The Outline of Science, Vol. 1 | Page 9

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.
§ 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 Milky Way, which spans the sky at night. It is true
that this is a particularly rich area of the Milky Way, but the entire belt
of light has been resolved in this way into masses or clouds of stars.
Astronomers have counted the stars in typical districts here and there,
and from these partial counts we get some idea of the total number of
stars. There are estimated to be between two and three thousand million
stars.
Yet these stars are separated by inconceivable distances from each
other, and it is one of the greatest triumphs of modern astronomy to
have mastered, so far, the scale of the universe. For several centuries
astronomers have known the relative distances from each other of the
sun and the planets. If they could discover the actual distance of any
one planet from any other, they could at once tell all the distances
within the Solar System.
The sun is, on the latest measurements, at an average distance of
92,830,000 miles from the earth, for as the orbit of the earth is not a

true circle, this distance varies. This means that in six months from now
the earth will be right at the opposite side of its path round the sun, or
185,000,000 miles away from where it is now. Viewed or
photographed from two positions so wide apart, the nearest stars show
a tiny "shift" against the background of the most distant stars, and that
is enough for the mathematician. He can calculate the distance of any
star near enough to show this "shift." We have found that the nearest
star to the earth, a recently discovered star, is twenty-five trillion miles
away. Only thirty stars are known to be within a hundred trillion miles
of us.
This way of measuring does not, however, take us very far away in the
heavens. There are only a few hundred stars within five hundred trillion
miles of the earth, and at that distance the "shift" of a star against the
background (parallax, the astronomer calls it) is so minute that figures
are very uncertain. At this point the astronomer takes up a new method.
He learns the different types of stars, and then he is able to deduce
more or less accurately the distance of a star of a known type from its
faintness. He, of course, has instruments for gauging their light. As a
result of twenty years work in this field, it is now known that the more
distant stars of the Milky Way are at least a hundred thousand trillion
(100,000,000,000,000,000) miles away from the sun.
Our sun is in a more or less central region of the universe, or a few
hundred trillion miles from the actual centre. The remainder of the stars,
which are all outside our Solar System, are spread out, apparently, in an
enormous disc-like collection, so vast that even a ray of light, which
travels at the rate of 186,000 miles a second, would take 50,000 years
to travel from one end of it to the