Great Astronomers | Page 5

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. 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.
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