A period of time based on the orbit of the Moon around the Earth. Several types of month are defined. The synodic month is equivalent to one lunation, the time taken for the Moon to go through one cycle of phases; its mean length is 29.530 59 days. The anomalistic month is the time between successive passages of the Moon through perigee; it lasts 27.554 64 days. The sidereal month is the time taken for the Moon to return to the same position against the star background; it lasts 27.321 66 days. The tropical month is the time between successive passages of the Moon through the vernal equinox; it lasts 27.321 58 days. The draconic month (or nodical month) is the time between successive passages of the Moon through its ascending node; it lasts 27.2122 days. The calendar month is an artificial unit consisting of a whole number of days; see Calendar.