July 8.
CHANGES IN THE COMET OF 1863.
Before we can completely accept the view that light-pressure forms this train of soot we must ascertain whether the pressure of light is capable of accounting for the flash-like rapidity with which a Comet’s tail changes.
A Comet may throw out a tail sixty million miles long in two days. Is it actually possible for light-pressure to accomplish that astonishing feat? Arrhenius has computed that 865,000 miles an hour is the speed of a light-flung particle of one-half the critical diameter. Because they are only one-eighteenth as large as this particle of critical diameter, the dust grains in a Comet’s tail would be propelled over the same 865,000 miles in less than four minutes. It follows that the solar radiation is amply strong enough to toss out a tail of sixty million miles in two days.
Photography in the hands of Prof. E. E. Barnard, of the Yerkes Observatory, has revealed some extraordinary changes in Comets’ tails, changes which are not apparent to the eye and which cannot be explained by light-pressure or by solar electrical forces. He has collected a formidable mass of photographic evidence which seems to show that there are other influences at work besides the Sun’s radiation, and that these influences manifest themselves in distorting and breaking a Comet’s tail. In some Comets of recent years, streams of matter have been shot out in large angles to the main direction of the tail without being at all bent by the pressure of light. In Morehouse’s Comet of 1908, tails were repeatedly formed and discarded to drift bodily out into space and melt away. Sometimes the photographic plate has shown the tail twisted like a corkscrew and sometimes it has revealed masses of matter at some distance from the head, where apparently no supply had reached it. At one time the entire tail of Morehouse’s Comet was thrown violently forward, a peculiarity so utterly opposed to the laws of gravitation that Professor Barnard suspects some unknown force at work in planetary space besides a force which undoubtedly resides in the Comet itself. If Halley’s Comet serves no other purpose than to throw light upon this mystery, its return will more than repay astronomers for all their observatory vigils.
From the fact that the matter is ejected from the head to form the tail, it would follow that, unless it has the means of rejuvenating itself, a comet must eventually be disintegrated. Instances of this fragmentation and, eventual disappearance of a Comet are not wanting in astronomical annals. It has been stated previously that when Biela’s Comet appeared in 1846 it became distorted and elongated, that it eventually split up into two separate bodies, that in 1852 it again appeared in its double form, and that it has since disappeared.
In a way, Comets may be said to bleed to death. At each return of Halley’s Comet, future astronomers will find it less brilliant than it was seventy-six or seventy-seven years before. Some time there will be no Halley’s Comet left, and the most famous Comet of its kind will be reduced to a shoal of meteors varying in weight from a few ounces to several tons and faithfully pursuing the orbit which their parent traced and retraced century after century.
COGGIA’S COMET, 1874: ON JULY 13.