month

Various periods of time based on the revolution of the moon about the Earth. All except the calendar month are the intervals of time between the instants when the moon passes through some reference point.

	days	days	hrs	min	sec
Synodic	29.53059	29	12	44	3
Tropical	27.32158	27	7	43	5
Sidereal	27.32166	27	7	43	12
Anomalistic	27.55455	27	13	18	33
Nodical	27.21222	27	5	5	36

In addition to the physical phenomena that show a monthly variation, such as the tides, many biological phenomena have periods of about a month, for example, women's menses. Globally, the temperature of the lowest 6 kilometers of the atmosphere is warmest about 5 to 8 days before the full moon, and coolest at the new moon, though the difference is only 2 to 3 hundredths of a degree.¹

1. Robert C. Balling and Randall S. Cerveny.
Geophysical Research Letters 22:3199-3201 (1995).

Calendar month

A division of the calendar year, of arbitrary length, but roughly equal to the various astronomical months. See calendar.

Synodic month

The mean interval between conjunctions of the Moon and Sun; that is, the average length of time the Moon takes to return to the point in its orbit in which it is closest to the Sun. The synodic month is the length of the cycle of phases of the moon; for example, the interval between new moon and new moon. The mean value may differ from any particular cycle by as much as seven hours.

For an estimate of the current length,

29.5305888531 + 0.00000021621T − 3.64 × 10⁻¹⁰T²

where T is ^{(the Julian day number − 2451545.0)}⁄₃₆₅₂₅

Tropical month

Imagine a line from pole to pole on the celestial sphere, passing through the center of the moon. It will cross the ecliptic, the circle on the celestial sphere where the plane of the earth's orbit cuts the sphere in half. A tropical month is the average interval between instants when the line through the moon crosses the point on the ecliptic called the vernal equinox.

The time required for the mean longitude of moon to increase 360 degrees.

Sidereal month

The time it takes the moon to return to the same position among distant stars.

Anomalistic month

The moon's orbit around the earth isn't circular. The point in its orbit at which it is closest to Earth is called the perigee. The anomalistic month is the average interval between successive passages of the moon through its perigee.

Nodical month

The average interval between successive northward passages of the moon across the ecliptic. It is sometimes called a draconic or draconitic month.