How long is the interval between two full moons?

The Moon needs 27.33 days to circle around the Earth. Something that is also referred to as “sidereal time”. But because the Earth orbits the Sun, just like the Moon orbits the Earth, the Moon has to travel two further days in order to resume the same position to the Earth and Sun. This is then called the “sidereal time”. In order to determine the point of time of the reoccurring full moon, the sidereal time serves as basis.

Which means: the Moon needs 29 days, 12 hours and 44 minutes, to be in the same precise position to the Sun and the Earth. If one assumes a specific position on earth at the time of full moon, it has to be exactly this time in order to be seen as full moon from the same position. However, the date of the synodic moon period can only be looked at as an average value, because in some months the synodic moon period takes a little longer than in others. Everyone who would like to know more details will have to hit the bookstore!

The moon month is mostly shorter than the calendar month. Apart from February. There, it is mostly reverse and it occasionally happens that fullmoon is cancelled in February (e.g. 1999)!

Photo: grublee/

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