Guided trip

Why the Moon changes shape

The Moon's month from space, then from your street. This trip moves the clock.

Open the trip: Why the Moon changes shape

The Moon's month, from above

We are far above the north pole. The ring is the path the Moon takes round the Earth, once a month, thirty Earths out. Both worlds are drawn larger than they are here, so you can see that the Sun lights the same half of each of them, all the time.

A sliver that grows

Now the Moon as the Earth sees it, with ten hours passing every second. Just after new Moon it stands almost between us and the Sun, so its lit half faces away and we see only a thin edge. Each evening it has moved a little further round, and shows us more.

Half, then full

A week after new we see half of the lit side. That is called the first quarter. A week later the Moon is on the far side of the Earth from the Sun, and the whole face we see is in daylight. That is full Moon, and it rises as the Sun sets.

And back again

After full, the dark comes in from the other edge. The Moon rises later every night, until it is a thin crescent in the dawn. Twenty-nine and a half days after it began, it is new again.

The side we never see

The Moon turns once for every trip round the Earth, so the same face always looks at us. This is the other one. It gets just as much sunlight as the side we know: there is no dark side of the Moon, only a far side.

The phases are not the Earth's shadow

This is the Earth's shadow on the Moon, and it is rare. The Moon's path is tilted five degrees against ours, so at most full Moons it passes above or below the shadow. A few times a year they line up, and that is a lunar eclipse.

The Moon over you tonight

This is the Moon from your own ground tonight. The lit side always points at the Sun, even when the Sun has set. Look again tomorrow at the same hour: it will be a little further east, and a little different in shape.

Everything is drawn where it really is, from the planets’ own orbits (astronomy-engine).