Month

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

The word month is derived from Old English monað, which is ultimately from Proto-Germanic *mēnōths.

Length

The mean month length of the Gregorian calendar is 30.436875 days.

Types of months in astronomy

In astronomy, the month is principally used to refer to the period of the Earth's moon's orbital period.

Sidereal month

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

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

The tropical month is the average time for the Moon to pass twice through the same equinox point of the sky.

Anomalistic month

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

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.

See also

• Lunar calendar
• Lunisolar calendar
• Solar calendar

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