by Phill Edwards
Nutation is an oscillation of the Earth's axis or rotation. It is largely due to the graviational pull of the Moon and the Sun. The largest componet of nutation has a period of 6798 days (18.61 years). This nutation is usually divided into two components, one parallel to the ecliptics and one perpendicular the to ecliptic. The parallel component is sometimes referred to as the precession of the equinoxes . The perpendicular component is referred to as nution of obliquity.
The direction of the Vernal Equinox is important in astronomy. It is the X axis or the zero of longitude or right ascension. The precession means that this direction is not fixed. A reference direction for the Vernal Equinox was defines for the J2000 epoch. It is defined in terms of the direction of fixed stars - typically distant quasars.
The 18.61 year nutation has the same period as the the precession of the Moon's orbital nodes.
The 18.61 year precession of the Moon's orbital nodes is the main component of nutation. Then mean longitude of the Moon's ascending node is given by: