Anomalistic month

The anomalistic month is the interval between two successive passages of the Moon through perigee, its closest point to Earth, lasting 27.5546 days. It is longer than the sidereal month (27.3217 days) because the Moon’s line of apsides rotates eastward, advancing perigee roughly 3 degrees per month and completing one revolution in about 8.85 years. The anomalistic period governs the Moon’s varying distance and therefore its changing horizontal parallax and semidiameter, both of which the navigator reads from the daily pages when correcting a lunar altitude. It also feeds the perigean spring tides that raise tidal range.