Editing Lunar phase (section)

==Timekeeping <span class="anchor" id="Calendar"></span> ==
{{main|Lunar calendar|Lunisolar calendar|Metonic cycle|Intercalation (timekeeping){{!}}Intercalation|History of calendars}}
Archaeologists have reconstructed methods of [[timekeeping]] that go back to prehistoric times, at least as old as the [[Neolithic]]. The natural units for timekeeping used by most historical societies are the [[day]], the [[solar year]] and the [[lunation]]. The first crescent of the new moon provides a clear and regular marker in time and pure lunar calendars (such as the Islamic [[Islamic calendar|Hijri calendar]]) rely completely on this metric. The fact, however, that a year of twelve lunar months is ten or eleven days shorter than the solar year means that a lunar calendar drifts out of step with the seasons. Lunisolar calendars resolve this issue with a year of thirteen lunar months every few years, or by restarting the count at the first new (or full) moon after the [[winter solstice]]. The [[Sumerian calendar]] is the first recorded to have used the former method; [[Chinese calendar]] uses the latter, despite delaying its start [[Chinese New Year#Dates in Chinese lunisolar calendar|until the second or even third new moon]] after the solstice. The [[Hindu calendar]], also a lunisolar calendar, further divides the month into [[Astronomical basis of the Hindu calendar|two fourteen day periods]] that mark the waxing moon and the waning moon. 

The ancient [[Roman calendar]] was broadly a lunisolar one; on the decree of [[Julius Caesar]] in the first century BCE, Rome changed to a [[solar calendar]] of twelve months, each of a fixed number of days except in a [[leap year]]. This, the [[Julian calendar]] (slightly revised in 1582 to correct the [[leap year]] rule), is the basis for the [[Gregorian calendar]] that is almost exclusively the [[civil calendar]] in use worldwide today.