Comets

Comets form a special class of heavenly bodies remarkable in many ways. They exist in great numbers, though it is only occasionally that they present an appearance to the unaided eye of an observer of the heavens. Records of such appearances are given in history, much fear and anxiety attending them during their short visits. But since more has been learnt of the nature of comets, and especially since the approximate time of the appearance of such a comet as happens to be a periodical visitor has been predicted by astronomers, much of the superstitious dread attaching to them has been removed. For it is the unexpected that alarms people, and comets are now no longer unexpected, though so far as their exact nature is concerned very little is known, and much still remains to be discovered.

The path of motion of a periodic visitor to our solar system is elliptic, the sun lying at one focus of the ellipse, and exerting a sufficient gravitational force on the body to prevent its rushing off never to return. But if the speed of the body could be accelerated when it is at perihelion or minimum distance from the sun, the path of motion would be a longer ellipse, and the body would take longer time to complete its orbit; and if the speed were increased to a certain definite degree, the intensity of gravity would be just insufficient to bring the body back to its starting point, the ellipse would extend to infinity, and we should get a parabolic orbit. Such a body, reaching its perihelion with such a speed, would take infinite time to accomplish its orbit, and would only visit our system once, departing, then for ever. Further, all bodies with speeds greater than their corresponding critical speeds would describe hyperbolic curves and would similarly visit our system once only.

The orbits of comets afford instances of all three types, elliptic, parabolic, and hyperbolic. In most cases the elliptic orbits of our regular visitors are so gigantic that they are practically parabolic.

Comets, whose orbits are hyperbolic, cannot have acquired their speed by reason of the sun's attraction only, otherwise they would describe parabolas. The effect of the attraction of the solar planets upon comets has also to be considered in the investigation of their motions. Perturbations are in this way produced which materially alter a comet's path; the converse perturbations, due to the reciprocal action of the comet upon the planets, are extremely small, and practically produce no effect in the elliptic orbits of the planets, for the mass of the comet is exceedingly small.

Another observed effect is that of the slight retardation of a comet as it rushes round the sun at perihelion. The period of Encke's comet is diminished by 2-1/2 hours on this account each time it effects a complete circuit round the sun; it is due apparently to some sort of frictional resistance in the medium surrounding the sun, as though the latter possessed a material envelope of extreme tenuity extending for millions of miles from its surface.

Comets vary considerably in appearance. There is generally a bright centre or nucleus, surrounded by fainter layers that stretch away into a sweeping tail of varying luminosity, and of such exceeding thinness that the brilliancy of a star when seen through a comet's tail seems unimpaired. It is observed that the tail of a comet, if it possess one, is always directed away from the sun. The mass of a comet is so slight in comparison with that of the heavenly bodies, whose masses we are able to compare and directly estimate, that no satisfactory means have been devised to obtain a numerical value thereof.

It is supposed that the nucleus consists of disconnected meteoric fragments, and that the tail consists of iron or chlorine, hydrocarbons, and hydrogen. Proximity to the sun probably volatilises part of the material available for the tail, and this is dissipated by subsequent visits to the sun till no more material remains to renew that which has frittered away. The comet is then tailless. The light of comets may be clue to electric discharges, to reflected solar light, and to incandescence of its ingredients. The repulsion of the tail from the sun seems to demand an electrical explanation. Twenty periodical comets are known, the more important of which are here tabulated: -

Besides these there are other comets worthy of notice. The 1811 comet was very brilliant; it describes an elliptic orbit of enormous dimensions, its maximum distance from the sun being about 40,000 million miles. Views of the great 1843 comet and of Donati's 1858 comet are given in the plate. The former approached dangerously near to the sun; its minimum distance from that body was only 32,000 miles. Its tail was over 200 million miles in length, and its orbit has since been followed by the 1880 comet. The earth passed through the tail of the great 1861 comet, but without any harm being done by the passage. Very many similar appearances have been observed, and their structure examined by telescope and spectroscope.