Comets may almost be regarded as the ghosts of heavenly bodies. The heads, in some cases, may consist of separate solid fragments, though on this astronomers are by no means agreed, but the tails at any rate are in fact of almost inconceivable tenuity. We know that a cloud a few hundred feet thick is sufficient to hide, not only the stars, but even the Sun himself. A Comet is thousands of miles in thickness, and yet even extremely minute stars can be seen through it with no appreciable diminution of brightness. This extreme tenuity of comets is moreover shown by their small weight. Enormous as they are I remember Sir G. Airy saying that there was probably more matter in a cricket ball than there is in a comet. No one, however, now doubts that the weight must be measured in tons; but it is so small, in relation to the size, as to be practically inappreciable. If indeed they were comparable in mass even to the planets, we should long ago have perished. The security of our system is due to the fact that the planets revolve round the Sun in one direction, almost in circles, and very nearly in the same plane. Comets, however, enter our system in all directions, and at all angles; they are so numerous that, as Kepler said, there are probably more Comets in the sky than there are fishes in the sea, and but for their extreme tenuity they would long ago have driven us into the Sun.
When they first come in sight Comets have generally no tail; it grows as they approach the Sun, from which it always points away. It is no mere optical illusion; but while the Comet as a whole is attracted by the Sun, the tail, how or why we know not, is repelled. When once driven off, moreover, the attraction of the Comet is not sufficient to recall it, and hence perhaps so many Comets have now no tails.
Donati's Comet, the great Comet of 1858, was first noticed on the 2d June as a faint nebulous spot. For three months it remained quite inconspicuous, and even at the end of August was scarcely visible to the naked eye. In September it grew rapidly, and by the middle of October the tail extended no less than 40 degrees, after which it gradually disappeared.
Faint as is the light emitted by Comets, it is yet their own, and spectrum analysis has detected the presence in them of carbon, hydrogen, nitrogen, sodium, and probably of iron.
Comets then remain as wonderful, and almost as mysterious, as ever, but we need no longer regard "a comet as a sign of impending calamity; we may rather look upon it as an interesting and a beautiful visitor, which comes to please us and to instruct us, but never to threaten or to destroy."[69] We are free, therefore, to admire them in peace, and beautiful, indeed, they are.
"The most wonderful sight I remember," says Hamerton, "as an effect of calm, was the inversion of Donati's Comet, in the year 1858, during the nights when it was sufficiently near the horizon to approach the rugged outline of Graiganunie, and be reflected beneath it in Loch Awe. In the sky was an enormous aigrette of diamond fire, in the water a second aigrette, scarcely less splendid, with its brilliant point directed upwards, and its broad, shadowy extremity ending indefinitely in the deep. To be out on the lake alone, in a tiny boat, and let it rest motionless on the glassy water, with that incomparable spectacle before one, was an experience to be remembered through a lifetime. I have seen many a glorious sight since that now distant year, but nothing to equal it in the association of solemnity with splendour."[70]
SHOOTING STARS
On almost any bright night, if we watch a short time some star will suddenly seem to drop from its place, and, after a short plunge, to disappear. This appearance is, however, partly illusory. While true stars are immense bodies at an enormous distance, Shooting Stars are very small, perhaps not larger than a paving stone, and are not visible until they come within the limits of our atmosphere, by the friction with which they are set on fire and dissipated. They are much more numerous on some nights than others. From the 9th to the 11th August we pass through one cluster which is known as the Perseids; and on the 13th and 14th November a still greater group called by astronomers the Leonids. The Leonids revolve round the Sun in a period of 33 years, and in an elliptic orbit, one focus of which is about at the same distance from the Sun as we are, the other at about that of Uranus. The shoal of stars is enormous; its diameter cannot be less than 100,000 miles, and its length many hundreds of thousands. There are, indeed, stragglers scattered over the whole orbit, with some of which we come in contact every year, but we pass through the main body three times in a century—last in 1866—capturing millions on each occasion. One of these has been graphically described by Humboldt:
"From half after two in the morning the most extraordinary luminary meteors were seen in the direction of the east. M. Bonpland, who had risen to enjoy the freshness of the air, perceived them first. Thousands of bodies and falling stars succeeded each other during the space of four hours. Their direction was very regular from north to south. They filled a space in the sky extending from due east 30° to north and south. In an amplitude of 60° the meteors were seen to rise above the horizon at east-north-east, and at east, to describe arcs more or less extended, and to fall towards the south, after having followed the direction of the meridian. Some of them attained a height of 40°, and all exceeded 25° or 30°. No trace of clouds was to be seen. M. Bonpland states that, from the first appearance of the phenomenon, there was not in the firmament a space equal in extent to three diameters of the moon which was not filled every instant with bolides and falling stars. The first were fewer in number, but as they were of different sizes it was impossible to fix the limit between these two classes of phenomena. All these meteors left luminous traces from five to ten degrees in length, as often happens in the equinoctial regions. The phosphorescence of these traces, or luminous bands, lasted seven or eight seconds. Many of the falling stars had a very distinct nucleus, as large as the disc of Jupiter, from which darted sparks of vivid light. The bodies seemed to burst as by explosion; but the largest, those from 1° to 1° 15' in diameter, disappeared without scintillation, leaving behind them phosphorescent bands (trabes), exceeding in breadth fifteen or twenty minutes. The light of these meteors was white, and not reddish, which must doubtless be attributed to the absence of vapour and the extreme transparency of the air."[71]