First look
The idea, in plain words
The Moon is the only world whose face you can study with your own eyes. The dark patches are plains of old lava. People once took them for water, and they are still called seas. The bright parts are older ground, covered in craters.
It is a big moon for a planet of our size. The Moon is a little more than a quarter as wide as the Earth, and it is 384,400 kilometres away. Thirty Earths would fit side by side in the gap. Light crosses it in a second and a quarter.
The Moon was made in a collision. About 4.5 billion years ago, when the Earth was new, something perhaps the size of Mars ran into it. The wreckage went into orbit round the Earth and gathered into the Moon. The rocks the astronauts brought home are the evidence: they are a very close match for the rocks of the Earth.
We only ever see one side. The Moon turns once in exactly the time it takes to go once round us, about 27 days, so the same face is always turned our way.
Nobody had seen the far side until a Soviet spacecraft photographed it in 1959. It was a surprise. It has hardly any dark seas, only craters upon craters. It is not dark, though. It gets as much sunlight as the side we see.
Twelve people have walked on the Moon, all of them between July 1969 and December 1972. Their footprints are still there. The Moon has no air and no weather to wear them away.
Go deeper
That was the idea. The same subject is written again below for readers who want more, each on a page of its own.
Words used here
- Moon
- A natural body that orbits a planet, a dwarf planet or an asteroid.
- Tidal locking
- When a moon turns exactly once for each orbit, so the same face always points at its planet.
- Impact crater
- A bowl blasted out of a surface by something striking it at many kilometres per second.
- Regolith
- The layer of loose dust and broken rock that covers the solid ground of an airless world.
- Phase
- How much of the sunlit half of a moon or planet is turned towards us.
- Tide
- The stretching of one body by another's gravity, which pulls harder on the near side than on the far side.
Where this comes from
The words are written for this site. The pictures are drawn by the page from the figures in the lesson, and each one says underneath what it is based on.