Including our own globe there are eight principal planets—viz., Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune. The two first-named being between us and the sun, are termed interior planets; the others are exterior. Mercury, Venus, and Mars are smaller than Earth. The other four are much larger.
Fig. 576.—An Orrery.
We have already described the planets as bodies wandering through the zodiac, and reflecting the sun’s light. Their orbits are very different from the moon’s; for instance, planets take a retrograde motion as well as a direct one. The sun and the planets revolving around him constitute the solar system.
We will commence our brief consideration of them with Mercury, the planet nearest to the sun.
The distance of Mercury from the sun is 35,000,000 of miles, less than half the distance our earth is from him, and so receives much more heat and light than we do. The sun to the Mercurians, if there be any inhabitants upon the planet, must appear about seven times larger than he does to us. Mercury’s year is about eighty-five days in length, so the seasons must be shorter if they follow the same rotation as ours. It passes through space with an exceedingly rapid motion, and so probably the ancients called the swift planet Mercury after the winged messenger of Jove.
Mercury is not an easy planet to observe, owing to its proximity to the sun, yet the ancients managed to descry it. But it can be seen just before sunrise and sunset in autumn, and in spring if the weather be clear. It possesses phases similar to our moon. Some authorities have stated that Mercury has an atmosphere, but this circumstance, as well as its formation, is still shrouded in mystery. Mercury’s day is a few minutes longer than ours.
Fig. 577.—Transit of Mercury.
A transit of Mercury is represented in the accompanying illustration (fig. 577). This phenomenon took place in 1845, but there have been many others noticed. The first recorded took place in November 1631, and these transits always occur in May or November.