Callisthenes obtained in Babylon a series of Chaldean astronomical observations ranging back through 1,903 years; these he sent to Aristotle.

Perhaps, since they were on burnt bricks, duplicates of them may be recovered by modern research in the clay libraries of the Assyrian kings.

Ptolemy, the Egyptian astronomer, possessed a Babylonian record of eclipses, going back 747 years before our era.

Long-continued and close observations were necessary, before some of these astronomical results that have reached our times could have been ascertained.

Thus the Babylonians had fixed the length of a tropical year within twenty-five seconds of the truth; their estimate of the sidereal year was barely two minutes in excess.
They had detected the precession of the equinoxes.