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 a good deal 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 in the duration of their day. Mercury and Mars are the babies of the system. The big brothers are Jupiter and Saturn. All the planets revolve around 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.

JUPITER, SATURN, URANUS, NEPTUNE

Our lecture to-day ought to make us take a very humble view of the size of our earth. Mercury, Venus, and Mars may be regarded as our peers, though we are slightly larger than Venus, and a good deal larger than Mercury or Mars; but all these four globes are utterly insignificant in comparison with the gigantic planets which lie in the outer parts of our system. These great bodies do not enjoy the benefits of the sun to the same extent that we are permitted to do; they are so far off that the sun's rays become greatly enfeebled before they can traverse the distance; but the gloom of their situation seems to matter but little, for it is highly improbable that these bodies could be inhabited. Though they get but little of the sun's heat, they seem to have ample heat of their own.

A view of parts of the paths of these four great planets is shown in Fig. 63. The innermost is Jupiter, which completes a circuit in about twelve years; then comes Saturn, revolving in an orbit so great that twenty-nine years and a half is required before the complete journey is finished. Still further outside is Uranus, which has a longer journey than Saturn, moves so much more slowly, that a man would have to live to the ripe old age of eighty-four if a complete revolution of Uranus was to be accomplished during his lifetime.

WE TRY TO MAKE A MAP

The group of bodies which cluster around our sun forms a little island, so to speak, in the extent of infinite space. We may illustrate this by a map in which we shall endeavour to show the stars placed at their proper relative distances. We first open the compasses one inch, and thus draw a little circle, which I intend to represent the path followed by our earth, the sun being at the centre of the circle. We are not going to put in all the planets. We take Neptune, the outermost, at once. To draw its path I open the compasses to thirty inches and draw a circle with that radius. That will do for our solar system, though the comets no doubt will roam beyond these limits. To complete our map we ought of course to put in some stars. There are a hundred million to choose from, and we shall begin with the brightest. It is often called the Dog star, but astronomers know it better as Sirius. Let us see where it is to be placed on our map. Sirius is beyond Neptune, so it must be outside somewhere. Indeed, it is a good deal further off than Neptune; so I try at the edge of the drawing-board: I have got a method of making a little calculation that I do not intend to trouble you with, but I can assure you that the results it leads me to are quite correct; they show me that this board is not big enough.