one is Aries the Ram, and the sun is seen projected
or thrown against that part of the sky where Aries is, in April, when we
begin spring; this is the first month to astronomers, and not January,
as you might suppose. Perhaps you will learn to recognize all the
constellations in the Zodiac one day; a few of them, such as the Bull
and the Heavenly Twins, you know already if you have followed this
chapter.

CHAPTER XII

WHAT THE STARS ARE MADE OF

How can we possibly tell what the stars are made of? If we think of the
vast oceans of space lying between them and us, and realize that we can
never cross those oceans, for in them there is no air, it would seem to
be a hopeless task to find out anything about the stars at all. But even
though we cannot traverse space ourselves, there is a messenger that
can, a messenger that needs no air to sustain him, that moves more
swiftly than our feeble minds can comprehend, and this messenger brings
us tidings of the stars--his name is Light. Light tells us many
marvellous things, and not the least marvellous is the news he gives us
of the workings of another force, the force of gravitation. In some ways
gravitation is perhaps more wonderful than light, for though light
speeds across airless space, it is stopped at once by any opaque
substance--that is to say, any substance not transparent, as you know
very well by your own shadows, which are caused by your bodies stopping
the light of the sun. Light striking on one side of the earth does not
penetrate through to the other, whereas gravitation does. You remember,
of course, what the force of gravitation is, for we read about that very
early in this book. It is a mysterious attraction existing between all
matter. Every atom pulls every other atom towards itself, more or less
strongly according to distance. Now, solid matter itself makes no
difference to the force of gravitation, which acts through it as though
it were not there. The sun is pulling the earth toward itself, and it
pulls the atoms on the far side of the earth just as strongly as it
would if there were nothing lying between it and them. Therefore, unlike
light, gravitation takes no heed of obstacles in the way, but acts in
spite of them. The gravitation of the earth holds you down just the
same, though you are on the upper floor of a house, with many layers of
wood and plaster between you and it. It cannot pull you down, for the
floor holds you up, but it is grav