PLANETOIDS. of ver}' small planets which, with one exception, travel' in orbits lying between those of JIais and Jupiter. Till tlie first year of the nineteenth century they remained undiscovered; but for some years before their existence had been sus- pected, mainly owing to the remarkable hiatus in the series of the planetary distances when com- pared with the law of J3ode (q.v. ). On the first day of January, ISOl, the first of them was de- tected by Piazzi at Palermo, and his success roused the astronomers to search for more plan- ets. Their search was successful, for Olbers (q.v.) discovered two in 1802 and 1807, and Harding one in 1804 ; but as all researches for .«onie time subsequent to 1807 were unavailing, astronomers gradually allowed themselves to set- tle down into the belief that no more planetoids remained to be discovered, when the detection of a fifth Ijy Hencke in 18-15 revived the hope of fresh discoveries, and from this period no year (excepting 1846) has passed w-ithout adding to the list. And since the application of photographic processes to observation, the number known to astronomers has increased with great rapidity. The first photographic asteroid was found by Wolf, of Heidelberg, Germany, December 20, ISOl, and was named by him Brucia, in honor of Katharine Wolft' Bruce", of Xew York, who con- tributed to the endowment of the Heidelberg observatory. Brucia received the number 323 in the list of planetoids. Since 1892 only a few planetoids have been found visually, though the total number in the list has been extended from 323 to over 500. It will be seen, therefore, that photography has almost completely replaced the . visual telescope in this kind of observation. The magnitudes of the planetoids have not been very accurately ascertained, but it is certain that they are exceedingly small compared even with Mercury, the smallest of the other planets, the diameter of the largest among them having been measured by Barnard as 485 miles, while most of the others are very much smaller than this. They also differ, generally speaking, from the rest of the planets in other respects ; their orbits are of greater eccentricit.y, are inclined to the ecliptic at a greater angle, and are interlaced in a most intricate manner, crossing each other so frequent- ly as to form, when viewed perpendicularly, a kind of network. Tlie consequence of this is that a mutual eclipsing of the sun at different periods by two planetoids must be of very frequent oc- currence. After the first two or three of these bodies had been discovered, the opinion was pro- pounded by Olbers that they were but the frag- ments of some large planet, and this hypothesis received corroboration from the intimate connec- tion which was shown to subsist among them : but this hypothesis is now rejected by most astrono- mers, and their existence is explained by the nebular hypothesis. See Nebui^e. Decidedly the most interesting planetoid is the one discovered August 13, 1898, by Witt of Ber- lin, and which has received the number 433 and the name Eros (q.v.). This small planet pos- sesses the remarkable orbital peculiarity that at perihelion it passes considerably within the orbit of IVIars, and thus approaches the earth nearer than any other kno«-n body except the moon, and is therefore especially ^vell suited for the deter- mination of solar parallax by the planetoid method. By means of long photosraphic exposure Wolf and Duaan. during !May and June. 1003, dis- VOL. XVI.— 7. 91 PLANETS. covered eight planetoids, provisionally indicated by 1903, LQ to LX, See Pakallax. PLANETS. Those solid spherical bodies which belong to our solar system, and revolve in elliptic orbits round the sun. The name planet is of considerable antiquity, and was applied to these dependents of the sun to dis- tinguish them from the other luminous bodies which stud the sky and present to the naked* eye no indication of change of relative place (see Stars), while the planets seem to wander about among them. The planets at present known are, in the order of their distance from the sun, Mer- cury, Venus, the earth, Mars, the planetoids, .Jupiter, Saturn, Uranus, and Neptune. Six of these. Mercury, Venus, the earth (which was not, however, then reckoned a planet). Mars, Ju- piter, and Saturn, were known to the ancients; Uranus was discovered by Sir William Herschel (q.v.) in 1781; and Neptune, after having its position and elements determined theoretically by Leverrier and Adams, was discovered by Galle iii 1846. The planetoids, of which over 500 are now known, have all been discovered since Janu- ary 1, 1801. Six of the planets, the earth, Mars, Jupiter, Saturn, Uranus, and Neptune, are at- tended by one or more satellites; Uranus (gen- erally), Neptune, almost all the planetoids, and all the satellites except our moon, are invisible to the naked eye. The visible planets can be at once distinguished from the fixed stars by their clear, steady light, while the latter have a sparkling or twinkling appearance. The com- ])arative proximity of the planets may be proved by examining them through a telescope of moder- ate power, when they appear as round luminous disks, hile the fixed stars show as mere points of light even in the most powerful telescopes. The planets as observed from the earth move sometimes from w-est to east, sometimes from east to west, and at times remain stationary at the point where the apparent motion changes from the one direction to the other. This irregularity in their movements was very puzzling to the ancient astronomers, who invented various hy- potheses to account for it. (See Ptolemaic System; Epicycle.) The system of Copernicus, by assuming the sun, and not the earth, as the centre of the system, explained with admirable simplicity what seemed before a maze of con- fusion, by showing that the planetary iijjparent motions are merely a result of real motions be- longing to the planets in general and to the earth carrying the observer. The planetary orbits differ considerably in their degrees of eccentricity, the planetoids, Mars, and Mercury being most, and the larger planets least, eccentric. No two planets move exactly in the same plane, though, as a general rule, the planes of the larger planets most nearly coincide with that of the ecliptic. The latter are consequently always to be found within a small strip of the heavens extending on both sides of the ecliptic (q.v.) : while the others have a far wider range. Pallas, one of the planetoids, has an angular elevation of orbit equal to 34° 41' above the ecliptic. According to Kepler's laws (see Astronomy), the nearer a planet is to the sun the shorter is the time of revolution. The arrangement of the planets in the solar system bears no known relation to their relative size or weiLdit. for though ilereury, Venus, and the earth follow the same order in size and distance
