n 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 other. This, then
is what we call our universe.

Are there other Universes?

Why do we say "our universe"? Why not _the_ universe? It is now believed
by many of our most distinguished astronomers that our colossal family
of stars is only one of many universes. By a universe an astronomer
means any collection of stars which are close enough to control each
other's movements by gravitation; and it is clear that there might be
many universes, in this sense, separated from each other by profound
abysses of space. Probably there are.

For a long time we have been familiar with certain strange objects in
the heavens which are called "spiral nebulae" (Fig 4). We shall see at a
later stage what a nebula is, and we shall see that some astronomers
regard these spiral nebulae as worlds "in the making." But some of the
most eminent astronomers believe that they are separate
universes--"island-universes" they call them--or great collections of
millions of stars like our universe. There are certain peculiarities in
the structure of the Milky Way which lead these astronomers to think
that our universe may be a spiral nebula, and that the other spiral
nebulae are "other universes."

[Illustration: _Photo: Harvard College Observatory._

FIG. 2.--THE MILKY WAY

Note the cloud-like effect.]

[Illustration: FIG. 3--THE MOON ENTERING THE SHADOW CAST BY THE EARTH

The diagram shows the Moon partially eclipsed.]

[Illustration: _From a photograph taken at the Yerkes Observatory_

FIG. 4.--THE GREAT NEBULA IN ANDROMEDA, MESSIER 31]

Vast as is the Solar System, then, it is excessively minute in
comparison with the Stellar System, the universe of the Stars, which is
on a scale far transcending anything the human mind can apprehend.

THE SOLAR SYSTEM

THE SUN

Sec. 1

But now let us turn to the Solar System, and consider the members of our
own little colony.

Within the Solar System there are a large number of problems that
interest us. What is the size, mass, and distance of each of the
planets? What satellites, like our Moon, do they possess? What are their
temperatur