Lunar precession


Lunar precession is the change in orientation of the lunar rotational axis with respect to a reference plane, following the normal rules of precession followed by spinning objects. The orbit of the Moon undergoes two important types of precessional motion: apsidal and nodal. The axis of the Moon also experiences precession.

Nodal precession

The first type of lunar precession is that of the plane of the Moon's orbit. The period of the lunar nodal precession is defined as the time it takes the ascending node to move through 360° relative to the vernal equinox. It is about 18.6 years and the direction of motion is westward, i.e. in the direction opposite to the Earth's orbit around the Sun if seen from the celestial north. This is the reason that a draconic month or nodal period is shorter than the sidereal month. After one nodal precession period, the number of draconic months exceeds the number of sidereal months by exactly one. This period is about 6,793 days.
As a result of this nodal precession, the time for the Sun to return to the same lunar node, the eclipse year, is about 18.6377 days shorter than a sidereal year. The number of solar orbits during one lunar nodal precession period equals the period of orbit divided by this difference, minus one: − 1.

Apsidal precession

The second kind of precession of the Moon's orbit is that of the major axis of the Moon's elliptic orbit, which precesses eastward by 360° in approximately 8.85 years. This is the reason that an anomalistic month is longer than the sidereal month. This apsidal precession completes one rotation in the same time as the number of sidereal months exceeds the number of anomalistic months by exactly one, after about 3,233 days.

Axial precession

The rotational axis of the Moon also undergoes precession. Since the Moon's axial tilt is only 1.5° with respect to the ecliptic, this effect is small. Once every 18.6 years, The lunar north pole describes a small circle around a point in the constellation Draco, while correspondingly, the lunar south pole describes a small circle around a point in the constellation Dorado.