# Q & A: The dark side of the Moon

Q:
If the moon rotates on its axis, then why do we always see the same side of the moon?
- Anonymous
A:
The short answer is: when the Moon rotates one cycle around its axis, it also completes one revolution around the Earth! Here's a diagram showing what's happening during one cycle:

(the distance is not to scale) Let's say the Moon starts out at the right of the diagram. During one revolution around the Earth, the "white" side of the Moon goes from left -> bottom -> right -> top and back to the left side of the Moon, completing one rotation around its axis.

The fact that the period of revolution is the same as the period of rotation for the Moon is not a coincidence. The gravitational field of the Earth is stronger on the part of the Moon nearer the Earth and weaker on the far side. That causes the Moon to distort its shape a little compared to what it would otherwise be. We can easily see the same sort of effect of the Moon on the Earth, since the liquid part of our planet distorts a lot from this "tidal force".

Back when the Moon was rotating enough to expose different sides to the Earth, the creaking of those tidal distortions provided a source of friction, gradually pulling energy out of the rotation until it was synched with the orbit. Similar effects are currently slowly changing the Earth's rotation and the Moon's orbit around it.

- Tsung +mbw

(published on 07/04/2011)

## Follow-Up #1: torque on and rotation of moon

Q:
Does the moon rotate on its axis? There are many articles on the internet that state that the moon spins on its axis. Surely these articles are wrong? The force acting on the moon is centripetal force, and this force works in 1 direction only. The moon is going in a straight line, but this line is bent by the effect of gravity. What is the force that is making the moon 'spin on its axis'? Similarly, if I fly an airplane around the Earth, the belly of the plane is always facing the Earth. The airplane engines push it forward only. What is the force that is 'twisting' the plane around its axis? If there's no force ... there's no spin!
- Joel (age 40)
Perth, Australia
A:
Actually the Moon does spin, just fast enough to keep the same face always toward the Earth.

Think of ordinary velocity. You don't need a force to keep the same velocity, you need a force to change the velocity. The same principle applies to spinning. The Moon doesn't need any torque to keep spinning. Torque is needed to change the spinning.

There's almost no torque on the Moon now, but in the past when it spun faster there was, caused by tidal forces from the Earth. That's described in the earlier parts of the thread in which we've put this question.

What about that airplane? The airplane wings are acted upon by the air, which gives enormous forces that can twist it upwards or downwards. The pilots control these forces (and hence torques) by moving ailerons on the wings, as you can see out the plane window. The reason the plane turns just enough to stay pointing about level is not that the torques are intrinsically small but that the pilots carefully adjust them to avoid crashing.

Mike W.

(published on 05/01/2013)

## Follow-Up #2: Why do we always see the same part of the moon?

Q:
Why do we always see the same part of the moon at night though it is on movement around the earth? Simply to say, why not the other part sometime while being on its orbiting the earth, please?
- Md. Anwar Hossain (age 44)
A:

This phenomenon is called 'tidal locking'.   It occurs in many  planetoidal systems.  For example Pluto and one of its moons Charon are both locked in a tidal embrace.

I suggest you look at the excellent detailed explanation in

LeeH

We also have some older answers on this:

MW

(published on 09/29/2015)