I never thought about this until recently, but our Moon is constantly rotating in such a way that it happens to perfectly be "looking at us" at all times. That sounds incredibly unlikely to me.

Couldn't we just as well have had a moon which constantly shifts patterns by rotating in a different manner, rather than us always seeing the same craters on the "moon's front side"?

I can very much understand that this mislead people "in olden times" to assume that this universe is constructed in a very different way from what it actually is. If the moon had been "shifting in pattern" from day to day, or even hour to hour, or even minute to minute, it would be much easier to conclude that it's a spinning orb far away, orbiting our orb. Or at least that's what it feels like to me.

The fact that the moon is always "looking at" us seems highly unlikely. It's rotating around us, but also rotating itself in exactly such a way that it appears to be "fixed" from our surface as we look at it. It's very strange to me.

How did we end up with such a moon? Wouldn't the far, far more likely case be a "spinning" moon rather than "fixed" one? Maybe the moon would orbit us but not rotate at all? (I'm unsure what that would look like from here.)

Or is there something I'm missing here?

  • 1
    $\begingroup$ The effect is not random chance. Its not pure luck that the moon just happens to have a rotation period that is exactly the same as its orbital period. Its called tidal locking (see also here) and is a rather well-researched phenomenon. $\endgroup$
    – Polygnome
    Jun 22, 2020 at 7:07
  • 1
    $\begingroup$ Does this answer your question? What causes objects to become tidally locked? $\endgroup$
    – Polygnome
    Jun 22, 2020 at 7:08
  • 2
    $\begingroup$ Isn't it absurdly unlikely that our Moon would constantly be “facing” us? As all of the larger moons in the solar system are similarly locked so that one face faces the plane they are orbiting, it's obviously not absurdly unlikely. You ended with Or is there something I'm missing here? The answer is a very emphatic yes. $\endgroup$ Jun 22, 2020 at 7:12

1 Answer 1


The key concept is called Tidal locking. Earth's gravity forced the Moon to rotate in that way during the first few tens of millions of years after the Moon formed and then kept it that way.

The wikipedia page describes the mechanism:

Consider a pair of co-orbiting objects, A and B. The change in rotation rate necessary to tidally lock body B to the larger body A is caused by the torque applied by A's gravity on bulges it has induced on B by tidal forces.[5]

The gravitational force from object A upon B will vary with distance, being greatest at the nearest surface to A and least at the most distant. This creates a gravitational gradient across object B that will distort its equilibrium shape slightly. The body of object B will become elongated along the axis oriented toward A, and conversely, slightly reduced in dimension in directions orthogonal to this axis. The elongated distortions are known as tidal bulges. (For the solid Earth, these bulges can reach displacements of up to around 0.4 metres (1.3 ft).[6]) When B is not yet tidally locked, the bulges travel over its surface due to orbital motions, with one of the two "high" tidal bulges traveling close to the point where body A is overhead. For large astronomical bodies that are nearly spherical due to self-gravitation, the tidal distortion produces a slightly prolate spheroid, i.e. an axially symmetric ellipsoid that is elongated along its major axis. Smaller bodies also experience distortion, but this distortion is less regular.

The material of B exerts resistance to this periodic reshaping caused by the tidal force. In effect, some time is required to reshape B to the gravitational equilibrium shape, by which time the forming bulges have already been carried some distance away from the A–B axis by B's rotation. Seen from a vantage point in space, the points of maximum bulge extension are displaced from the axis oriented toward A. If B's rotation period is shorter than its orbital period, the bulges are carried forward of the axis oriented toward A in the direction of rotation, whereas if B's rotation period is longer, the bulges instead lag behind.

Because the bulges are now displaced from the A–B axis, A's gravitational pull on the mass in them exerts a torque on B. The torque on the A-facing bulge acts to bring B's rotation in line with its orbital period, whereas the "back" bulge, which faces away from A, acts in the opposite sense. However, the bulge on the A-facing side is closer to A than the back bulge by a distance of approximately B's diameter, and so experiences a slightly stronger gravitational force and torque. The net resulting torque from both bulges, then, is always in the direction that acts to synchronize B's rotation with its orbital period, leading eventually to tidal locking.

The same process is, very slowly, slowing the Earth's rotation to synchronize with the Moon, although I'm not sure that process will ever complete.


Not the answer you're looking for? Browse other questions tagged .