A month is a unit of time, used with calendars, that is approximately as long as a natural orbital period of the Moon, the words month and Moon are cognates. The traditional concept arose with the cycle of Moon phases, such lunar months ('lunations') are synodic months and last approximately 29. 53 days. From excavated tally sticks, researchers have deduced that people counted days in relation to the Moon's phases as early as the Paleolithic age. Synodic months, based on the Moon's orbital period with respect to the Earth-Sun line, are still the basis of many calendars today, and are used to divide the year. Types of months in astronomy The following types of months are mainly of significance in astronomy, most of them (but not the distinction between sidereal and tropical months) first recognized in Babylonian lunar astronomy. The sidereal month is defined as the Moon's orbital period in a non-rotating frame of reference (which on average is equal to its rotation period in the same frame). It is about 27. 32166 days (27 days, 7 hours, 43 minutes, 11. 6 seconds). It is closely equal to the time it takes the Moon to pass twice a 'fixed' star (different stars give different results because all have a very small proper motion and are not really fixed in position). A synodic month is the most familiar lunar cycle, defined as the time interval between two consecutive occurrences of a particular phase (such as new moon or full moon) as seen by an observer on Earth. The mean length of the synodic month is 29. 53059 days (29 days, 12 hours, 44 minutes, 2. 8 seconds).