all and the other
nearly as large as the earth, circle round and round the sun in orbits
lying inside ours. Now if we want to place objects around our lamp-sun
which will represent these planets in size, and to put them in places
corresponding to their real positions, we should find no room large
enough to give us the space we ought to have. We must take the lamp out
into a great open field, where we shall not be limited by walls. Then
the smallest planet, named Mercury, which lies nearest of all to the
sun, would have to be represented by a pea comparatively close to the
sun; Venus, the next, would be a greengage plum, and would be about
twice as far away; then would come the earth, a slightly larger plum,
about half as far again as Venus. After this there would be a lesser
planet, called Mars, like a marble. These are the first four, all
comparatively small; beyond them there is a vast gap, in which we find
the asteroids, and after this we come to four larger planets, mighty
indeed as regards ourselves, for if our earth were a greengage plum,
the first of these, Jupiter, would have to be the size of a football at
least, and the next, Saturn, a smaller football, while Uranus and
Neptune, the two furthest out, would be about the size of the toy
balloons children play with. The outermost one, Neptune, would be thirty
times as far from the sun as we are.

[Illustration: COMPARATIVE SIZES OF THE PLANETS.]

This is the solar system, and in it the only thing that shines by its
own light is the sun; all the rest, the planets and their moons, shine
only because the rays of light from the sun strike on their surfaces and
are reflected off again. Our earth shines like that, and from the nearer
planets must appear as a brilliant star. The little solar system is
separated by distances beyond the realm of thought from the rest of the
universe. Vast as are the intervals between ourselves and our planetary
neighbours, they are as nothing to the space that separates us from the
nearest of the steady shining fixed stars. Why, removed as far from us
as the stars, the sun himself would have sunk to a point of light; and
as for the planets, the largest of them, Jupiter, could not possibly be
seen. Thus, when we look at those stars across the great gulf of space,
we know that though we see them they cannot see us, and that to them our
sun must seem only a star; consequently we argue that perhaps these
stars themselves are suns with famili