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Latest revision as of 21:22, 23 February 2025
Month
A month is a unit of time, used with calendars, which is approximately as long as a natural period known as the lunar month. In the Gregorian calendar, introduced in 1582 and now used almost universally, a month is a period of 28, 30, or 31 days.
Etymology[edit]
The word month is derived from Old English monað, which is ultimately from Proto-Germanic *mēnōths.
Length[edit]
The mean month length of the Gregorian calendar is 30.436875 days.
Types of months in astronomy[edit]
In astronomy, the month is principally used to refer to the period of the Earth's moon's orbital period.
Sidereal month[edit]
The sidereal month is the time the Moon takes to complete one full revolution around the Earth with respect to the background stars.
Synodic month[edit]
The synodic month is the period of the Moon's phases, because the Moon's appearance depends on the position of the Earth and the Sun.
Tropical month[edit]
The tropical month is the average time for the Moon to pass twice through the same equinox point of the sky.
Anomalistic month[edit]
The anomalistic month is the average time the Moon takes to go from perigee to perigee - the point in the Moon's orbit when it is closest to Earth.
Draconic month[edit]
The draconic month is also known as the nodical month or nodal period. It is the period in which the Moon returns to the same node of its orbit; the nodes are the two points where the Moon's orbit crosses the plane of the Earth's orbit.
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