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Lunar precession
From Wikipedia, the free encyclopedia
There are two important precessional motions in the Orbit of the Moon.
The long axis (line of the apsides: perigee and apogee) of the moon's elliptical orbit precesses about once in just under 9 years. It is caused by the solar tide. This precession period is equal to the time that number of sidereal months counted exceeds the number of anomalistic months counted by exactly one. This happens after about 3233 days.
This precession causes the full moon cycle to be over a month longer than a sidereal year.
There are approximately two such lunar precession cycles in a saros cycle.
This is to be distinguished from precession of the lunar nodes of the lunar orbit on the plane of the ecliptic. This is mainly caused by the oblation of the Earth; it is the period of the main nutation term in the orientation of the polar axis of the Earth. This nodal period is about twice as long as the apsidal precession period discussed above. After the nodal period, the number of draconic months counted exceed the number of sidereal months counted by exactly one: this happens after about 6793 days (18.6 years).
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