were
formed of crystal spheres in which the lights were placed, much as
lanterns may be hung upon a ceiling. These spheres were conceived to
be one above the other; the planets were on the lower of them, and the
fixed stars on the higher, the several crystal roofs revolving about
the earth. So long as the earth was supposed to be a flat and
limitless expanse, forming the centre of the universe, it was
impossible for the students of the heavens to attain any more rational
view as to their plan.

The fact that the earth was globular in form was understood by the
Greek men of science. They may, indeed, have derived the opinion from
the Egyptian philosophers. The discovery rested upon the readily
observed fact that on a given day the shadow of objects of a certain
height was longer in high latitude than in low. Within the tropics,
when the sun was vertical, there would be no shadow, while as far
north as Athens it would be of considerable length. The conclusion
that the earth was a sphere appears to have been the first large
discovery made by our race. It was, indeed, one of the most important
intellectual acquisitions of man.

Understanding the globular form of the earth, the next and most
natural step was to learn that the earth was not the centre of the
planetary system, much less of the universe, but that that centre was
the sun, around which the earth and the other planets revolved. The
Greeks appear to have had some idea that this was the case, and their
spirit of inquiry would probably have led them to the whole truth but
for the overthrow of their thought by the Roman conquest and the
spread of Christianity. It was therefore not until after the revival
of learning that astronomers won their way to our modern understanding
concerning the relation of the planets to the sun. With Galileo this
opinion was affirmed. Although for a time the Church, resting its
opposition on the interpretation of certain passages of Scripture,
resisted this view, and even punished the men who held it, it
steadfastly made its way, and for more than two centuries has been the
foundation of all the great discoveries in the stellar realm. Yet long
after the fact that the sun was the centre of the solar system was
well established no one understood why the planets should move in
their ceaseless, orderly procession around the central mass. To Newton
we owe the studies on the law of gravitation which brought us to our
present large conception