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
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