It will be understood that we can always
define the position of a point on the sky with reference to the
surrounding stars. No doubt we do not see the stars near the sun
when the sun is shining, but they are there nevertheless. The
ingenuity of Hipparchus enabled him to determine the positions of
each of the two equinoxes relatively to the stars which lie in its
immediate vicinity. After examination of the celestial places of
these points at different periods, he was led to the conclusion that
each equinox was moving relatively to the stars, though that movement
was so slow that twenty five thousand years would necessarily elapse
before a complete circuit of the heavens was accomplished. Hipparchus
traced out this phenomenon, and established it on an impregnable
basis, so that all astronomers have ever since recognised the
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.
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