Satellites

Satellites are small celestial bodies attendant. on ~he planets. They rotate round the planet, which is often called the primary, and which controls their motion. The interior planets, Mercury and Venus, are unaccompanied by any satellites, while the Earth's attendant, the Moon, is naturally by far the best known of all. [MOON.] Venus was formerly believed to possess a satellite, first pointed out by Fontana in 1645, and many astronomers in the 16th and 17th centuries testified to its existence. Later work has, however, proved that some of the astronomers mistook certain stars for the satellite, while the others must be considered as the victims of illusion, since Venus has no obvious moon. Since the Middle Ages Mars has been credited with two satellites, which, however, do not appear to have been actually seen till 1877, since when they have been repeatedly observed. They are called Deimos and Phobos, and their diameters have been estimated as 6 and 7 miles; they are therefore the smallest known satellites. Jupiter has five satellites, four of which were discovered by Galileo, while the fifth was first noted in September, 1892, by Professor Bernard at the Lick Observatory. All lie very nearly in the plane of Jupiter's equator. The first four are visible even with the feeblest telescope, but the fifth is so small, being only about 100 miles across, and moves so rapidly, that it is nearly always invisible, for it fades away in the presence of the slightest amount of light from Jupiter. All Jupiter's satellites revolve more rapidly than does our Moon, the last discovered taking rather less than 12 hours, only 2 hours longer than Jupiter's own period of rotation. Between the first three satellites there are curious relationships. The mean motion of the first, together with twice that of the third, is equal to three times that of the second, and also the mean longitnde of the first, together with twice that of the third, is equal to three times that of the second, increased by 180°; hence they cannot be all three eclipsed at one time, although each is eclipsed at every revolution. The times of the eclipses of these satellites have been recorded over a very long period, and their recurrences predicted. Careful observation led to the discovery that a certain definite difference was obtained between the observed time of an eclipse and the predicted time; the eclipse occurred before it was expected when Jupiter was nearest to the Earth, whereas it happened later when Jupiter was farthest away. This led to the idea that light took a definite time to travel, and, since the distance of Jupiter from the Earth in the two cases was known, this gave a means for measuring the velocity of light. The satellites are not only rendered invisible to us because they pass into the dark shadow of Jupiter cast by the sun, but they may actually pass behind the planet himself, in which case they are occulted. The moments when an occultation begins or ends are not nearly so sharply defined as the time of an eclipse, since it is difficult to see the satellite when it is at the very edge of Jupiter's disc. It is similarly difficult to see the satellite when it is pursuing a transit in front of the planet, but it generally casts a shadow, which is seen as a small dark moving spot on the planet's face. The detection of Saturn's satellites has extended over many years, from the discovery of the first by Huyghens in 1655, followed by the finding of four more by Cassini later in the same century and another two by Herschell, to the simultaneous observations made by Lassell at Liverpool and Bond in America on September 19th, 1848, of a small moon far away from the planet, and the further discovery of a ninth in 1899. Huyghens' satellite is the largest one known, its diameter being about 3,300 miles, The four moons of Uranus have their orbits in the same plane, and this plane is nearly perpendicular to the plane of the planet's orbit. This fact is curious and unique, while it is also remarkable that the orbits of the satellites appear to be perfectly spherical. Neptune, like the Earth, possesses only one satellite, which revolves round its master in about six days.