At the simplest level, most well-known lunar calendars are based on the initial approximation that 2 lunations last 59 days: a 30-day full month followed by a 29-day hollow month — but this is only roughly accurate, and eventually needs correction by using larger cycles, or the equivalent of leap days. Additionally, the synodic month does not fit easily into the year, which makes accurate, rule-based lunisolar calendars complicated. The most common solution to this problem is the Metonic cycle, which takes advantage of the fact that 235 lunations are approximately 19 tropical years (which add up to not quite 6940 days). However, a Metonic calendar will drift against the seasons by about 1 day every 200 years. Metonic calendars include the calendar used in the Antikythera Mechanism about 2000 years ago, and the Hebrew calendar.
The complexity required in an accurate lunisolar calendar may explain why solar calendars, with months which no longer relate to the phase of the Moon, but are based only on the motion of the Sun relative to the equinoxes and solstices, have generally replaced lunar calendars for civil use in most societies.
Intro Types of months in astronomy Calendrical consequences Months in various calendars See also References
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