A 5th Century B.C. Athenian by the name of Meton noticed that the Moon’s phases returned to the same dates of the year after 19 years ( i.e. 228 calendar months = 235 lunar months ). The Metonic period is important for anyone who would devise a calendar which respected the 29.53 day lunar month or sunth, and yet reconcile it with Earth’s 365.25 day year at least periodically.