Metonic Cycle

meh-TON-ik SY-kuhl

Definition

The Metonic cycle is a near-match between Sun and Moon: 19 tropical years come out almost exactly equal to 235 synodic months — that is, 235 cycles of New Moon to New Moon. After 19 years the phases of the Moon fall on roughly the same calendar dates again. This near-match is the basis on which several ancient calendars decided when to add a leap month, and it is named after Meton of Athens, who brought it into Greek calendar practice in 432 BCE.

In Tradition

In the history of Babylonian and Greek astronomy, Francesca Rochberg argues that the 19-year Sun-Moon period "begins to be employed for the Babylonian calendar around 500 b.c., somewhat earlier than the date for Meton of Athens" — making the cycle a Babylonian calendar relation that was later passed to Greece. Hunger and Pingree treat the same relation as the standard leap-month cycle of the Neo-Babylonian and Achaemenid calendars.

In Practice

For calendar and ephemeris work, the cycle predicts that a given lunar phase falling near a given solar date will return, after 19 years, to nearly the same date. In astrological timing, it is the period over which the Moon's phase relative to the Sun — and so the New or Full Moon falling near a person's birthday or near a fixed star — comes back to a similar arrangement; ages 19, 38, and 57 are often picked out as the years when the birth lunar phase is roughly repeated. Families of eclipses that recur at intervals tied to the Metonic and Saros cycles are tracked for forecasting. The 19-year figure is only approximate — the small leftover difference from 19 exact years causes a slow drift in phase.

Historical Origin

The 19-year leap-month relation is attested in cuneiform Babylonian calendar practice from about the 5th century BCE, predating its introduction to the Greek calendar by Meton of Athens in 432 BCE, per Rochberg. It is treated in Hunger and Pingree's Astral Sciences in Mesopotamia (1999) and in Rochberg's Heavenly Writing (2004). The cycle passed into the medieval Christian computus for fixing the date of Easter and remains in use in the Hebrew calendar.

Further Reading

  • Francesca Rochberg, The Heavenly Writing: Divination, Horoscopy, and Astronomy in Mesopotamian Culture
  • Hermann Hunger, Astral Sciences in Mesopotamia