tropical month

The tropical month is a measure of time related to the Moon's revolution around the Earth, specifically the period between the Moon's passages through the same celestial longitude. It is approximately 27 days, 7 hours, 43 minutes, and 5 seconds of mean solar time. This is only slightly shorter than the sidereal month, which is the time it takes for the Moon to return to the same position relative to the stars. The difference arises because the Earth-Moon system is also orbiting the Sun. The tropical month is distinct from the synodic month, which is the time for the Moon to complete a cycle of phases as seen from Earth, averaging about 29.5 days.12