Moon phase

The moon phase or lunar phase is the shape of the sunlit portion of the Moon when you look at it from Earth. These phases change cyclically as the Moon orbits the Earth, according to the changing positions of the Moon and Sun relative to the Earth.

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The Moon’s rotation is tidally locked by the Earth’s gravity, therefore the same lunar surface always faces Earth. This face is variously sunlit depending on the position of the Moon in its orbit. There are four principal lunar phases: new moon (0% sunlit), first quarter, full moon (100% sunlit), and third quarter (also known as last quarter).

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Timing of the Moon’s phases shifts

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The average calendar month, which is  1⁄12 of a tropical year, is about 30.44 days, while the Moon’s phase (synodic) cycle repeats on average every 29.53 days. Therefore, the timing of the Moon’s phases shifts by an average of almost one day for each successive month. (Lunar year=354 days.)

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The approximate moon phase can be calculated for any date by calculating the number of days since a known new moon (such as January 1, 1900 or August 11, 1999) and reducing this modulo 29.530588853 (the length of a synodic month). Can you imagine how watch makers manage to create a complication like this?

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Watches with a moon phase

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