Titan, the largest satellite, is 771,000 miles distant from the planet. The nearest one is only 117,000 miles away but there are also three others which are closer to the planet than our moon is to us. Phœbe, the most distant one, is almost 8,000,000 miles away and takes nearly 18 months to make a revolution. It appears as a full moon only once in a year and a half—quite a contrast to Mars' little Phobos who goes through his phases one and a half times every night! Phœbe seems to be possessed with a streak of contrariness for she moves in her orbit around Saturn in the opposite direction from all of the other moons.
Japetus, the second largest satellite, has the interesting peculiarity of appearing three or four times as bright when on the western side of Saturn as when on the eastern side of it. This is explained by the supposition that one of its sides has a greater reflecting power than the other and, like our moon, always has the same side toward its planet. We would, therefore, see one side of it when east of Saturn and the other side of it when west of it.
Themis, the tenth moon, was first recognized by W. H. Pickering in 1905 but it is only recently that its discovery was fully confirmed. Professor Pickering also discovered the ninth moon of Saturn.
CHAPTER XV
MERCURY, URANUS AND NEPTUNE
THE TINY PLANET OF MERCURY, THE MESSENGER OF THE GODS
Diameter—3030 miles
MERCURY, the smallest and fleetest and hottest of the planets, shines as a star of the first magnitude. It lies closer to the sun than any other planet, which increases the difficulty of studying it. Most of the time it is either hidden behind the sun or is passing between us and the sun, and the only time when it is observable is when it is to the east or the west of the sun. Indeed it is so often hidden or lost in the brightness of the sun that it is rarely seen except by a professional astronomer.
Like Venus, Mercury is seen first on one side of the sun and then on the other. In the early spring it appears in the west almost invisible in the twilight; in the early autumn, in the east as a morning star just preceding or mingled in the rays of the dawn. It is always seen very close to the eastern or western horizon, never more than two hours before the sun in the morning nor more than two hours after it in the evening.
Because of the great inclination of the orbit of Mercury, it is seen in a transit across the face of the sun not more than thirteen times in a hundred years. This transit may only be seen in a telescope. If it so happens that it travels across the center of the sun, the journey consumes about eight hours, although the little black dot is skimming across the big, bright disk at the rate of 100,000 miles an hour. The next five transits will occur on November 8th, 1927; May 10th, 1937; November 12th, 1940; May 13th, 1953, and November 6th, 1960.
The eccentricity of the orbit of Mercury is so pronounced that its greatest and least distance from the sun varies nearly 15,000,000 miles. This causes considerable variation in the speed of this planet, for as the sun does not lie in the center of the orbit but at one focus, it is not compelled to go as fast on the far side as on the side nearest to this tremendous mass. Being so close to the great attracting force of the sun, it is however, forced to move faster than any other planet in the solar system in order that the right balance of speed may be obtained to counteract the gravitational pull which would otherwise draw him instantly to the burning surface. Mercury's average rate of speed around its orbit is 35 miles a second. Neptune, seventy-five times as far off, feels so much less this relentless pull that it can roll leisurely along at the comfortable rate of 3.4 miles a second with no fear of being drawn to destruction.