In Europe the frequency near the equinoxes rises above that at midwinter, and we have a distinct double period, with a principal minimum at midsummer and a secondary minimum at midwinter.
To the Greek astronomer Hipparchus belongs the credit of the discovery (c. 130 B.C.) of the theory of the precession of the equinoxes, for a knowledge of which among the Babylonians we find no definite proof; but such a signal advance in pure science did not prevent the Greeks from developing in a most elaborate manner the theory of the influence of the planets upon the fate of the individual.
The ecliptic intersects the celestial equator at two opposite points, the equinoxes, at an angle of 23° 27'.
Already, in the third millennium B.C., equinoxes and solstices were determined in China by means of culminating stars.
Professor Newcomb, who has compiled an instructive table of the equinoxes severally observed by Hipparchus and Ptolemy, with their errors deduced from Leverrier's solar tables, finds palpable evidence that the discrepancies between the two series were artificially reconciled on the basis of a year 6 m too long, adopted by Ptolemy on trust from his predecessor.