HIPPARCHUS
255
that the lengths of the mean synodic, the sidereal, the
anomalistic and the draconitic month obtained by Hipparchus
agree exactly with Babylonian cuneiform tables of date not
later than Hipparchus, and it is clear that Hipparchus was
in full possession of all the results established by Babylonian
astronomy.
Improved estimates of sizes and distances of sun
and moon.
4. Hipparchus improved on Aristarchus’s calculations of the
sizes and distances of the sun and moon, determining the
apparent diameters more exactly and noting the changes in
them ; he made the mean distance of the sun 1,2451), the mean
distance of the moon 33| D, the diameters of the sun and
moon 12-| D and ^ D respectively, where D is the mean
diameter of the earth.
Epicycles and eccentrics.
5. Hipparchus, in investigating the motions of the sun, moon
and planets, proceeded on the alternative hypotheses of epi
cycles and eccentrics; he did not invent these hypotheses,
which were already fully understood and discussed by
Apollonius, While the motions of the sun and moon could
with difficulty be accounted for by the simple epicycle and
eccentric hypotheses, Hipparchus found that for the planets it
was necessary to combine the two, i.e. to superadd epicycles to
motion in eccentric circles.
Catalogue of stars.
6. He compiled a catalogue of fixed stars including 850 or
more such stars; apparently he was the first to state their
positions in terms of coordinates in relation to the ecliptic
(latitude and longitude), and his table distinguished the
apparent sizes of the stars. His work was continued by
Ptolemy, who produced a catalogue of 1,022 stars which,
owing to an error in his solar tables affecting all his longi
tudes, has by many erroneously been supposed to be a mere
reproduction of Hipparchus’s catalogue. That Ptolemy took
many observations himself seems certain. 1
1 See two papers by Dr. J. L. E. Dreyer in the Monthly Notices of the
Boyal Astronomical Society, 1917, pp. 528-39, and 1918, pp. 343-9.
