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        Science | Astronomy Terms

Month, a unit of time. The calendar month is approximately 1/12 of the calendar year. According to the Gregorian calendar, which is used in everyday life throughout most of the world, the various months do not have an equal number of days but contain 28, 29 (leap year), 30, or 31 days.

There are various other types of months, which are based on movements of the moon. These include:

Lunar, or Synodic, Month, the interval between two consecutive phases of the moon; for example, from full moon to full moon. The motion of the moon is irregular, however, and such a month may vary as much as 12 hours or more. The average synodic month is 29.53059 days. The Muslim calendar is based on the lunar month. The Jewish calendar is similarly based but is adjusted to the solar year.

Sidereal Month, the period required for the moon to travel from a point in front of a particular star back to the same point as viewed from the center of the earth. This period may vary in length by several hours. The average sidereal month is 27.32166 days.

Nodical, or Draconic, Month, the time required for the moon to travel from one of the lunar nodes back to the same node. (A node is a point where the plane of the earth's orbit crosses the plane of the moon's orbit.) The average length of the nodical month is 27.21222 days. This kind of month is used in calculating eclipses.

Tropical Month, the time required for the moon to travel from one celestial longitude back to the same longitude. It averages 27.32158 days.

Anomalistic Month, the time required for the moon to travel from perigee (the lowest point in its orbit) to perigee. The average length of the anomalistic month is 27.55455 days.