/munth/, n.1. Also called calendar month. any of the twelve parts, as January or February, into which the calendar year is divided.2. the time from any day of one calendar month to the corresponding day of the next.3. a period of four weeks or 30 days.4. Also called solar month. one-twelfth of a solar or tropical year.5. Also called lunar month. the period of a complete revolution of the moon around the earth, as the period between successive new moons (synodic month), equal to 29.531 days, or the period between successive conjunctions with a star (sidereal month), equal to 27.322 days, or the period between successive perigees (anomalistic month), equal to 27.555 days, or the period between successive similar nodes (nodical month or draconic month), equal to 27.212 days.6. an unusually long period of time of indefinite length: I haven't seen him for months.[bef. 900; ME; OE monath; c. OHG manod, ON manathr. See MOON]
* * *▪ time measurementa measure of time corresponding or nearly corresponding to the length of time required by the Moon to revolve once around the Earth.The synodic month, or complete cycle of phases of the Moon as seen from Earth, averages 29.530588 mean solar days in length (i.e., 29 days 12 hours 44 minutes 3 seconds); because of perturbations in the Moon's orbit, the lengths of all astronomical months vary slightly. The sidereal month is the time needed for the Moon to return to the same place against the background of the stars, 27.321661 days (i.e., 27 days 7 hours 43 minutes 12 seconds); the difference between synodic and sidereal lengths is due to the orbital movement of the Earth–Moon system around the Sun. The tropical month, 27.321582 days (i.e., 27 days 7 hours 43 minutes 5 seconds), only 7 seconds shorter than the sidereal month, is the time between passages of the Moon through the same celestial longitude. The draconic, or nodical, month of 27.212220 days (i.e., 27 days 5 hours 5 minutes 35.8 seconds) is the time between the Moon's passages through the same node, or intersection of its orbit with the ecliptic, the apparent pathway of the Sun.Months of the yearAs a calendrical period, the month is derived from the lunation—i.e., the time elapsing between successive new moons (or other phases of the moon). A total of 12 lunations amounts to 354 days and is, roughly, a year. A period of 12 lunations was therefore used by some primitive peoples to make their calendrical year. As is obvious, the lunar-based year (and a calendar derived from it) cannot be accurately correlated with a solar-based year, and the month's continued use in the Gregorian calendar of modern times is merely a recognition of its convenience as a calendar division (see Table (Months of the year)).
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