Metonic cycle named after fifth-century B.C. astronomer Meton of Athens

The Greek astronomer Meton of Athens developed an effective system to adjust lunar calendars to the solar year [1]:

Meton supposedly observed that 19 solar years corresponded almost exactly to 235 lunar months. Since 19 lunar years consist of 228 months, it was thus necessary to add 7 intermediate months into 19 lunar years to put the lunar and solar calendars into phase.

Before Meton, the Babylonians made similar (or the same) observations. Nevertheless, the 19-year, 235-lunar-month cycle is named Metonic cycle or Meton's cycle. It became the basis for the Greek calendar until the Julian calendar was introduced in 46 B.C. [2]. The Metonic cycle was adapted by the Jews in the 4th century A.D., who established a sophisticated lunisolar calendar. The Metonic cycle is still used today to determine the dates of the Jewish festival of Rosh Hashanah and the Christian festival of Easter [3].

Keywords: astronomy, history, calendars, dating

References
[1] Jacqueline de Bourgoing: The Calendar • History, Lore, and Legend. Harry N. Abrams, Inc., Publishers, 2001; page 22.
[2] Eric W. Weisstein: Metonic Cycle.
[3] Tony Freeth: Decoding an Ancient Computer. Sci. Am. December 2009, 301 (6), pp. 76-83.