Satellites

Satellites are small members of the solar system, taking the place of attendants of the larger planets, by which their motions are controlled. In relation to them, the controlling planet is called their 'Primary.' For a discussion of the satellite of the earth the reader is referred to the article MOON. The satellites of Mars are interesting as by far the smallest known. One of them, Phobos, revolves round its primary so quickly that its rising and setting are determined chiefly by its own motion. To an observer on Mars it will rise in the west, and cross the sky in a few hours. Both were discovered in 1877. The first three of Jupiter's satellites are eclipsed at every revolution, the fourth less frequently. Their frequent passages before and behind their primary form one of the most attractive spectacles for small telescopes. Their orbits differ but little from circles, and between the first three a curious relation exists—viz. the mean sidereal motion of the 1st added to twice that of the 3d is equal to three times that of the 2d; so that, except at a vast interval, the three cannot all be eclipsed at once, although each is eclipsed once in every period. Long observation and careful calculation have enabled the places of these satellites to be so accurately predicted that their eclipses have been used to determine the velocity of light. Jupiter, owing to the earth's orbital movement, is at one time 183,000,000 miles nearer us than at another. The eclipses of his satellites are therefore delayed or hastened, according to the velocity of light, as the strokes of a hammer at a distance are delayed in reaching us by the finite velocity of sound (see LIGHT, SUN). From this difference (about 500 seconds) the velocity of light has been calculated. One of the satellites of Saturn, Titan, is much the largest of all, approaching in size the smaller planets. The motion of the satellites of Uranus is retrograde. The table sums up what was known prior to the discovery at the Lick Observatory in 1892 of a fifth (very small) satellite of Jupiter, which ranks first (as nearest its primary), probably under 100 miles in diameter.