years had been discovered by Babylonian astronomers prior to 502 BC. This
led Hipparchus to determine precession circa 150 BC.
The length of the tropical year was determined with a gnomen between
successive solar solstices. The length of the sidereal year was determined
from successive heliacal risings.
From Time in History by G. J. Whitrow.
Hipparchus found the interval between successive equinoxes was 365.25 days
- 1/300 = 365.2497, in excess by 0.0044 days or about 6 minutes. Hipparchus
had found values for the tropical year and for the sidereal year by
comparing his observations of the angular separations of stars (Spica and
Regulus) from equinoctial or solstitial points with those of Timocharis and
Aristyllos some 160 years before. The stars had shifted 2 degrees eastward.
Precession was between 1 degree in 100 years and 1 degree in 78 years. The
sidereal year was 365.2569 days, about 0.78 minutes too long.
He may have had sextants, quadrants, equatorial armillae, armillary spheres
and/or planispheric astrolabes.
(Further details of interest but omitted.)
From Early Astronomy from Babylonia to Copernicus by W. M. O'Neil.
Gordon
Gordon Uber [EMAIL PROTECTED] San Diego, California USA
Webmaster: Clocks and Time: http://www.ubr.com/clocks
