Why haven’t we seen the other side of the moon?

Answer by Award-Winning Scifi Author C Stuart Hardwick

The far side of the moon was first photographed by Luna 3 in 1959. The above is a recent image captured by NASA’s LRO.

If you are asking why we can only see the near side from Earth, that’s because the moon tidally locked. It used to spin faster than it orbits, but just as its gravity causes a tidal bulge in Earth, our gravity causes a bulge in the moon.

Tidal bulges are carried along by the rotation of the body in question, and so create a slight gravitational imbalance. Today, this imbalance is pushing the moon away as a rate of 3.5 cm per year and slowing Earth’s rotation by about a millisecond per century. In the past, that same process slowed the moon’s rotation until it’s rotational and orbital periods became the same.

Now the moon’s stuck with it’s tidal bulge facing us. Any perturbation that would tend to turn the moon with respect to Earth gets cancelled out by the pull of our gravity on the moon’s bulge, and nothing is ever likely to disturb it enough to break free.

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