Month - Calendrical Consequences

Calendrical Consequences

For more details on this topic, see lunar calendar and lunisolar calendar.

At the simplest level, all lunar calendars are based on the approximation that 2 lunations last 59 days: a 30 day full month followed by a 29 day hollow month — but this is only marginally accurate and quickly needs correction by using larger cycles, or the equivalent of leap days. One exception is the Hebrew calendar, which was calculated very precisely over a thousand years ago, based on a lunation of 29 days, 12 hours, 44 minutes and 3⅓ seconds, which differs from modern calculations by only one day in 15000 years.

Second, the synodic month does not fit easily into the year, which makes constructing accurate, rule-based lunisolar calendars difficult. 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 (such as the Hebrew calendar) will drift against the seasons by about 1 day every 200 years.

The problems of creating reliable lunar calendars may explain why solar calendars, having months which no longer relate to the phase of the Moon, and being based only on the motion of the Sun against the sky, have generally replaced lunar calendars for civil use in most societies.