Hipparchus possessed one of the master-minds of all time was the detection
of that remarkable celestial movement known as the precession of the
equinoxes.
most profound investigation, especially when it is remembered that in
the days of Hipparchus the means of observation of the heavenly
bodies were only of the rudest description, and the available
observations of earlier dates were extremely scanty.
We can but look with astonishment on the genius of the man who, in
spite of such difficulties, was able to detect such a phenomenon as
the precession, and to exhibit its actual magnitude. I shall endeavour
to explain the nature of this singular celestial movement, for it may be
said to offer the first instance in the history of science in which we find
that combination of accurate observation with skilful interpretation,
of which, in the subsequent development of astronomy, we have so many
splendid examples.
The word equinox implies the condition that the night is equal to the
day. To a resident on the equator the night is no doubt equal to the
day at all times in the year, but to one who lives on any other part
of the earth, in either hemisphere, the night and the day are not
generally equal. There is, however, one occasion in spring, and
another in autumn, on which the day and the night are each twelve
hours at all places on the earth. When the night and day are equal
in spring, the point which the sun occupies on the heavens is termed
the vernal equinox. There is similarly another point in which the
sun is situated at the time of the autumnal equinox.
In any investigation of the celestial movements the positions of these two
equinoxes on the heavens are of primary importance, and Hipparchus,
with the instinct of genius, perceived their significance, and
commenced to study them. 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.
Tags: Astronomers, Great Astronomers
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