ssity for more accurate places of the sun and
moon, and the liberality of the Ptolemys who ruled Egypt, combined to
provide regular observations at Alexandria, so that, when Hipparchus
came upon the scene, there was a considerable amount of material for him
to use. His discoveries marked a great advance in the science of
astronomy. He noted the irregular motion of the sun, and, to explain it,
assumed that it revolved uniformly not exactly about the earth but
about a point some distance away, called the "excentric".[1] The line
joining the centre of the earth to the excentric passes through the
apses of the sun's orbit, where its distance from the earth is greatest
and least. The same result he could obtain by assuming that the sun
moved round a small circle, whose centre described a larger circle about
the earth; this larger circle carrying the other was called the
"deferent": so that the actual motion of the sun was in an epicycle. Of
the two methods of expression Hipparchus ultimately preferred the
second. He applied the same process to the moon but found that he could
depend upon its being right only at new and full moon. The irregularity
at first and third quarters he left to be investigated by his
successors. He also considered the planetary observations at his
disposal insufficient and so gave up the attempt at a complete planetary
theory. He made improved determinations of some of the elements of the
motions of the sun and moon, and discovered the Precession of the
Equinoxes, from the Alexandrian observations which showed that each year
as the sun came to cross the equator at the vernal equinox it did so at
a point about fifty seconds of arc earlier on the ecliptic, thus
producing in 150 years an unmistakable change of a couple of degrees, or
four times the sun's diameter. He also invented trigonometry. His star
catalogue was due to the appearance of a new star which caused him to
search for possible previous similar phenomena, and also to prepare for
checking future ones. No advance was made in theoretical astronomy for
260 years, the interval between Hipparchus and Ptolemy of Alexandria.
Ptolemy accepted the spherical form of the earth but denied its rotation
or any other movement. He made no advance on Hipparchus in regard to the
sun, though the lapse of time had largely increased the errors of the
elements adopted by the latter. In the case of the moon, however,
Ptolemy traced the variable inequality noticed so