Our Closest Astronomical Neighbor

The Moon is the only natural satellite of Earth:

orbit distance: 384,400 km from Earth
diameter: 3476 km
mass: 7.35x1022 kg
orbital period: 29.5 days
rotational period: 29.5 days

Called Luna by the Romans, Selene and Artemis by the Greeks, and many other names in other mythologies.

The Moon has been known since prehistoric times. It is the second brightest object in the sky after the Sun. As the Moon orbits around the Earth once per month, the angle between the Earth, the Moon and the Sun changes; we see this as the cycle of the Moon's phases. The time between successive new moons is 29.5 days.

Due to its size and composition, the Moon is sometimes classified as a terrestrial "planet" along with Mercury, Venus, Earth and Mars.

The gravitational forces between the Earth and the Moon cause the tides. The Moon's gravitational pull is stronger on the side of the Earth nearest to the Moon and weaker on the opposite side. Since the Earth and it's oceans are somewhat flexible, they are stretched out along the line toward the Moon. From our perspective on the Earth's surface, we see two small bulges: one in the direction of the Moon and one directly opposite. Since the effect is much stronger in water than the solid crust, the water bulges are higher. And because the Earth rotates much faster than the Moon moves in its orbit, the bulges move around the Earth about once a day giving two high tides per day.

But the Earth is not really fluid either. Friction between the water and the land carries the Earth's bulges slightly ahead of the point directly opposite the Moon. This means that the force between the Earth and the Moon is not exactly along the line between their centers - producing a torque on the Earth and an accelerating force on the Moon! This transfers rotational energy from the Earth to the Moon, slowing down the Earth's rotation by about 1.5 milliseconds/century and raising the Moon into a higher orbit by about 3.8 centimeters per year.

The asymmetric nature of this gravitational interaction is also responsible for the fact that the Moon rotation is locked in phase with its orbit, so that the same side is always facing toward the Earth. Just as the Earth's rotation is now being slowed by the Moon's influence, in the distant past the Moon's rotation was slowed by the action of the Earth. When the Moon's rotation rate was slowed to match its orbital period (so that the bulge always faced toward the Earth) there was no longer an off-center torque on the Moon. Once the torque ended, the Moon's rotation and orbit were stable.