The time taken for the Moon to complete one full orbit of the Earth, some 29 and a half days (see Lunation). However, the motion of the Moon in its orbit is very complicated and its period is not constant. There are a number of different kinds of month used in astronomy, astrology and calendrics:

Sidereal Month: the time it takes the Moon to return to the same position on the celestial sphere among the fixed stars (Latin: sidus): 27.321661 days (27 d 7 h 43 min 11.5 s) or 27 1/3 days. This type appeared among cultures in the Middle East, India, and China as they divided the sky in 27 or 28 lunar mansions, characterized by asterisms, one for each day the Moon follows its track among the stars.

Tropical Month: the time taken with regard to the vernal point, or "lunar equinox". Because of precession, this point moves back slowly along the ecliptic. It takes the Moon less time to return to an ecliptic longitude of zero than to the same point amidst the fixed stars: 27.321582 days (27 d 7 h 43 min 4.7 s). This slightly shorter period is known as the tropical month.

Anomalistic Month: the Moon's orbit is an ellipse rather than a circle, but its orientation and shape are not fixed. In particular, the position of the extreme points (the line of the apsides: perigee and apogee), makes a full circle (lunar precession) in about nine years. It takes the Moon longer to return to the same apsis because it moved ahead during one revolution. This longer period is called the anomalistic month, and has an average length of 27.554551 days (27 d 13 h 18 min 33.2 s), or about 27½ days.

Draconic Month: the plane of the Moon's orbit is tilted by approx 5° with respect to the plane of the ecliptic. The line of intersection of these planes defines the ascending and descending nodes. The plane of the Moon's orbit precesses over a full circle in about 18.6 years, so the nodes move backwards over the ecliptic with the same period. Hence the time it takes the Moon to return to the same node is again shorter than a sidereal month: this is called the draconic, nodical, or draconitic month. It lasts 27.212220 days (27 d 5 h 5 min 35.8 s), or about 27 1/5 days. It is important for predicting eclipses: these take place when the Sun, Earth and Moon are on a line (i.e., in syzygy). The three bodies are only on a line when the Moon is on the ecliptic, i.e. when it is at one of the nodes. The "draconic" month refers to the mythological dragon that lives in the nodes and regularly eats the Sun or Moon during an eclipse.

Synodic Month: Moon phases occur because we see the part of the Moon that is illuminated by the Sun from different angles as the Moon traverses its orbit. Because the Earth orbits the Sun, it takes the Moon extra time (after completing a sidereal month, i.e. a full circle) to catch up and return to the same position with respect to the Sun. This longer period is called the synodic month. Because of the perturbations of the orbits of the Earth and Moon, the actual time between lunations may range from about 29.27 to about 29.83 days. The long-term average duration is 29.530588 days (29 d 12 h 44 min 2.8 s), or about 29½ days.

Calendar Month: Divisions of the solar year (approx 365.25 days) of either of 30 or 31 days in the Western Gregorian Calendar, excepting February which is normally 28 days, with an extra day added every leap year (to balance the account for the extra four quarters of a day).

Moon

The Earth's sole natural satellite. The Moon is classed as a planet ("wanderer") in astrology and represents matters to do with women, the mother, the public, the emotions, habit patterns and so on. A feminine planet, she rules Cancer and is exalted in Taurus. Several other planets have recently been determined by astronomers to have moons, but these are not generally considered in astrology. More about the Moon.