Fom our viewpoint on Earth, the moon always shows the same side. There’s no permanently dark side of the moon, but there is a far side that we’d never seen until 1959 when the Russians put the space probe Luna 3 in orbit around the moon. But if the moon is rotating, why don’t we see all of it?
This is a hard one to get your head around as several things are moving at once. The Moon is actually rotating on its own axis. But it is also orbiting the Earth. Over time, the gravity of the Earth has slowed the moon’s rotation down. This is known as tidal locking.
What this means is that although it’s rotating, the Moon always keeps the same face facing towards the Earth. This is down to two important facts:
1. The moon rotates on its own axis, and it takes approximately 28 days to make one full rotation
2. The moon orbits around the earth and takes approximately 28 days to make one full orbit.
As you can see, this means that the time to orbit the Earth and the time to spin on it’s axis are the same. So from Earth, the moon always shows the same side.
Still not convinced? These animations might help!
In the animation below, I have coloured sections of the moon to make it clearer. This is the view from above as it rotates.
As the moon rotates, it is also travelling around the Earth. The Moon has become locked into a pattern – the time it takes to orbit the Earth is the same as the time it takes to make one full rotation.
In the animation below, I’ve taken the spinning moon and made it move in a circle. Suddenly it looks like it’s not rotating, but it is, honest!
As you can see, as the moon travels around the Earth, it is also rotating. The Red/Yellow face is always kept facing towards the Earth.
If we were standing on the Earth looking at the Moon we would only see the Red/Yellow face. We would never be able to see the Blue/Green face.
Another thing we could add into this model is the effect of the Sun. The sun shines on one half of the moon. The side facing away from the Sun is in shadow.
As the moon orbits the Earth the same side is facing the Earth, but the shadow moves across the whole of the moon. If you had a moon base you would spend 14 days in sunshine and 14 days in darkness. There’s no such thing as a permanently dark side of the moon.
(Note – very much not to scale!)
See also – Moon Phases with Oreos
What a great explanation to a question I would have previously struggled to answer!
I couldn’t picture the moon rotating until I saw your animation. You’re a born teacher. Thanks!
I have taken pictures of the moon and they always come out different. The ‘spots’ are in different places. How is it possible??
Because the orbit of the moon is elliptical – it appears to wobble a bit over the course of the year. Wee still see the same face, but it’s not always orientated in exactly the same way. See here: https://svs.gsfc.nasa.gov/10836