springtime to have seen the Pleiades and Arcturus setting late, which
seemed to early commentators a proof of Homer's inaccuracy. Likewise
Homer, both in the _Odyssey_ [2] (v. 272-5) and in the _Iliad_
(xviii. 489), asserts that the Great Bear never set in those
latitudes. Now it has been found that the precession of the equinoxes
explains all these puzzles; shows that in springtime on the
Mediterranean the Bear was just above the horizon, near the sea but
not touching it, between 750 B.C. and 1000 B.C.; and fixes the date of
the poems, thus confirming other evidence, and establishing Homer's
character for accuracy. [3]

(4) The orientation of Egyptian temples and Druidical stones is such
that possibly they were so placed as to assist in the observation of
the heliacal risings [4] of certain stars. If the star were known,
this would give an approximate date. Up to the present the results of
these investigations are far from being conclusive.

Ptolemy (130 A.D.) wrote the Suntaxis, or Almagest, which includes a
cyclopedia of astronomy, containing a summary of knowledge at that
date. We have no evidence beyond his own statement that he was a
practical observer. He theorised on the planetary motions, and held
that the earth is fixed in the centre of the universe. He adopted the
excentric and equant of Hipparchus to explain the unequal motions of
the sun and moon. He adopted the epicycles and deferents which had
been used by Apollonius and others to explain the retrograde motions
of the planets. We, who know that the earth revolves round the sun
once in a year, can understand that the apparent motion of a planet is
only its motion relative to the earth. If, then, we suppose the earth
fixed and the sun to revolve round it once a year, and the planets
each in its own period, it is only necessary to impose upon each of