He proceeded in the same way to compute Lunar tables. Making use of
Chaldaean eclipses, he was able to get an accurate value of the moon's
mean motion. [Halley, in 1693, compared this value with his own
measurements, and so discovered the acceleration of the moon's mean
motion. This was conclusively established, but could not be explained
by the Newtonian theory for quite a long time.] He determined the
plane of the moon's orbit and its inclination to the ecliptic. The
motion of this plane round the pole of the ecliptic once in eighteen
years complicated the problem. He located the moon's excentric as he
had done the sun's. He also discovered some of the minor
irregularities of the moon's motion, due, as Newton's theory proves,
to the disturbing action of the sun's attraction.

In the year 134 B.C. Hipparchus observed a new star. This upset every
notion about the permanence of the fixed stars. He then set to work to
catalogue all the principal stars so as to know if any others appeared
or disappeared. Here his experiences resembled those of several later
astronomers, who, when in search of some special object, have been
rewarded by a discovery in a totally different direction. On comparing
his star positions with those of Timocharis and Aristillus he found no
stars that had appeared or disappeared in the interval of 150 years;
but he found that all the stars seemed to have changed their places
with reference to that point in the heavens where the ecliptic is 90
degrees from the poles of the earth--i.e., the equinox. He found that
this could be explained by a motion of the equinox in the direction of
the apparent diurnal motion of the stars. This discovery of
_precession of the equinoxes_, which takes place at the rate of 52".1
every year, was necessary for the progress of accurate astronomical
observations. It is due to a steady revolution of the earth's pole
round the pole of the ecliptic once in 26,000 years in the opposite