Com′ets, the name given to those large and more or less bright bodies which now and then enter the solar system, sweep around the sun, and again return to more distant regions of space where they become invisible as before. In general comets are composed of three parts. First, there is a luminous cloud, the coma, from which these bodies take their name. In its center is generally found a brighter nucleus, the part which the astronomer observes when he determines the orbit of the comet. Thirdly, there is the tail, which is a great streamer of light, following the comet as it approaches the sun and preceding the comet as it leaves the sun. Considered with respect to their brightness, comets may be divided into two classes: telescopic comets and naked-eye comets. By far the greater number belong to the first class and can be seen only by the aid of a telescope. Considered with reference to their orbits, comets may be divided into three classes: (1) those whose orbits are elliptical; (2) those having parabolic orbits; and (3) those having hyperbolic orbits.
Nearly all comets appear to move along a parabola, having the sun at the focus; some 20 per cent., however, of those studied move in elliptical paths. Since a parabola is a curve which has two branches extending off to infinity, we do not expect ever to see again a comet which has a parabolic orbit. But an ellipse is a closed curve, and accordingly some of those comets having elliptical orbits have been observed many times.
Nature of comets
Concerning the composition or structure of comets very little is known, and that little has been learned almost entirely from the spectroscope. This instrument shows that a very small portion of the comet’s light is reflected sunlight. The comet shines by its own light, giving a spectrum which is described as follows by Professor Young of Princeton:
“The spectrum of most comets consists of a more or less faint continuous spectrum (which may be due to reflected sunlight, though it usually is too faint to show the Fraunhofer lines) overlaid by three bright bands,—one in the yellow, one in the green and the third in the blue. A fourth band is sometimes visible in the violet. The green band, which is much the brightest of the three, is in some cases crossed by a number of fine, bright lines, and there are traces of similar lines in the yellow and blue bands. This spectrum is absolutely identical with that given by the blue blaze of an ordinary gas or candle flame; or, better, by the blue flame of a Bunsen burner consuming ordinary illuminating gas. Almost beyond question it indicates the presence in the comet of some gaseous hydrocarbon, which in some way is made to shine; either by a general heating of the whole body to the point of luminosity (which is hardly probable), or by electric discharges within it, or by local heatings due to the action of sunlight; or none of these surmises may be correct, and we may have to seek some other explanation not yet suggested.
It therefore, on the whole, appears highly probable that comets are bodies made up of small discrete particles.
One great mystery in connection with these bodies is the fact that the tail is repelled by the sun. But recently the Swedish physicist, Arrhenius, has given an explanation of this fact, which is quite plausible. He bases his explanation upon the fact, discovered by Maxwell, that a beam of light exerts a pressure in the direction in which the light is traveling and that the amount of this pressure is numerically equal to the amount of energy, in unit volume, of the beam of light.
If, now, the particles of which the comet is composed are very small, they will present a surface which is very large compared with their mass. Since this pressure increases very rapidly as the comet approaches the sun, it is an easy matter to compute, for particles of any given size, just how near the sun they must go in order that this repulsive pressure due to sunlight may equal the gravitational attraction of the sun for the particle. If the comet should come nearer the sun than this limiting distance, repulsion will ensue. Or at any given distance we can easily compute the size which the particles must not exceed in order to be repelled by the sunlight. This explanation is quite is accordance with the results obtained by means of the spectroscope.
Remarkable comets
In recent times the most brilliant comets are those of 1858 and 1882. The former is known from its discoverer as Donati’s. To the astronomer no comet has more interest, perhaps, than that of Halley (1682), whose return to the solar system in 1759 was predicted by its discoverer. The comet actually returned, as predicted, and in 1835 returned for a second time. Its next return occurred in 1910. Eneke’s comet is also one of great interest, returning every three and one-half years. See Miss Clerke’s History of Astronomy during the 19th Century, Chaps. 5, 10 and 11.