What Are the Motions of the Moon?

Due to the Earth's gravitational pull and to the larger forces exerted in the Solar System on the Earth itself, the Moon has three major motions, as well as a set of rocking and nodding motions called libration. The motions of the moon are directed by synchronous rotation and revolution, which always keeps the same side facing Earth.
  1. The Moon's Orbit

    • The primary motion of the Moon is its orbit around the Earth. The Moon circles the Earth once every 27.3 days and follows an elliptical motion, keeping within 5 degrees of the ecliptic, the geometric plane containing the orbit of the Earth around the Sun. When the Moon's path crosses the ecliptic at full or new phases, eclipses occur.

    The Moon In Earth's Orbit

    • Because the Earth and Moon share a common center of gravity, the Moon joins the Earth in its orbit around the Sun, while holding its own orbital pattern around the Earth. Since the Moon's rotation and revolution are synchronous, the Moon always keeps the same side facing the Earth as it traces its circuit around the Sun.

    The Moon's Rotation on Its Axis

    • In addition to its cycles around the Earth and the Sun, the Moon also rotates on its own axis, its orientation fixed by gravitational forces from the Earth. At 655.728 hours, the Moon's rotation period is much slower than Earth's and the rotational speed at its equator is about 10 miles per hour.

    The Moon's Libration

    • Thanks to the elliptical path of the Moon's orbit, which results in a faster speed at perigee (the closest point on the ellipse) and a slower one at apogee (the most distant), and the tilt in the axis of the Moon's rotation, over the course of a month, the Moon also displays a set of movements called libration. The Moon rocks east to west by about 7.9 degrees in the course of a month and "nods" north to south by about 7 degrees over the same period.

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