Meaning of Metonic cycle

Me•ton'ic cy'cle

Pronunciation: (mi-ton'ik), [key]
— Astron. Astron.
  1. a cycle of 235 synodic months, very nearly equal to 19 years, after which the new moon occurs on the same day of the year as at the beginning of the cycle with perhaps a shift of one day, depending on the number of leap years in the cycle.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.