y 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 regio