For
astronomy and
calendar studies, the
Metonic cycle or
Enneadecaeteris (from , "nineteen years") is a period of very close to 19 years that is remarkable for being nearly a
common multiple of the solar
year and the synodic (lunar) month. The
Greek astronomer Meton of Athens (fifth century BC) observed that a period of 19 years is almost exactly equal to 235
synodic months and, rounded to full days, counts 6,940 days. The difference between the two periods (of 19 years and 235 synodic months) is only a few hours, depending on the definition of the year.