A lunar month is the amount of time it takes for the Moon to pass through each of its phases (new moon, half, full moon), and then return back to its original position. It takes 29 days, 12 hours, 44 minutes and 3 seconds for the Moon to complete one lunar month.

The Moon. Image credit: ESA

The Moon. Image credit: ESA

You might have heard that the Moon only takes 27.3 days to complete one orbit around the Earth. So why is a lunar month more than 2 days longer than the orbit of the Moon?

 

A lunar month is the amount of time it takes for the Moon to get from a specific phase, like a new moon, back to the same phase. In other words, the Moon has to get back to the point in its orbit where the Sun is in the same position from our point of view. Since the Moon is going around the Sun with the Earth as part of its orbit, the Moon has to catch up a little bit on each orbit. It takes 2.2 additional days each orbit of the Moon to catch up.

 

This method of measuring a lunar month, from new moon to new moon, is known as a synodic month. A new moon is defined as when the Moon has the same ecliptic longitude as the Sun, as seen from the center of the Earth; when the Sun, Moon and Earth are perfectly lined up.

 

Simulated view of the moon’s phases. The period of time from new moon to new moon is known as the lunar month, lunation or synodic month. From the years 1760 to 2200, the longest lunar month spans 29 days 19 hours and 58 minutes (Dec. 9, 1787 to Jan. 8, 1778) while the shortest lasts for 29 days 6 hours and 34 minutes (June 12 to July 12, 1885).