The Moon

The Moon is the only natural satellite of Earth. It has no formal name other than "The Moon" although it is occasionally called Luna (Latin for moon) to distinguish it from the generic "moon". Its symbol is a crescent. Apart from the word lunar, the terms selene/seleno and cynthion (from the Lunar deities Selene and Cynthia) refer also to the Moon (aposelene, selenocentric, pericynthion, etc.).

The average distance from the Moon to the Earth is 384,403 kilometres (238,857 miles). The Moon's diameter is 3,476 kilometres (2,160 miles).

In 1969, Neil Armstrong and Buzz Aldrin became the first humans to land on the Moon.

The two sides

The Moon has synchronous rotation. As a result, one side of the Moon (the "near side") is permanently turned towards Earth. The other side, the "far side", mostly cannot be seen from Earth, except for small portions near the limb which can be seen occasionally due to libration. Most of the far side was completely unknown until the era of space probes. This synchronous rotation is a result of torque having slowed down the Moon's rotation in its early history, a process known as tidal locking.

The far side is sometimes called the "dark side". In this case "dark" means "unknown and hidden" and not "lacking light"; in fact the far side receives as much sunlight as the near side, but at opposite times. Spacecraft are cut off from direct radio communication with the Earth when on the far side of the Moon.

One distinguishing feature of the far side is its almost complete lack of maria (singular: mare), which are the dark albedo features.

Orbit

The Moon makes a complete orbit about once a month. Each hour the Moon moves relative to the stars by an amount roughly equal to its angular diameter, or by about 0.5°. The Moon differs from most satellites of other planets in that its orbit is close to the plane of the ecliptic and not in the Earth's equatorial plane.

The time it takes to make a complete orbit with respect to the stars is a sidereal month; the time it takes to reach the same phase is called a synodic month. These differ because in the meantime the Earth and Moon have both orbited some distance around the Sun.

The gravitational attraction that the Moon exerts on Earth is the cause of tides in the sea. Tidal flow is synchronized to the Moon's orbit around Earth. This synchronous rotation is only true on average because the Moon's orbit has definite eccentricity. When the Moon is at its perigee, its rotation is slower than its orbital motion, and this allows us to see up to an extra eight degrees of longitude of its East (right) side. Conversely, when the Moon reaches its apogee, its rotation is faster than its orbital motion and reveals another eight degrees of longitude of its West (left) side. This is called longitudinal libration. The tidal bulges on Earth caused by the Moon's gravity lag behind the apparent position of the Moon, due to the impedance of the ocean system - effectively the inertia of the water and the friction as it slides over the ocean bottom and into or out of bays and estuaries. As a result, some of the Earth's rotational momentum is gradually being transferred to the Moon's orbital momentum, resulting in the Moon slowly receding from Earth at the rate of approximately 38 mm per year. At the same time the Earth's rotation is gradually slowing, the Earth's day thus lengthens by about 15 µs every year. Because the lunar orbit is also inclined to the Earth's equator, the Moon seems to oscillate up and down (as a person's head does when indicating "yes") as it moves in celestial latitude (declination). This is called latitudinal libration and reveals the Moon's polar zones over about seven degrees of latitude. Finally, because the Moon is only at about 60 Earth radii distance, an observer at the equator who observes the Moon throughout the night moves by an Earth diameter sideways. This is diurnal libration and reveals about one degree's worth of lunar longitude.

Earth and Moon orbit about their barycentre, or common centre of mass, which lies about 4700 km from Earth's centre (about 3/4 of the way to the surface). Since the barycentre is located below the Earth's surface, Earth's motion is more commonly described as a "wobble". When viewed from Earth's North pole, Earth and Moon rotate counter-clockwise about their axes; the Moon orbits Earth counter-clockwise and Earth orbits the Sun counter-clockwise.

It may seem curious that the inclination of the lunar orbit and the tilt of the Moon's axis of rotation are listed as varying considerably. One must be reminded here that the orbital inclination is measured with respect to the primary's equatorial plane (in this case the Earth's), and that the axis of rotation's tilt is measured with respect to the normal to the satellite's orbital plane (the Moon's). For most planetary satellites, but not for the Moon, these conventions model physical reality and the values are therefore stable.