Month, period of time, originally measured with reference to the revolution of the moon; today the reference is to the simplified calendar month. In astronomy, the lunar, or synodic, month is the time required for the moon to pass from a particular phase, such as the full moon, back to the same phase; it averages 29.53059 days. The sidereal month is the time taken for the moon to make one revolution from a particular star back to the same star, as estimated from the center of the earth; it averages 27.32166 days. Other types of months used in astronomy are the tropical month, the mean time of revolution from one celestial longitude back to the same longitude, averaging 27.32156 days; the nodical month, the mean time of revolution from one node back to the same node, averaging 27.21222 days; and the solar month, equivalent to one-twelfth of a tropical year or 30.43685 days. The calendar month is exactly 28, 29, 30, or 31 days. See Calendar.
See also Day; Moon; Time; Year.
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