Moon’s motion includes more than just circular motion or rotation around the Earth. Here are some other types of motion that the Moon exhibits:
- Orbit around the Earth: The Moon orbits the Earth in an elliptical path. It completes one revolution relative to the Vernal Equinox and the stars in about 27.32 days (a tropical month and sidereal month) and one revolution relative to the Sun in about 29.53 days (a synodic month). The Moon’s orbit is close to the ecliptic plane instead of Earth’s equatorial plane, and it is inclined by about 5.1° with respect to the ecliptic plane. This inclination is responsible for the Moon passing above or below the Sun during eclipses.
- Tidal evolution: The Moon’s orbit is gradually evolving due to the conservation of angular momentum. The Earth-Moon distance is increasing at a rate of approximately 38 millimeters per year. As a result, Earth’s axial rotation is slowing down, lengthening the day by approximately 24 microseconds every year. This tidal evolution is caused by the gravitational interaction between the Earth and the Moon.
- Elongation: Elongation refers to the Moon’s angular distance east of the Sun at any given time. It determines the Moon’s phase and position in the sky. At new moon, the elongation is zero, and the Moon is in conjunction with the Sun. At full moon, the elongation is 180°, and the Moon is in opposition to the Sun. When the elongation is either 90° or 270°, the Moon is in quadrature.
- Precession: The Moon’s orbit experiences apsidal precession, which is the rotation of the orbit within the orbital plane. The major axis of the Moon’s orbit makes one complete revolution every 8.85 Earth years, or 3,232.6054 days. This precession is distinct from the nodal precession of the orbital plane and the axial precession of the Moon itself.
- Libration: The Moon is in synchronous rotation with the Earth, meaning it always keeps the same face toward Earth. However, due to its eccentric orbit, there are slight variations in the Moon’s rotation and orbital motion, leading to libration. There are several types of libration:
- Optical libration in longitude: The Moon’s apparent size changes as it moves toward and away from an observer on Earth. This creates a perspective effect, allowing us to see up to eight degrees of longitude of the Moon’s far side.
- Optical libration in latitude: The Moon’s axis of rotation is inclined, creating a perspective effect that allows us to see almost 7° of latitude beyond the pole on the Moon’s far side.
- Diurnal libration: Observers at the Earth’s equator who observe the Moon throughout the night move laterally by one Earth diameter, allowing them to view an additional one degree’s worth of lunar longitude.
- Physical librations: These are actual nutations of the Moon’s rotation axis in space, but they are very small.
- Path of Earth and Moon around the Sun: When viewed from the north celestial pole, the Moon orbits Earth counterclockwise, and Earth orbits the Sun counterclockwise. The Moon and Earth also rotate on their own axes counterclockwise. The Sun’s gravitational effect on the Moon is more than twice that of Earth’s, causing the Moon’s trajectory to always be convex as seen from a great distance outside the Earth-Moon solar orbit.
It’s important to note that these motions can be visualized from different perspectives and are described differently depending on the reference frame. The Moon’s motion is complex and influenced by various factors, including the gravitational interactions between the Earth, Moon, and Sun.
