Moon

The Moon follows an elliptical orbit around Earth at an average distance of about 384,400 kilometres. It rotates once in roughly the time it takes to orbit Earth, a condition called synchronous rotation, so observers on Earth usually see the same hemisphere. The far side is not permanently dark: sunlight illuminates different portions of the Moon as it moves around Earth. Those changing viewing angles create the phases, from new moon to full moon and back. The Moon's gravity, together with the Sun's, is a major influence on Earth's ocean tides.

The lunar surface preserves a long record of impacts and volcanic activity. Bright, elevated highlands contrast with broad, darker plains called maria; these plains formed when lava flooded some large impact basins and later cooled. The Moon is a differentiated body with a crust, mantle and core. It has only a very thin, weak atmosphere, so its surface is not shielded from meteoroid impacts or the Sun's radiation as Earth's surface is by its atmosphere.

The leading family of explanations holds that the Moon formed from debris produced by a massive collision involving the young Earth about 4.5 billion years ago. In NASA's account, the impactor may have been a single body about the size of Mars or several objects; the precise details of the event remain under investigation. Lunar rocks returned by the Apollo missions helped researchers test formation models: their chemistry and other evidence indicate that the early Moon was once molten and that its materials have close similarities to Earth's.

People have observed the Moon for millennia, using its recurring phases to track time and mark calendars. Robotic spacecraft and the crewed Apollo missions transformed it from a distant object of observation into a world that can be studied directly. Apollo astronauts returned about 382 kilograms of lunar material, while later orbiters and landers have mapped the surface and examined its environment. These investigations make the Moon an important natural record of the processes that shaped Earth and the early Solar System.