bout the same distance
from the earth, he readily perceives that it circles about our sphere.
This the people knew of old, but they made of it an evidence that the
sun also went around our sphere. Here, then, is the critical point.
Why does the sun not behave in the same manner as the moon? At this
stage of his inquiry the student best notes what takes place in the
motions of the planets between the earth and the sun. He observes that
those so-called inferior planets Mercury and Venus are never very far
away from the central body; that they appear to rise up from it, and
then to go back to it, and that they have phases like the moon. Now
and then Venus may be observed as a black spot crossing the disk of
the sun. A little consideration will show that on the theory that
bodies revolve round each other in the solar system these movements of
the inner planets can only be explained on the supposition that they
at least travel around the great central fire. Now, taking up the
outer planets, we observe that they occasionally appear very bright,
and that they are then at a place in the heavens where we see that
they are far from the solar centre. Gradually they move down toward
the sunset and disappear from view. Here, too, the movement, though
less clearly so, is best reconcilable with the idea that these bodies
travel in orbits, such as those which are traversed by the inner
planets. The wonder is that with these simple facts before them, and
with ample time to think the matter over, the early astronomers did
not learn the great truth about the solar system--namely, that the sun
is the centre about which the planets circled. Their difficulty lay
mainly in the fact that they did not conceive the earth as a sphere,
and even after they attained that conception they believed that our
globe was vastly larger than the planets, or even than the sun. This
misconception kept even the thoughtful Greeks, who knew that the earth
was spherical in form, from a clear notion as to the structure of our
system. It was not, indeed, until mathematical astronomy attained a
considerable advance, and men began to measure the distances in the
solar system, and until the Newtonian theory of gravitation was
developed, that the planetary orbits and the relation of the various
bodies in the solar system to each other could be perfectly discerned.

Care has been taken in the above statements to give the student
indices which may assist him in working ou