astronomer and mathematician who may have discovered the precession of the equinoxes (equinoxes, precession of the), the slow rotation of the Earth's axis that results in slight variations in the length of the year. Head of the astronomical school at Sippar, Kidinnu was probably responsible for introducing the 19-year cycle into the Babylonian calendar in 383 BC. In this system each year had 12 lunar months, but 7 extra months were inserted at intervals during the 19-year period. The 19-year cycle of intercalations was adopted by the Jews and has remained in use.

Kidinnu also developed the more refined system (called System B) used by the Babylonians to describe more clearly the motions of the Sun and planets. This system utilized steadily increasing and decreasing values for the planetary positions, sometimes called zigzag functions. Kidinnu's calculation of the length of the synodic month (from New Moon to New Moon) yielded a value of 29.530614 days, which differs by less than one second from the true value.