Moon

The Moon takes exactly the same time to rotate on its axis as it does to orbit Earth (27.3 days), a phenomenon called synchronous rotation caused by tidal forces. This means we never see the far side from Earth's surface. The far side was first photographed by the Soviet Luna 3 probe in 1959.

The Moon is moving away from Earth at about 3.8 cm per year. Around 1.4 billion years ago, it was much closer and Earth days lasted only 18 hours. Over time it will keep receding, and total eclipses will become impossible once its apparent disk is too small to cover the Sun.

Between 1969 and 1972, six Apollo missions brought 12 astronauts to the lunar surface. Neil Armstrong was the first on July 20, 1969. No human has returned since, although several agencies are working on new crewed missions.

Seismometers installed by the Apollo missions detected lunar seismic activity, known as moonquakes. They are caused by temperature changes, meteorite impacts and Earth's gravitational influence. Some moonquakes last half an hour, much longer than earthquakes, because the Moon's cold, dry rock vibrates for longer.

Orbital observations and the intentional impact of the LCROSS probe in 2009 confirmed water ice in permanently shadowed craters near the lunar poles. This discovery is key for future lunar bases, since water could be used for drinking, oxygen production and fuel.

The most accepted theory is the Giant Impact: about 4.5 billion years ago, a Mars-sized protoplanet called Theia collided with the young Earth. Debris thrown into space gradually gathered by gravity and formed the Moon. This explains why the Moon has a composition similar to Earth's crust and why its iron core is proportionally small.

The dark patches visible to the naked eye are called maria, or seas, although they contain no water. They are basaltic plains formed by ancient volcanic eruptions that flooded large crater basins between 3 and 4 billion years ago.

Lunar gravity pulls ocean water toward the side of Earth closest to the Moon, creating a bulge of water, or high tide. The opposite side also has high tide because of inertia. As Earth rotates, these bulges create the cycles of high and low tides. The Sun also contributes, though to a lesser degree.

Artemis is NASA's program to return astronauts to the Moon. Its goal is to establish a sustained human presence in the lunar environment, including an orbital station called Gateway and surface bases.

The Moon does not have a global magnetic field like Earth. However, studies of lunar rocks show that around 3.5 billion years ago it did have one, generated by an active liquid core. Today, local residual magnetization remains in some crustal regions, but there is no dipole field protecting it from the solar wind.