Moon Essentials: Orbit

The visualizations on this page display the shape and tilt of the Moon's orbit and the ways these change over the course of eight and a half years. Initially, we see the orbit edge-on, but this quickly moves to a view from above (or north of) the plane of the Earth's orbit around the Sun, called the ecliptic, here colored purple. The light blue ring represents the plane of the Moon's orbit, which is tilted about five degrees to the ecliptic. The darker half is below (south of) the ecliptic. The yellow arrow points toward the Sun.

The orbit itself, the white line inside the light blue ring, is almost circular. Drawn so that it fills a standard sheet of paper, the orbit deviates from a perfect circle by less than the width of a pencil point. But it is in fact an ellipse, with the Earth not at the center but at one of the two foci. As a result, every month brings the Moon through near and far points called perigee and apogee. The thickness of the light blue ring represents the range of distances bracketed by these two points. Collectively, perigee and apogee are called the apses, or often the fancier apsides (AP-sih-deez).

Another pair of important points on the Moon's path are the nodes. These are the points where the orbit intersects the ecliptic. At the ascending node, the Moon crosses the ecliptic while moving north; at the descending node it's moving south. Solar and lunar eclipses can only happen when the Moon is near one of the nodes. At other places in its orbit, it is too far above or below the ecliptic to line up with the Earth and Sun.

Over time, both the nodes and the apses precess or rotate, although they do this in opposite directions. The nodes precess clockwise when viewed from the north, while the apses precess counterclockwise. In both cases, this motion is caused by the perturbing effect of the Sun's gravity. The more common mental image of these motions is the simplified model depicted in the first animation on this page, in which the precession occurs at a uniform rate. In reality, the nodal precession speeds up and slows down, depending on the angle of the nodes to the Sun, while the true apogee and perigee hop around somewhat chaotically, as the second animation shows.

The Moon's elliptical orbit is constantly being distorted by the Sun and, to a lesser extent, by other bodies in the solar system. The numbers you might look up that describe the orbit – the semimajor axis, eccentricity, inclination, precession rates, and the lengths of synodic, sidereal, draconic, and anomalistic months – are all averages. They vary continuously, a fact that for hundreds of years vexed astronomers trying to precisely predict the Moon's motion. Since the 1960s, the Moon's position has been precisely measured by bouncing lasers off of retroreflectors left on the Moon by Apollo astronauts and Soviet landers. Using these measurements, sophisticated computer models of the solar system can predict the Moon's position to a precision of centimeters.