Lunar month
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- This should perhaps be merged with Month.
In lunar calendars, a lunar month is the time between two successive similar syzygies (new moons or full moons). There are many variations. In Middle-Eastern and European traditions, the month starts when the young crescent moon becomes first visible at evening after conjunction with the Sun 1 or 2 days before that evening (e.g. in the Islamic calendar). Others use a reckoned moon (e.g. the Hebrew calendar), or use a tabular scheme (Gregorian calendar). Yet others have months run from full moon to full moon. Calendars count integer days, so months may be 29 or 30 days in length, in some regular or irregular sequence. But all lunar months approximate the mean length of the synodic month of approximately 29.53059 days (29 days, 12 hours, 44 minutes and 3 seconds).
There are several different ways of expressing the lunar month.
1) The Moon's rotation period equals its orbital period - both are 27.32 days (27 days, 7 hours, 43 minutes, 11.6 seconds). This is known as a sidereal month and is measured by observing how long it takes the Moon to pass a fixed star on the celestial sphere.
2) A synodic month (29 days, 12 hours, 44 minutes, 2.8 seconds) and is measured from New Moon to New Moon. A New Moon occurs when the Moon intersects an imaginary plane between the Sun and Earth - that is, when the Moon is directly between the Sun and Earth. A synodic month is longer than a sidereal month because the Earth-Moon system is orbiting the Sun in the same direction as the Moon is orbiting the Earth. Therefore, it takes about 2.2 days longer for the Moon to return to the intersection where the New Moon occurs. The synodic month is the most common way of expressing the lunar cycle.
3) An anomalistic month is measured from perigee to perigee - the point in the Moon's orbit when it is closest to Earth. An anomalistic month is 27.55 days.
4) Two other types of lunar months are the tropical month (lunar equinox to lunar equinox) and the draconic, or nodal, month (node to node, or the point where the Moon's orbit crosses the Earth's orbit).
- See also: month, tithi, vedic timekeeping