Fig. 128.—Comet Showing Nucleus, Coma and Tail.
A comet is really made up of three parts, which are, (1) a bright head or core, called a nucleus, and this is covered with (2) a layer of hazy light, called a coma, to which there is attached (3) a luminous tail, all of which is shown in [Fig. 128]. The nucleus of a comet is formed of a bunch of stones and pieces of iron, all widely separated and which are held together by attraction as they speed through space. As a comet nears the Sun it begins to get hot and to throw off burning gases, which make up its coma, and these bright gases streaming along form its tail.
There are several ways in which a comet can be told from the planets. In the first place, new comets, that is, comets which have never been discovered before, appear suddenly and in any part of the sky, though they may be very dim at first, and after a while they fade away, sometimes never to return again.
Fig. 129.—An Ellipse, Parabola and Hyperbola.
Second, they shine chiefly with their own light like the stars; third, they do not travel in small circles around the Sun like the planets; but the paths they take are either long ovals, called ellipses, or great curves whose ends never meet, called parabolas and hyperbolas, as shown in [Fig. 129]; fourth, they do not travel through space in a line with the planets and Sun, but shoot in and out of our solar system from and to every direction; and fifth and last, they travel at enormously high speeds.
Now, a comet, however great it may grow to be, never bursts into view, big, bright and beautiful, but when one is found it is usually seen as a little, dim patch of light, and it would quite likely be mistaken for a nebula, if it did not move along so swiftly.
Fig. 130.—Head and Tail of Comet Do Not Obey the Same Laws.
A comet can be told by its movement and its movement can be plotted in the same way I have described for plotting the position of a planet in [Chapter IV]; and so, if you see a dim, little ball of light in the sky and find that it changes its position when compared with the fixed stars near it you may guess that you have discovered a comet.