Metonic cycle

Depiction of the 19 years of the Metonic cycle as a wheel, with the Julian date of the Easter New Moon, from a 9th-century computistic manuscript made in St. Emmeram's Abbey (Clm 14456, fol. 71r)
For example, by the 19-year Metonic cycle, the full moon repeats on or near Christmas between 1711 and 2300.[1][2] A small horizontal libration is visible comparing their appearances. A red color shows full moons that are also lunar eclipses.

The Metonic cycle or enneadecaeteris (from Ancient Greek: ἐννεακαιδεκαετηρίς, from ἐννεακαίδεκα, "nineteen") is a period of almost exactly 19 years after which the lunar phases recur at the same time of the year. The recurrence is not perfect, and by precise observation the Metonic cycle defined as 235 synodic months is just 2 hours, 4 minutes and 58 seconds longer than 19 tropical years. Meton of Athens, in the 5th century BC, judged the cycle to be a whole number of days, 6,940.[3] Using these whole numbers facilitates the construction of a lunisolar calendar.

A tropical year (about 365.24 days) is longer than 12 lunar months (about 354.36 days) and shorter than 13 of them (about 383.90 days). In a Metonic calendar (a type of lunisolar calendar), there are twelve years of 12 lunar months and seven years of 13 lunar months.

  1. ^ "Rare Full Moon on Christmas Day". NASA. 17 December 2015. Archived from the original on 8 November 2023.
  2. ^ Skilling, Tom (20 December 2015). "Ask Tom: How unusual is a full moon on Christmas Day?". Chicago Tribune. Archived from the original on 22 December 2015.
  3. ^ needs citation