MERCURY, VENUS, AND MARS.
We can hardly think of either the sun or the moon as a world in the sense in which our earth is a world, but there are some bodies called planets which seem more like worlds, and it is about them that we are now going to talk. Besides our Earth there are seven planets of considerable size, and a whole host of insignificant little ones. These planets are like ours in a good many respects. One of them, Venus, is about the same size as this earth; but the two others, Mercury and Mars, are very much smaller. There are also some planets very much larger than any of these, namely, Jupiter, Saturn, Uranus, and Neptune. We shall in this lecture chiefly discuss three bodies, namely, Mercury, Venus, and Mars, which, with the earth, form the group of “inner” planets.
The planets are all members of the great family dependent on the sun. Venus and the earth may be considered the pair of twins, alike in size and weight. Mercury and Mars are the babies of the system. The big brothers are Jupiter and Saturn. All the planets revolve round the sun, and derive their light and their heat from his beams. We should like to get a little closer to some of our fellow-planets and learn their actual geography. Unfortunately, even under the most favorable circumstances, they are a very long way off. They are many millions of miles distant, and are always at least a hundred times as far as the moon. But far as the planets may be, astronomers have been familiar with their existence for ages past. I can give you a curious proof of this. You remember how we said the first and the second days of the week were called after the sun and the moon, Sun-day and Moon-day, or Monday, respectively. Let us see about the other days. Tuesday is not quite so obvious, but translate it into French and we have at once Mardi; this word means nothing but Mars’ day, and our Tuesday means exactly the same. Wednesday is also readily interpreted by the French word Mercredi, or Mercury’s day, while Venus corresponds to Friday. Jupiter’s day is Thursday, while Saturn’s day is naturally Saturday. The familiar names of the days of the week are thus associated with the seven moving celestial bodies which have been known for uncounted ages.