precession of the equinoxes as one of the fundamental facts of
astronomy. Not until nearly two thousand years after Hipparchus had
made this splendid discovery was the explanation of its cause given
by Newton.

From the days of Hipparchus down to the present hour the science of
astronomy has steadily grown. One great observer after another has
appeared from time to time, to reveal some new phenomenon with regard
to the celestial bodies or their movements, while from time to time
one commanding intellect after another has arisen to explain the true
import of the facts of observations. The history of astronomy thus
becomes inseparable from the history of the great men to whose
labours its development is due.

In the ensuing chapters we have endeavoured to sketch the lives and
the work of the great philosophers, by whose labours the science of
astronomy has been created. We shall commence with Ptolemy, who,
after the foundations of the science had been laid by Hipparchus,
gave to astronomy the form in which it was taught throughout the
Middle Ages. We shall next see the mighty revolution in our
conceptions of the universe which are associated with the name of
Copernicus. We then pass to those periods illumined by the genius of
Galileo and Newton, and afterwards we shall trace the careers of
other more recent discoverers, by whose industry and genius the
boundaries of human knowledge have been so greatly extended. Our
history will be brought down late enough to include some of the
illustrious astronomers who laboured in the generation which has just
passed away.