The heavenly bodies fall into two very distinct classes so far as their
relation to our Earth is concerned; the one class, a very small one,
comprises a sort of colony of which the Earth is a member. These bodies
are called _planets_, or wanderers. There are eight of them, including
the Earth, and they all circle round the sun. Their names, in the order
of their distance from the sun, are Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, Neptune, and of these Mercury, the nearest to
the sun, is rarely seen by the naked eye. Uranus is practically
invisible, and Neptune quite so. These eight planets, together with the
sun, constitute, as we have said, a sort of little colony; this colony
is called the Solar System.
The second class of heavenly bodies are those which lie _outside_ the
solar system. Every one of those glittering points we see on a starlit
night is at an immensely greater distance from us than is any member of
the Solar System. Yet the members of this little colony of ours, judged
by terrestrial standards, are at enormous distances from one another. If
a shell were shot in a straight line from one side of Neptune's orbit to
the other it would take five hundred years to complete its journey. Yet
this distance, the greatest in the Solar System as now known (excepting
the far swing of some of the comets), is insignificant compared to the
distances of the stars. One of the nearest stars to the earth that we
know of is Alpha Centauri, estimated to be some twenty-five million
millions of miles away. Sirius, the brightest star in the firmament, is
double this distance from the earth.
We must imagine the colony of planets to which we belong as a compact
little family swimming in an immense void. At distances which would take
our shell, not hundreds, but millions of years to traverse, we reach
the stars--or rather, a star, for the distances between stars are as
great as the distance between the nearest of them and our Sun. The
Earth, the planet on which we live, is a mighty globe bounded by a crust
of rock many miles in thickness; the great volumes of water which we
call our oceans lie in the deeper hollows of the crust. Above the
surface an ocean of invisible gas, the atmosphere, rises to a height of
about three hundred miles, getting thinner and thinner as it ascends.
[Illustration: LAPLACE
One of the greatest mathematical astronomers of all time and the
originator of the nebular theory.]
[Illustration: _Photo: Royal Astronomical Society._