Page:The story of the comets.djvu/26

4 "Tip" as applied to the tail of a comet is generally little more than a figure of speech, because it is, as a rule, impossible to say what is the tip, that is, to say where the tail comes to an end. Occasionally the tail increases to a length, it may be, of 10 or 20 degrees of arc or more. In the case of comets of great size and brilliancy this tail sometimes spreads across a large portion of the heavens; sometimes there are more tails than one. An ordinary tail presents the appearance of a stream of milky-white light which is always fainter and usually broader the further from the head that one examines it. Occasionally the broadening of the tail towards its extremity becomes a very marked feature.

The nucleus of a small comet is generally circular, as indeed is the whole comet, but a nucleus is sometimes oval, and, in very rare cases, may present a radiated appearance. The nucleus, if visible to the naked eye (the comet itself being a small one), generally looks like, and may easily be mistaken for a star or a planet, the coma not being visible until a telescope is brought to bear on the comet. But in a telescope such a comet will show as a point of light surrounded by a fog of light. Sometimes, of course, the foggy appearance may reveal itself even to the naked eye if the comet as a whole is sufficiently luminous. Arago remarked that the nucleus is generally eccentrically placed in the head, lying towards the margin nearest the Sun. I do not, however, think that this can be considered an established law applicable to the majority of the small comets; and under any circumstances it would seem to betoken the forthcoming appearance of something of the nature of a tail. Sometimes a comet will have 2 or more nuclei or bright centres of light, but one is the normal number.

The newly found comet approaches the Sun in a curvilinear path which frequently differs but little from a straight line. It generally crosses that part of the heavens in which the Sun is situated so near the Sun as to be lost in its rays, but it emerges again on the other side frequently with increased brilliancy and increased length of tail. The phenomena of disappearance are then not unlike those which marked the