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 degrees and 27 degrees
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 Chaldaeans 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