Metonic cycle
19-year cycle aligning lunar phases with the solar year.
The Metonic cycle, also known as the enneadecaeteris, is a period of almost exactly 19 years after which the lunar phases recur at the same time of the year. It was judged by Meton of Athens in the 5th century BC to be a whole number of days, 6,940, facilitating the construction of a lunisolar calendar. The cycle is the most accurate cycle of time under 100 years for synchronizing the tropical year and the lunar month through intercalation of a thirteenth lunar month.
- field
- Astronomy, Calendar Systems
- known_for
- 19-year lunisolar cycle synchronizing lunar phases and tropical year
- cycle_length
- 19 tropical years (6,939.602 days) vs 235 synodic months (6,939.689 days)
- intercalation
- 7 leap years (13 months) in 19-year cycle
- total_days
- 6,940 days (125 full months, 110 deficient months)
Lore & Background
Meton of Athens introduced the 19-year cycle to the Attic calendar in 432 BC. The earliest evidence for the Babylonian 19-year cycle dates to the 5th century BC, not the late 6th as sometimes claimed. According to Livy, the second king of Rome, Numa Pompilius (reigned 715–673 BC), inserted intercalary months so that 'in the twentieth year the days should fall in with the same position of the sun from which they had started'—this describes a 20-year cycle, not a 19-year one, and the attribution to Numa is anachronistic. Diodorus Siculus reports that Apollo is said to have visited the Hyperboreans once every 19 years.
Reader's Guide
The Metonic cycle forms the basis of the Greek and Hebrew calendars and is used for the computation of the date of Easter each year. In the Babylonian and Hebrew lunisolar calendars, the years 3, 6, 8, 11, 14, 17, and 19 are the long (13-month) years. The cycle was implemented in the 2nd century BC Antikythera mechanism, offering evidence for its popularity. Around AD 260, Anatolius devised a method for determining Easter Sunday, but a later version of the Metonic 19-year lunar cycle became the basic structure of Dionysius Exiguus' and Bede's Easter tables, prevailing until the Gregorian calendar was introduced in 1582. The Coligny calendar, a Celtic lunisolar calendar using the Metonic cycle, dates from c. AD 200 but points to older origins. The Runic calendar is a perpetual calendar based on the 19-year Metonic cycle, with the oldest known example from the 13th century. The Bahá'í calendar is also based on cycles of 19 solar years. The Hebrew calendar uses a Small Maḥzor, a 19-year cycle similar to but slightly different from the Greek Metonic cycle, likely derived from the Babylonian calendar.
Did You Know?
- The Metonic cycle is 2 hours, 4 minutes and 58 seconds longer than 19 tropical years.
- The cycle was implemented in the 2nd century BC Antikythera mechanism.
- The Coligny calendar, a Celtic lunisolar calendar, uses the Metonic cycle and dates from c. AD 200.
- The Runic calendar, also known as a Rune staff, is a perpetual calendar based on the 19-year Metonic cycle.
More in Ancient Greek astronomy 1-24
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record