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AukusRuckus (talk | contribs) Disambiguated Intercalation → Intercalation (timekeeping) in section {{main}} template. Moved December 2022 {{confusing}} template up |
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{{short description|Shape of the Moon's directly sunlit portion as viewed from Earth}}
[[File:2023-Moon-Phases-Northern-Hemisphere-4K.webm|thumb|300px|The lunar phases and [[libration]]s in 2023 as viewed from the [[Northern Hemisphere]] at hourly intervals, with titles and supplemental graphics]]
[[File:2023 Moon Phases - Southern Hemisphere - 4K.webm|thumb|300px|The lunar phases and [[libration]]s in 2023 as viewed from the [[Southern Hemisphere]] at hourly intervals, with titles and supplemental graphics]]
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[[File:Moon phases en.jpg|thumb|The phases of the Moon as viewed looking southward from the [[Northern Hemisphere]]. Each phase would be rotated 180° if seen looking northward from the [[Southern Hemisphere]]. The upper part of the diagram is not to scale, as the Moon, the Earth, and the Moon's orbit are all much smaller relative to the Earth's orbit than shown here.]]
There are four ''principal'' (primary, or major) lunar phases: the [[new moon]], first quarter, [[full moon]], and last quarter (also known as third or final quarter), when the Moon's [[ecliptic longitude]] is at an angle to the Sun (as viewed from the center of the Earth) of 0°, 90°, 180°, and 270° respectively.{{sfn|Seidelmann|1992|p=478}}{{efn|The quarter phases happen when the observer–Moon–Sun angle is 90°{{citation needed|date=December 2022}}, also known as [[quadrature (astronomy)| quadrature]]{{dubious|date=December 2022}}. This is not the same as a [[right angle]]
Between the principal phases are ''intermediate'' phases, during which the apparent shape of the illuminated Moon is either [[crescent]] or [[wikt: gibbous|gibbous]]. On average, the intermediate phases last one-quarter of a [[synodic month]], or 7.38 days.{{efn|Their durations [[orbital eccentricity |vary slightly]] because the [[orbit of the Moon| Moon's orbit]] is somewhat [[elliptic orbit| elliptical]], so its [[orbital speed]] is not constant.}}
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==Timekeeping <span class="anchor" id="Calendar"></span> ==
{{main|Lunar calendar|Lunisolar calendar|Metonic cycle|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.
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