If at any moment the sun ceased to exert this
pull the earth would instantly fly off into space straight in the
direction in which it was moving at the time, that is to say, at a
tangent. This tendency to fly off at a tangent is continuous. It is the
balance between it and the sun's pull which keeps the earth to her
almost circular orbit. In the same way the seven other planets are held
to their orbits.

Circling round the earth, in the same way as the earth circles round the
sun, is our moon. Sometimes the moon passes directly between us and the
sun, and cuts off the light from us. We then have a total or partial
eclipse of the sun. At other times the earth passes directly between the
sun and the moon, and causes an eclipse of the moon. The great ball of
the earth naturally trails a mighty shadow across space, and the moon is
"eclipsed" when it passes into this.

The other seven planets, five of which have moons of their own, circle
round the sun as the earth does. The sun's mass is immensely larger than
that of all the planets put together, and all of them would be drawn
into it and perish if they did not travel rapidly round it in gigantic
orbits. So the eight planets, spinning round on their axes, follow their
fixed paths round the sun. The planets are secondary bodies, but they
are most important, because they are the only globes in which there can
be life, as we know life.

If we could be transported in some magical way to an immense distance in
space above the sun, we should see our Solar System as it is drawn in
the accompanying diagram (Fig. 1), except that the planets would be mere
specks, faintly visible in the light which they receive from the sun.
(This diagram is drawn approximately to scale.) If we moved still
farther away, trillions of miles away, the planets would fade entirely
out of view, and the sun would shrink into a point of fire, a star. And
here you begin to realize the nature of the universe. _The sun is a
star. The stars are suns._ Our sun looks big simply because of its
comparative nearness to us. The universe is a stupendous collection of
millions of stars or suns, many of which may have planetary families
like ours.

Sec. 2

The Scale of the Universe

How many stars are there? A glance at a photograph of star-clouds will
tell at once that it is quite impossible to count them. The fine
photograph reproduced in Figure 2 represents a very small patch of that
pale-white belt, the Milk