The Moon: how it works

	The Moon	Compared with the Earth
Width	3,476 km	A little over a quarter
Mass		About one eighty-first
Pull of gravity at the surface	1.62 metres per second, each second	One sixth
Distance from the Earth	384,400 km on average	About 30 Earths side by side
Nearest and furthest	363,300 km and 405,500 km
One orbit, measured against the stars	27.3 days
From one full Moon to the next	29.5 days
In sunlight, and in the dark	About 127 °C, and about −173 °C

The width is twice the radius NASA gives, and the fraction for the mass is one divided by NASA’s 0.0123. Both were worked out here.

The whole face of the Moon in grey, with large dark smooth patches across the upper half and bright, heavily cratered ground in the lower half. — **A photograph.** The side of the Moon we always see, pieced together from about 1,300 pictures taken by NASA's Lunar Reconnaissance Orbiter in December 2010. The dark plains are the seas. Credit: NASA/GSFC/Arizona State University.

Seas and highlands. The seas are great hollows blasted out by impacts and later flooded with lava, which set into dark, smooth rock. That happened between about 4.2 and 1.2 billion years ago. The bright highlands are older. They are the Moon’s first crust, which set from an ocean of molten rock that once covered the whole Moon, about 4.4 billion years ago. The highlands have been collecting craters ever since, which is why they are so battered.

The collision. The body that hit the Earth has been given a name, Theia. The best evidence for it came back with the Apollo crews, who brought home 382 kilograms of rock and soil. The Moon’s rocks turned out to be very like the Earth’s, and one fine detail of their oxygen matches too closely to be chance. The Moon also has only a small iron core. Both make sense if the Moon was built from the outer, rocky layers of the Earth and of the body that struck it. Rocks from the Moon show that it formed about 60 million years after the Solar System began.

Why one face. The Earth’s gravity pulls a little harder on the near side of the Moon than on the far side. Long ago that pull slowed the Moon’s spin until one turn matched one orbit, and it has stayed that way. Many moons of other planets have ended up the same.

Not a film from a camera: a flight made from real measurements. Every hill and crater here is where NASA’s Lunar Reconnaissance Orbiter measured it to be, and the surface is built from its photographs. The tour passes the great basins, the bright young crater Tycho, the south pole and the valley where Apollo 17 landed. About 5 minutes, with a spoken commentary. Credit: NASA’s Scientific Visualization Studio.

The other face of the Moon in grey: craters crowded edge to edge, with only two or three small dark patches. — **A photograph.** The far side of the Moon, which can never be seen from the Earth, from pictures by NASA's Lunar Reconnaissance Orbiter. Credit: NASA/GSFC/Arizona State University.

Two different faces. The crust is thicker on the far side than on the near side, and the near side has had far more volcanoes. That is why the seas are nearly all on the side that faces us. Why the two halves came to differ is still being worked out.

Slowly leaving. The Moon raises the tides in our seas, and the tides push back. The effect is to move the Moon away from us by 3.8 centimetres a year.

People on the Moon. Six Apollo missions landed, and two people from each walked on the surface. The first were Neil Armstrong and Buzz Aldrin, on 20 July 1969. The last left in December 1972.

Water. The Moon is very dry, but not completely. There is ice in craters near the poles whose floors never see the Sun.

How it works

How it works

Where this comes from