found that the comet in question completed its journey round the sun in a period of about 600 years.

In 1682 there appeared a great comet, which has become famous under the name of Halley's Comet, in consequence of the profound investigations made into its motion by the great astronomer, Edmund Halley. He fixed its period of revolution around the sun at about seventy-five years, and predicted that it would reappear in the early part of 1759. He did not, however, live to see this fulfilled, but the comet duly returned--_the first body of the kind to verify such a prediction_--and was detected on Christmas Day, 1758, by George Palitzch, an amateur observer living near Dresden. Halley also investigated the past history of the comet, and traced it back to the year 1456. The orbit of Halley's comet passes out slightly beyond the orbit of Neptune. At its last visit in 1835, this comet passed comparatively close to us, namely, within five million miles of the earth. According to the calculations of Messrs P.H. Cowell and A.C.D. Crommelin of Greenwich Observatory, its next return will be in the spring of 1910; the nearest approach to the earth taking place about May 12.

On the 26th of March, 1811, a great comet appeared, which remained visible for nearly a year and a half. It was a magnificent object; the tail being about 100 millions of miles in length, and the head about 127,000 miles in diameter. A detailed study which he gave to this comet prompted Olbers to put forward that theory of electrical repulsion which, as we have seen, has since been so carefully worked out by Bredikhine. Olbers had noticed that the particles expelled from the head appeared to travel to the end of the tail in about eleven minutes, thus showing a velocity per second very similar to that of light.

The discovery in 1819 of the comet known as Encke's, because its orbit was determined by an astronomer of that name, drew attention for the first time to Jupiter's comet family, and, indeed, to short-period comets in general. This comet revolves around the sun in the shortest known period of any of these bodies, namely, 3-1/3 years. Encke predicted that it would return in 1822. This duly occurred, the comet passing at its nearest to the sun within three hours of the time indicated; being thus the second instance of the fulfilment of a prediction of the kind. A certain degree of irregularity which Encke's Comet displays in the dates of its returns to the sun, has been supposed to indicate that it passes in the course of its orbit through some retarding medium, but no definite conclusions have so far been arrived at in this matter.

A comet, which appeared in 1826, goes by the name of Biela's Comet, because of its discovery by an Austrian military officer, Wilhelm von Biela. This comet was found to have a period of between six and seven years. Certain calculations made by Olbers showed that, at its return in 1832, it would pass _through the earth's orbit_. The announcement of this gave rise to a panic; for people did not wait to inquire whether the earth would be anywhere near that part of its orbit when the comet passed. The panic, however, subsided when the French astronomer, Arago, showed that at the moment in question the earth would be some 50 millions of miles away from the point indicated!

In 1846, shortly after one of its returns, Biela's Comet divided into two portions. At its next appearance (1852) these portions had separated to a distance of about 1-1/2 millions of miles from each other. This comet, or rather its constituents, have never since been seen.

Perhaps the most remarkable comet of recent times was that of 1858, known as Donati's, it having been discovered at Florence by the Italian astronomer, G.B. Donati. This comet, a magnificent object, was visible for more than three months with the naked eye. Its tail was then 54 millions of miles in length. It was found to revolve around the sun in a period of over 2000 years, and to go out in its journey to about 5-1/2 times the distance of Neptune. Its motion is retrograde, that is to say, in the contrary direction to the usual movement in the solar system. A number of beautiful drawings of Donati's Comet were made by the American astronomer, G.P. Bond. One of the best of these is reproduced on Plate XVII., p. 256.

In 1861 there appeared a great comet. On the 30th of June of that year the earth and moon actually passed through its tail; but no effects were noticed, other than a peculiar luminosity in the sky.

In the year 1881 there appeared another large comet, known as Tebbutt's Comet, from the name of its discoverer. This was the _first comet of which a satisfactory photograph was obtained_. The photograph in question was taken by the late M. Janssen.

The comet of 1882 was of vast size and brilliance. It approached so close to the sun that it passed through some 100,000 miles of the solar corona. Though its orbit was not found to have been altered by this experience, its nucleus displayed signs of breaking up. Some very fine photographs of this comet were obtained at the Cape of Good Hope by Mr. (now Sir David) Gill.

The comet of 1889 was followed with the telescope nearly up to the orbit of Saturn, which seems to be the greatest distance at which a comet has ever been seen.

The _first discovery of a comet by photographic means_[25] was made by Professor Barnard in 1892; and, since then, photography has been employed with marked success in the detection of small periodic comets.

The best comet seen in the Northern hemisphere since that of 1882, appears to have been Daniel's Comet of 1907 (see Plate XVIII., p. 258). This comet was discovered on June 9, 1907, by Mr. Z. Daniel, at Princeton Observatory, New Jersey, U.S.A. It became visible to the naked eye about mid-July of that year, and reached its greatest brilliancy about the end of August. It did not, however, attract much popular attention, as its position in the sky allowed it to be seen only just before dawn.

[24] With the exception, of course, of such an anomaly as the retrograde motion of the ninth satellite of Saturn.

[25] If we except the case of the comet which was photographed near the solar corona in the eclipse of 1882.