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.