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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 is 29.53 days (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 2.1 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 |
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