Credited to Greece. The cycle carries the name of the Athenian astronomer Meton, but Britannica notes that Babylonian calendar makers used the same 19-year pattern of seven extra months and that Meton followed Babylonian precedent.
How it began
The cycle is named for Meton, an Athenian astronomer who was active in 432 BCE. Encyclopaedia Britannica credits him with discovering it. He is dated by his observation of the summer solstice on 27 June 432 BCE, and he began his 19-year cycle from that point. The period he set out is one after which the phases of the Moon fall again on the same days of the solar year.
Before it existed
A month measured by the Moon and a year measured by the seasons do not divide evenly into each other. Calendars that kept months in step with the Moon had to add an extra month every so often to stay in step with the Sun. According to Britannica, the Sumerians and Babylonians at first added these months haphazardly, with no fixed rule, so nobody could say far in advance which years would be long ones.
How it works
Nineteen years of the seasons contain almost exactly 235 lunar months. Computed from modern data, 235 lunar months come to 6,939 days and 16.5 hours, and 19 solar years to 6,939 days and 14.5 hours, a difference of about two hours. A calendar that gives 12 of the 19 years twelve months and the other 7 years thirteen months therefore ends each cycle with the Moon and the seasons back where they began.
Who pushed back
How much of the idea was Meton's own is open to question. Britannica states that from about 380 BCE the Babylonians had fixed rules that spread seven extra months at set points across 19 years, and it says that Meton followed Babylonian precedent. The record here keeps the Athenian attribution because the cycle bears his name and his dated observation, and marks it as disputed.
What lasted
The cycle outlived the Athenian calendar. The Jewish calendar follows it, with an added month in 7 years out of every 19. In the Christian reckoning of dates, the place of a year within the 19 is called its golden number. Britannica treats the cycle under chronology for this reason: it is the rule by which calendars that count months by the Moon are kept in step with the year of the seasons.
