The Moon

The Moon is about a quarter of Earth's diameter, which makes it unusually large compared with its planet. It orbits at an average distance of about 384,400 kilometres, and light reflected from its surface takes just over a second to reach us.

The Moon has almost no atmosphere, so there is no weather to erode its surface. Craters formed billions of years ago remain clearly visible. The dark patches, called maria or seas, are vast plains of ancient solidified lava.

The leading explanation, the giant impact hypothesis, holds that about 4.5 billion years ago a Mars-sized body, sometimes called Theia, collided with the young Earth. Debris thrown into orbit gathered together to form the Moon.

Evidence for this includes Moon rocks brought back by the Apollo missions, which have a chemical makeup very similar to Earth's mantle, and the Moon's small iron core, which fits the idea that it formed mainly from Earth's outer rocky layers.

The Moon produces no light of its own; it reflects sunlight. Half of it is always lit by the sun, but as it orbits Earth over about 29.5 days we see different portions of that lit half, producing phases: new moon, waxing crescent, first quarter, waxing gibbous, full moon, and then the waning phases back to new.

The Moon always shows us the same face because it is tidally locked: it rotates exactly once for each orbit. The side we never see from Earth is often called the dark side, but it receives just as much sunlight as the near side. It was first photographed by the Soviet probe Luna 3 in 1959, and in 2019 China's Chang'e 4 made the first soft landing there.

The Moon's gravity pulls on Earth's oceans. Water on the side facing the Moon is pulled slightly more than Earth's centre, and water on the far side is pulled slightly less, creating two bulges. As Earth rotates through these bulges, most coastlines experience two high tides and two low tides roughly every day.

The sun affects tides too. When the sun, Moon and Earth line up at new and full moon, their pulls combine to give larger spring tides; when they are at right angles, the smaller neap tides occur. Tidal friction is also slowly lengthening Earth's day and pushing the Moon away by about 3.8 centimetres a year.

• A solar eclipse happens when the Moon passes between Earth and the sun, casting its shadow on Earth.
• By coincidence, the sun is about 400 times wider than the Moon and about 400 times farther away, so they appear almost the same size in the sky, allowing total solar eclipses.
• A lunar eclipse happens when Earth passes between the sun and the Moon, and Earth's shadow falls on the Moon, often turning it a coppery red.
• Eclipses do not happen every month because the Moon's orbit is tilted about 5 degrees relative to Earth's orbit around the sun.

On 20 July 1969, during NASA's Apollo 11 mission, Neil Armstrong and Buzz Aldrin became the first people to walk on the Moon, while Michael Collins orbited above. Between 1969 and 1972, twelve astronauts walked on the lunar surface across six missions and returned about 382 kilograms of rock and soil.

Robotic missions have since found water ice in permanently shadowed craters near the poles, which could supply drinking water, oxygen and rocket fuel for future explorers. Several countries and companies are now planning crewed and uncrewed missions, including NASA's Artemis programme, which aims to return astronauts to the Moon.