He also studied the movements of the sun
and the moon, and framed theories to account for the incessant
changes which he saw in progress. He found a much more difficult
problem in his attempt to interpret satisfactorily the complicated
movements of the planets. With the view of constructing a theory
which should give some coherent account of the subject, he made many
observations of the places of these wandering stars. How great were
the advances which Hipparchus accomplished may be appreciated if we
reflect that, as a preliminary task to his more purely astronomical
labours, he had to invent that branch of mathematical science by
which alone the problems he proposed could be solved. It was for
this purpose that he devised the indispensable method of calculation
which we now know so well as trigonometry. Without the aid rendered
by this beautiful art it would have been impossible for any really
important advance in astronomical calculation to have been effected.
But the discovery which shows, beyond all others, that 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. The inquiry which conducted to this discovery involved a
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.
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