Metonic cycle
A 19-year period equalling 235 lunar months and nearly 6940 days, after which lunar phases recur on the same solar calendar dates.
Synodic month: The 29.53-day cycle of lunar phases, from new moon to new moon, the basic unit of the cycle. Multiplying this period by 235 gives the 19-year span the cycle reconciles with the solar year.
Meton of Athens: Fifth-century BC Athenian astronomer who introduced the 19-year lunisolar cycle, likely with Euctemon. The cycle's namesake and probable first public proponent, working in Athens around 432 BC.
Computus: The medieval calculation, based on the 19-year cycle, that fixes the date of Easter each year. Easter's rule of the first Sunday after a full moon after the equinox rests directly on the cycle.
Callippic cycle: A 76-year refinement of Meton's cycle that drops one day every four Metonic cycles. Its only rival within Greek astronomy, adopted because Meton's cycle ran about a day long.
Tropical year: The 365.2422-day cycle of the seasons, the solar period the lunar calendar must be fitted to. Nineteen tropical years differ from 235 lunar months by only about two hours.
Euctemon: Athenian astronomer of the fifth century BC who measured solstices and compiled the first parapegma-style star calendars. Meton's probable collaborator in the solstice observations that anchored the 19-year count.
Golden number: A year's position from 1 to 19 in the Metonic cycle, used in calendrical computation. The numbering scheme that made the cycle operational in medieval Easter tables.
Hipparchic lunar cycle: Hipparchus's more precise month length, derived by comparing Babylonian and Greek observations centuries apart. Hipparchus measured Meton's two-hour error and built a cycle accurate to minutes.
Epact: The age of the moon on a given calendar date, used to track lunar months against solar years. The cycle advances the epact by eleven each year, generating the pattern of embolismic months.
Babylonian astronomy: The Mesopotamian tradition of systematic lunar and planetary observation, the mathematical bedrock of Greek calendrics. Babylonian lunar records may have supplied the raw 19-year relation before Meton published it.