History of Astronomy | Page 9

90° 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 direction to the planetary
revolutions.
Hipparchus was also the inventor of trigonometry, both plane and
spherical. He explained the method of using eclipses for determining
the longitude.

In connection with Hipparchus' great discovery it may be mentioned
that modern astronomers have often attempted to fix dates in history by
the effects of precession of the equinoxes. (1) At about the date when
the Great Pyramid may have been built gamma Draconis was near to
the pole, and must have been used as the pole-star. In the north face of
the Great Pyramid is the entrance to an inclined passage, and six of the
nine pyramids at Gizeh possess the same feature; all the passages being
inclined at an angle between 26° and 27° to the horizon and in the plane
of the meridian. It also appears that 4,000 years ago--i.e., about 2100
B.C.--an observer at the lower end of the passage would be able to see
gamma Draconis, the then pole-star, at its lower culmination.[1] It has
been suggested that the passage was made for this purpose. On other
grounds the date assigned to the Great Pyramid is 2123 B.C.
(2) The Chaldæans gave names to constellations now invisible from
Babylon which would have been visible in 2000 B.C., at which date it
is claimed that these people were studying astronomy.
(3) In the Odyssey, Calypso directs Odysseus, in accordance with
Phoenician rules for navigating the Mediterranean, to keep the Great
Bear "ever on the left as he traversed the deep" when sailing from the
pillars of Hercules (Gibraltar) to Corfu. Yet such a course taken now
would land the traveller in Africa. Odysseus is said in his voyage in
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 these an additional annual
motion to enable us to represent truly the apparent motions. This way
of looking at the apparent motions shows why each planet, when
nearest to the earth, seems to move for a time in a retrograde direction.
The attempts of Ptolemy and others of his time to explain the
retrograde motion in this way were only approximate. Let us suppose
each planet to have a bar with one end centred at the earth. If at the
other end of the bar one end of a shorter bar is pivotted, having the
planet at its other end, then the planet is given an annual motion in the
secondary circle (the epicycle), whose centre revolves round