planets, and by this means practised their art with an
apparent degree of truthfulness.

Although over one hundred years had elapsed since the death of
Copernicus, his theory of the solar system did not find many supporters,
and the old forms of astronomical belief still retained their hold on
the minds of the majority of philosophic thinkers. This can be partly
accounted for, as many of the Ptolemaic doctrines were at first
associated with the Copernican theory, nor was it until a later period
that they were eliminated from the system.

Though Copernicus deserved the credit of having transferred the centre
of our system from the Earth to the Sun, yet his theory was imperfect in
its details, and contained many inaccuracies. He believed that the
planets could only move round the Sun in circular paths, nor was he
capable of conceiving of any other form of orbit in which they could
perform their revolutions. He was therefore compelled to retain the use
of cycles and epicycles, in order to account for irregularities in the
uniformly circular motions of those bodies.

We are indebted to the genius of Kepler for having placed the Copernican
system upon a sure and irremovable basis, and for having raised
astronomy to the position of a true physical science. By his discovery
that the planets travel round the Sun in elliptical orbits, he was
enabled to abolish cycles and epicycles, which created such confusion
and entanglement in the system, and to explain many apparent
irregularities of motion by ascribing to the Sun his true position with
regard to the motions of the planets.

After the death of Kepler, which occurred in 1630, the most eminent
supporter of the Copernican theory was the illustrious Galileo, whose
belief in its accuracy and truthfulness was confirmed by his own
discoveries.

Five of the planets were known at this time--viz. Mercury, Venus, Mars,
Jupiter, and Saturn; the latter, which revolves in its orbit at a
profound distance from the Sun, formed what at that time was believed to
be the boundary of the planetary system. The distance of the Earth from
the Sun was approximately known, and the orb was observed to rotate on
his axis.

It was also ascertained that the Moon shone by reflected light, and that
her surface was varied by inequalities resembling those of our Earth.
The elliptical form of her orbit had been discovered by Horrox, and her
elements were computed with a certain degree of accuracy.

T