The mean interval of time between successive passages of the Moon through its closest point to Earth (its perigee), equal to 27.55464 days. The Moon's orbit is not fixed in space but can be regarded as a slowly rotating ellipse, the rotation due principally to perturbation by the Sun. Hence the perigee point advances slowly around the orbit, completing one circuit in 8.85 years. As a consequence, the anomalistic month is about 5.5 hours longer than the sidereal month.
Subjects: Astronomy and Astrophysics.