Editing Month (section)

== Calendrical consequences ==
{{Further|lunar calendar|lunisolar calendar}}

At the simplest level, most well-known lunar calendars are based on the initial approximation that 2 lunations last 59 [[solar day]]s: a 30-day '''full month''' followed by a 29-day '''hollow month''' — but this is only roughly accurate and regularly needs [[intercalation (timekeeping)|intercalation]] (correction) by a [[leap day]].

Additionally, the synodic month does not fit easily into the [[tropical year|solar (or 'tropical') year]], which makes accurate, rule-based [[lunisolar calendar]]s that combine the two cycles 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 6,940 days): 12 years have 12 lunar months, and 7 years are 13 lunar months long. However, a [[Metonic calendar]] based year will drift against the seasons by about one day every 2 centuries. Metonic calendars include the calendar used in the [[Antikythera Mechanism]] about 21 centuries ago, and the [[Hebrew calendar]].

Alternatively in a pure [[lunar calendar]], years are defined as having always 12 lunations, so a year is 354 or 355 days long: the [[Islamic calendar]] is the prime example.  Consequently, an Islamic year is about 11 days shorter than a solar year and cycles through the seasons in about 33 solar = 34 lunar years: the [[Islamic New Year]] has a different [[Gregorian calendar]] date in each (solar) year.

Purely [[solar calendar]]s often have 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, or are purely conventional like in the widely used [[Gregorian calendar]].

The complexity required in an accurate lunisolar calendar may explain why solar calendars have generally replaced lunisolar and lunar calendars for civil use in most societies.