COMET (Greek, ((hair,'" alluding to the luminous appendage or tail with which the brighter comets are always associated, and which formerly won for them the name of ohairy stare). Comets may for convenience be divided into two classes, periodic and un expected. From 2 to 3 of the former, and from 3 to 4 of the latter, appear on the average every year, but the number varies greatly. In 1898 as many as 10 were discovered, 5 being found in 12 days. Of the 10, 7 -were un expected.
Every comet, no matter how magnificent it may subsequently be, when first discovered, if remote from the sun., appears as a small, very faint, hazy ball. As it comes nearer it becomes brighter, and generally larger, although some times it contracts in size. If it is going to be a fine comet it gradually lengthens out, and develops a short tail. The tail rapidly brightens and lengthens as the comet ap proaches perihelion, as the point in its orbit is called where it is nearest the sun. If the earth is favorably situated the comet appears at its best a few days after the passage of perihelion, and then gradually fades out and disappears as it came. About one unexpected cornet in eight is visible to the naked eye. The periodic comets move in elliptical orbits about the sun in planes not in general greatly in clined to the ecliptic. The longest period so far definitely known, that of Halley's comet, is 76 years. The unexpected comets have still larg-er orbits, lying in all planes, with much longer periods. Many of them have a retro grade motion, and occasionally one visits the sun never again to return to it. When an un expected comet arrives, its orbit is first com puted from three observations made on dif ferent nights, on the assumption that it is moving in a parabolic orbit. If we are able to observe it through a long period we frequently find that its path differs slightly from a para bola. It is not likely that any comet moves in an exact parabola. If it moves a little slower than the parabolic velocity, its orbit is an ellipse, and the comet really becomes periodic, although it is not classed as such until it has been certainly identified at another return. If
it moves a little faster than the parabolic velocity, its orbit is a hyperbola, and the comet will recede into space never to return.
Shduld cpmet happen to pass near one of the larger-planets, its orbit may be somewhat changed by the approach and its speed may be either increased or diminished. In this manner new members of the periodic class of comets are from time to time added to the list. On the other hand, its velocity may be so far accel erated as to change its orbit, or even drive it out of our system, never to return to it. In this way some of the members of our periodic family of comets are subject to change; others, how ever, have been permanently added to the solar system. Out of 33 well-known periodic comets 24 have, or did have, periods lying between five and nine years. Many of these were doubtless brought into our system by the giant planet Jupiter. Some of them have already disap peared, owing to a change in their orbits, while some have lost a large part of their gaseous constituents and thus become invisible.
This last statement naturally brings us to the question, What is a comet? • In early times comets were supposed to be objects within our own atmosphere, presaging famines, wars and the death of kings. It was first shown by Tycho Brahe that they were celestial bodies independent of the earth, and Newton proved that, excepting their tails, they were bodies subject to the law of gravitation. In the middle of the last century it was shown by H. A. Newton, Schiaparelli and others that our chief meteoric showers, those of 10 August and 14 November, were due to great swarms of meteoric bodies moving in elliptical orbits, practically coincident with the orbits of two well-known periodic comets, 1862, III (Tut tle's), and 1866, I (Tempel's).