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.
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