Earth & Space
Why do we mostly see the same side of the Moon?
The Moon turns, yet shows us nearly the same face. Puschel explores the puzzle.
The short answer
The Moon turns once on its axis while completing one trip around Earth. Both take about the same time. That keeps almost the same lunar side facing Earth. The far side still receives sunlight too.
One orbit, one spin
1. Starting to the right of Earth
Blue stands for Earth and grey for the Moon. The orange dot marks one lunar side. Here it points left towards Earth. Remember its direction in the picture.
2. A quarter of the way round
The Moon is now above Earth. The orange dot points down towards Earth. Its direction in the picture has changed: the Moon has also spun on its own axis.
3. Halfway round
Now the Moon is to the left of Earth. The dot points right towards Earth. Since the start, the Moon has completed half a spin and half an orbit.
4. Back at the start
After a whole orbit the Moon is on the right again. The dot points left again. The Moon has also completed one whole spin, keeping the same marked side facing Earth.
All four pictures look at the orbit from the same direction. The dot is an imaginary marker, not a bright lunar phase.
An orbit means going around Earth; a spin means turning on its own axis. These take about the same time for the Moon.
Small variations are omitted, so in reality we say almost the same view. The far side also receives sunlight.
Would a Moon with no spin keep the same side facing Earth?
No. Without a spin, the dot in our drawing would always point in the same direction, such as left. On the other side of Earth it would then point away from Earth. Our Moon spins as it orbits: one spin per orbit.
An imagined Wusel scene
Puschel and Flauschi investigate
Puschel carries a marked ball around an Earth model. First he keeps its mark facing the same wall; from Earth, Flauschi sees different sides. Next he keeps the mark facing Earth. This time the ball has also turned once during its trip.
Try explaining it
What must a Moon model do to keep its mark facing Earth?
Choose a), b) or c), then explain your choice.
- a)Spin twice on its axis during an orbit.
- b)Turn once on its axis during one orbit.
- c)Not spin on its axis during an orbit.
Show the explanation
Correct: b): Turn once on its axis during one orbit.
The Moon turns once on its axis while completing one trip around Earth. Both take about the same time. That keeps almost the same lunar side facing Earth. The far side still receives sunlight too.
Word help
- orbit
- The path of one body around another.
- rotation
- Turning around one's own axis.
- axis
- An imaginary line around which something turns; in machines it may also be a physical part.
- circuit
- A closed path through which electric current can flow.
Explore in more detail
Here you will find longer explanations and further details. You can explore them together.
Understand step by step
How it works – simply explained
The Moon circles Earth
One orbit goes all the way around Earth. The Moon also rotates on its own axis.
Two timings match
One rotation takes about as long as one orbit, keeping nearly the same face towards Earth. Without rotation we would gradually see different faces.
The other side gets sunlight
Far side does not mean permanently dark. It receives sunlight during the month too. A turning diagram shows orientation, not true sizes.
Why the Moon rotates despite showing one face
Imagine walking around a table while keeping your face towards its centre. You must turn your own body as you go. After a complete circuit you have also turned once. The Moon similarly takes about the same time to orbit and rotate, so we see almost the same face. This is a thought model, not a spinning challenge.
References for this explanation: [1]
The key points again
Discovery 1
The Moon rotates on its axis while orbiting Earth.
Source: [1]
Discovery 2
One rotation takes about as long as one orbit. This keeps almost the same side facing Earth.
Source: [1]
Discovery 3
The far side is not always dark. It also receives sunlight during the month.
Source: [1]