rth on which we live and move extends to
a great distance in every direction, and that the heaven is an immense
dome upon the inner surface of which the stars are fixed. Such must
needs have been the idea of the universe held by men in the earliest
times. In their view the earth was of paramount importance. The sun and
moon were mere lamps for the day and for the night; and these, if not
gods themselves, were at any rate under the charge of special deities,
whose task it was to guide their motions across the vaulted sky.

Little by little, however, this simple estimate of nature began to be
overturned. Difficult problems agitated the human mind. On what, for
instance, did the solid earth rest, and what prevented the vaulted
heaven from falling in upon men and crushing them out of existence?
Fantastic myths sprang from the vain attempts to solve these riddles.
The Hindoos, for example, imagined the earth as supported by four
elephants which stood upon the back of a gigantic tortoise, which, in
its turn, floated on the surface of an elemental ocean. The early
Western civilisations conceived the fable of the Titan Atlas, who, as a
punishment for revolt against the Olympian gods, was condemned to hold
up the expanse of sky for ever and ever.

Later on glimmerings of the true light began to break in upon men. The
Greek philosophers, who busied themselves much with such matters,
gradually became convinced that the earth was spherical in shape, that
is to say, round like a ball. In this opinion we now know that they were
right; but in their other important belief, viz. that the earth was
placed at the centre of all things, they were indeed very far from the
truth.

By the second century of the Christian era, the ideas of the early
philosophers had become hardened into a definite theory, which, though
it appears very incorrect to us to-day, nevertheless demands exceptional
notice from the fact that it was everywhere accepted as the true
explanation until so late as some four centuries ago. This theory of the
universe is known by the name of the Ptolemaic System, because it was
first set forth in definite terms by one of the most famous of the
astronomers of antiquity, Claudius Ptolemaeus Pelusinensis (100-170
A.D.), better known as Ptolemy of Alexandria.

In his system the Earth occupied the centre; while around it circled in
order outwards the Moon, the planets Mercury and Venus, the Sun, and
then the planets Mars, Jupiter