ASTEROIDS, a group of small planetary bodies known also as minor planets which revolve round the sun between the orbits of Mars and Jupiter. The most remarkable feature of these bodies is that they occupy a vacant place in the solar system in which a planet would naturally belong. Between the orbit of Mars, the fourth planet in order, and that of Jupiter, the fifth, there is a space more than double the radius of the orbit of Mars. This gap was noticed from the time that the distances of the planets were laid down by Kepler. It was long suspected that a planet would be found occupying such space and an organized effort was made to discover it. The discovery of a planet which was sup posed to be the long-sought body was made by Piazzi at Palermo on the first day of the 19th century. To the great surprise of astronomers three other planets were found in the course of the next sixyears to be revolving in the same region. The smallness of the four bodies gave rise to the celebrated hypothesis of Olbers that these bodies were the frag ments of a single planet which had been dis rupted by some cataclysm. Some 40 years elapsed when in 1846 a fifth asteroid was dis covered. Others soon followed. More pow erful telescopes were applied to the search, a thorough system was introduced and in this way the number known went on increasing until it mounted into the hundreds, and now a dozen or more are frequently added to the list in a single year.
When photography was applied to form a permanent picture of the stars in the it was found that this method was the easiest by which discoveries of these objects could be made. Whatever method may be used, the difficulty of discovering an asteroid arises from the fact that it is impossible to distin guish such a body from a fixed star by its mere aspect. It can be detected only by its motion among the stars and therefore re quires that the same body should be observed at different times. But a photograph enables the motion to be detected in a very simple way, as follows: If a telescope, mounted so as to serve as a camera, is pointed at a given region of the sky for half an hour or more the images, of the stars which fall on the plate remain im movable, and these bodies are photographed as simple points of light. But if an asteroid
is in the field, its motion during the exposure is quite appreciable; and its picture appears as a short line, equal in length to the amount of motion during the exposure of the plate, which can be detected at sight. A decided improvement to this photographic method has been recently devised which consists in sub jecting the plate to a slight, uniform, side wise motion equal to that of the teroid in the part of the sky under examina tion. The stars are thus photographed upon the plate as trails while the asteroid appears as a very much shorter trail or as a point of light; its image is thus rendered far more intense than if it were allowed to trail, and numerous asteroids have been thus discovered which would have been far too faint to be detected by the older method. During the last 10 years the number discovered has earned the total list up to more than 800, a number so great that it is almost impossible to com pute the orbits or motions or even to find separate and appropriate names for the bodies.
The asteroids are distinguished from the major planets by several distinct and inter esting features. One of these concerns their orbits. While the major planets, with the exception of Mercury, all move in nearly cir cular orbits, the orbits of the asteroids are, in the general average, markedly eccentric.
In some cases the asteroid is twice as far from the sun at aphelion as at perihelion. One result of this is that they appear several times brighter when seen in 'opposition at perihelion than at aphelion. The inclination of the orbits is also frequently very large. That of Pallas, one of the original four, is inclined no less than to the elliptic. The result of this is that many of these bodies wander far outside of the limits of the Zo diac; indeed, in many cases they are seen north of the zenith in our latitudes when near est the earth.