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A month is a unit of time, used with calendars, that is approximately as long as a natural orbital period of the Moon; the words month and Moon are cognates.The traditional concept of months arose with the cycle of Moon phases; such lunar months ("lunations") are synodic months and last approximately 29.53 days, making for roughly 12.37 such months in one Earth year.
After completing its § Sidereal month, the Moon must move a little further to reach the new position having the same angular distance from the Sun, appearing to move with respect to the stars since the previous month. Consequently, at 27 days, 7 hours, 43 minutes and 11.5 seconds, [5] the sidereal month is about 2.2 days shorter than the ...
Since each lunation is approximately 29 + 1 ⁄ 2 days, [1] it is common for the months of a lunar calendar to alternate between 29 and 30 days. Since the period of 12 such lunations, a lunar year , is 354 days, 8 hours, 48 minutes, 34 seconds (354.36707 days), [ 1 ] purely lunar calendars are 11 to 12 days shorter than the solar year .
The formal lunar day is therefore the time of a full lunar day-night cycle. Due to tidal locking, this equals the time that the Moon takes to complete one synodic orbit around Earth, a synodic lunar month, returning to the same lunar phase. The synodic period is about 29 + 1 ⁄ 2 Earth days, which is about 2.2 days longer than its sidereal period.
The approximate age of the Moon, and hence the approximate phase, can be calculated for any date by calculating the number of days since a known new moon (such as 1 January 1900 or 11 August 1999) and reducing this modulo 29.53059 days (the mean length of a synodic month).
The actual length of a synodic month varies from about 29 days 6 hours and 30 minutes (29.27 days) to about 29 days and 20 hours (29.83 days), a variation range of about 13 hours and 30 minutes. Accordingly, for convenience, the Hebrew calendar uses a long-term average month length, known as the molad interval , which equals the mean synodic ...
List of mammalian gestation durations. This is a collection of lists of mammal gestation period estimated by experts in their fields. The mammals included are only viviparous ( marsupials and placentals) as some mammals, which are monotremes (including platypuses and echidnas) lay their eggs. A marsupial has a short gestation period, typically ...
There is no specified length of any particular Chinese month, so the first month could have 29 days (short month, 小月) in some years and 30 days (long month, 大月) in other years. A 12-month-year using this system has 354 days, which would drift significantly from the tropical year. To fix this, traditional Chinese years have a 13-month ...