The discovery that shows, beyond all others, that Hipparchus possessed one of the masterminds of all time was the detection of that remarkable celestial movement known as the precession of the equinoxes. The inquiry that led 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. 才差運動 地球が太陽の周りを公転する際、他の惑星の重力の影響を受け、地球の回転軸がほんのわずかながらずれて、長い期間でみた場合、コマの首ふり運動のような動きをします。その周期はなんと、25800年！で、地球の回転軸が１回転します。プラネタリウムは地球からみた宇宙図ですが、この宇宙図において、春分点、秋分点は重要であり、その位置が一周することをギリシャのヒッパカスは発見したのです。いやー、昔のギリシャ人はすごいもんですな。
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 endeavor 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 skillful 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 25,000 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 recognized the precession of the equinoxes as one of the fundamental facts of astronomy. Not until nearly 2,000 years after Hipparchus had made this splendid discovery was the explanation of its cause given by Newton.