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A year has about 365.24 solar days but 366.24 sidereal days. Therefore, there is one fewer solar day per year than there are sidereal days, similar to an observation of the coin rotation paradox. [5] This makes a sidereal day approximately 365.24 / 366.24 times the length of the 24-hour solar day.
W1 is the ecliptic longitude of the Moon w.r.t. the fixed ICRS equinox: its period is the sidereal month. If we add the rate of precession to the sidereal angular velocity, we get the angular velocity w.r.t. the Equinox of the Date: its period is the tropical month (which is rarely used). l is the mean anomaly: its period is the anomalistic month.
27.321661 days [7] (equal to sidereal orbital period due to spin-orbit locking, a sidereal lunar month) 27 d 7 h 43 m 11.5 s 29.530588 days [ 7 ] (equal to synodic orbital period , due to spin-orbit locking, a synodic lunar month )
The sidereal year is 20 min 24.5 s longer than the mean tropical year at J2000.0 (365.242 190 402 ephemeris days). [ 1 ] At present, the rate of axial precession corresponds to a period of 25,772 years, [ 3 ] so sidereal year is longer than tropical year by 1,224.5 seconds (20 min 24.5 s, ~365.24219*86400/25772).
Informally, a lunar day and a lunar night is each approx. 14 Earth days. The formal lunar day is therefore the time of a full lunar day-night cycle. Due to tidal locking, this equals the time that the Moon takes to complete one synodic orbit around Earth, a synodic lunar month, returning to the same lunar phase. The synodic period is about 29 ...
Timekeeping on the Moon is an issue of synchronized human activity on the Moon and contact with such. The two main differences to timekeeping on Earth are the length of a day on the Moon, being the lunar day or lunar month, observable from Earth as the lunar phases, and the rate at which time progresses, with 24 hours on the Moon being 58.7 microseconds (0.0000587 seconds) faster, [1 ...
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[20] [21] Based on geological studies of tidal rhythmites, the day was 21.9±0.4 hours long 620 million years ago and there were 13.1±0.1 synodic months/year and 400±7 solar days/year. The average recession rate of the Moon between then and now has been 2.17±0.31 cm/year, which is about half the present rate.