Apsidal Precession

The orbit that turns

No orbit is a perfect circle. The Moon’s path is an ellipse with Earth at one focus, so there is a near point (perigee) and a far point (apogee). The straight line through both is the line of apsides. Apsidal precession is the slow rotation of that line: the whole ellipse pivots in its plane, carrying perigee and apogee around with it.

The Moon’s apsides turn forward (the same way the Moon orbits), completing a full turn in about 8.85 years. That is the opposite sense to the Moon’s nodes, which regress over 18.6 years. They are two different slow turnings of the same tilted, elliptical orbit.

Why it matters: longer months, super-Moons, and eclipses

• The anomalistic month. Because perigee creeps forward to meet the returning Moon, the anomalistic month (perigee to perigee, 27.55 days) is a touch longer than the 27.32-day sidereal month. You will find both on the cycles by length page.
• Super-Moons. When a full or new Moon lands near perigee, the Moon is closest and looks largest, a “super-Moon.” Near apogee it is a smaller “micro-Moon.” The distance swings from roughly 356,500 km to 406,700 km, changing the apparent size by about 14%.
• Total vs annular eclipses. A solar eclipse near perigee shows a large Moon that fully covers the Sun (total); near apogee the Moon is too small and leaves a bright ring (annular). The Moon’s changing distance, paced by the apsides, decides which you get. See eclipses.

Apsidal precession of the planets

Every planet’s orbit precesses too. The steady gravitational tugs of the other planets make each perihelion (the orbit’s closest point to the Sun) creep forward, century after century. The amounts are tiny, measured in arcseconds per century, but they are real, and they were mapped out long before anyone could explain them.

Each orbit comes full circle over tens of thousands to many millions of years, a span called its apsidal period. That motion is almost entirely Newtonian, but general relativity adds a tiny extra advance on top, largest for the planets closest to the Sun. The table shows both:

The apsidal period is the time for the perihelion to circle once relative to the stars, from current secular rates (JPL); it is set almost entirely by the gravitational pull of the other planets, of which the general-relativistic part (last column) is only a tiny slice. *Venus’s orbit is so nearly circular that its perihelion barely moves, so its period is extremely long and not well defined. †Saturn’s perihelion is currently drifting backward. For comparison, the Moon’s apsides come full circle in just 8.85 years.

Mercury’s 43 arcseconds: a clue to relativity

Mercury’s perihelion advances about 574″ per century relative to the fixed stars. Painstaking 19th-century calculations of the other planets’ pull accounted for about 531″, leaving a stubborn 43″ per century that Newton’s gravity simply could not explain. Astronomers even proposed an unseen planet, “Vulcan,” to make up the difference. In 1915, Einstein’s general relativity predicted an extra advance of almost exactly 43″ for Mercury, with no new planet required. It was one of the first and most celebrated confirmations of the theory.