Sometimes there is no nucleus, and sometimes there is no tail; but there is always a coma--a soft hazy cloud of light, perhaps small enough to look like a dim star at first. But a comet watched from the Earth can never be so far off as the stars. Even the very largest comets are seldom to be seen farther away than Jupiter. There are comets of all sizes, from the huge to the tiny. Perhaps one would find as much difference between comets in the sky as between a whale and a minnow in water. Under a certain size they are hidden from us; but tinier comets than we can see may float in the sky by myriads. Of those which we can see, the thickest and heaviest part of the whole--the “nucleus”--may be only about fifty or a hundred miles through, or it may be some thousands of miles. The coma, or bright fog surrounding this thickest part of the comet, is generally as much as ten thousand miles across; and sometimes it is a hundred thousand miles. As for the bright train, it is, when fully formed, seldom less than ten millions of miles long, and sometimes it is a hundred millions of miles. Such a tail as this would reach the whole way from our Earth to the Sun. Yet a comet is not heavy. Its make is most wonderfully light; far more so than the very lightest world in the Sun’s kingdom. Saturn is not so solid or so heavy as water; but a comet really almost seems to be less solid and heavy than a mist. Very faint stars can be seen shining through thousands of miles of comet-thickness; while it does not take much of an earth-mist to hide the light of even the brightest star. Not long ago people were much frightened at comets. If a big one appeared in the sky it was thought to be a sign of something dreadful about to happen. Nobody then had any idea what immense numbers of comets are always in the sky. It was feared that, if a comet should run against our Earth, the whole world would be destroyed. Nobody knew how very light and delicate in its make a comet is. If such a thing ever did happen, which is most unlikely, one cannot say that no harm would be done; but certainly our Earth would not be destroyed. These comets seem to shine partly in the sunlight, and partly by their own brightness. You must not think that a comet always has a tail. More often, when one is first seen in the distance, it is only as a little hazy patch, or like a dim star, with no train of light at all. But as it comes hastening out of cold and darkness into the warmth and glare of the Sun great changes take place in its shape. The nucleus very often gets a little smaller; and why this should be I cannot tell you. But the coma gets larger, and takes to throwing out bright jets. Then the tail begins to grow; and day by day it becomes larger and larger. A comet drawing nearer to the Sun travels head-foremost, with the tail following after the head. This is only what one would expect.