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A month is a unit of time, used with calendars, that is approximately as long as a natural phase cycle 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.
This longer period is called the anomalistic month [15] and has an average length of 27.554 551 days (27 d 13 h 18 min 33.2 s). The apparent diameter of the Moon varies with this period, so this type has some relevance for the prediction of eclipses (see Saros ), whose extent, duration, and appearance (whether total or annular) depend on 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 .
Celestial sphere-based: as in sidereal time, where the apparent movement of the stars and constellations across the sky is used to calculate the length of a year. These units do not have a consistent relationship with each other and require intercalation. For example, the year cannot be divided into twelve 28-day months since 12 times 28 is 336 ...
On average, this synodic period lasts 29 days, 12 hours, 44 minutes, 3 seconds, [1] the length of a lunar month on Earth. The exact length varies over time because the speed of the Earth–Moon system around the Sun varies slightly during a year due to the eccentricity of its elliptical orbit, variances in orbital velocity, and a number of ...
The period between two new moons is 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.
The Gregorian calendar, like the Julian calendar, is a solar calendar with 12 months of 28–31 days each. The year in both calendars consists of 365 days, with a leap day being added to February in the leap years. The months and length of months in the Gregorian calendar are the same as for the Julian calendar.
In the so-called five-phase calendar, the year consists of 10 months and a transition, each month being 36 days long, and the transitions 5 or 6 days. During the Warring States period (~475–220 BC), the primitive lunisolar calendars were established under the Zhou Dynasty, known as the six ancient calendars ( simplified Chinese : 古六历 ...