Tidal Locking | Why Do We Only See One Side of the Moon?

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0:01In 1959, the Soviet spacecraft Luna 3 beamed back images of something Earthlings had never

0:06before seen: the far side of the moon.

0:09We always see the same old side of the moon because the moon rotates exactly once on its

0:14axis each time it orbits Earth.

0:15If it weren’t spinning at all, we’d get at least one 360 degree view of its surface

0:20with each lap.

0:21If it were spinning twice as fast, we’d also see the moon’s entire surface more

0:24than once per orbit.

0:26But instead, our moon’s motions – like the spin and orbit of most other moons in

0:30our solar system – are, remarkably, in perfect sync.

0:34This wasn’t always the case: our best guess is that our own moon formed due to a massive

0:38asteroid impact, and its initial spin and dizzying 10-hour orbit were almost certainly

0:42not in sync with each other – though we don’t know which was faster.

0:46At such close range, Earth’s gravity deformed the moon into a slight oval, with one of its

0:50bulges facing Earth.

0:52Those bulges quickly swung out of alignment, thanks to the moon's asynchronous spin and

0:56orbit, but Earth’s gravity continually squeezed them back again.

1:00What’s more, this gravitational tugging would have influenced the moon’s rotation

1:03rate: if it was spinning more than once per orbit, earth would pull at a slight angle

1:07against the moon’s direction of rotation, slowing its spin; if the moon was spinning

1:12less than once per orbit, Earth would have pulled the other way, speeding its rotation.

1:16Whatever the case, it took just 1000 years for the Earth’s pull to adjust the moon’s

1:20spin enough that one rotation of the moon corresponded to one trip around the earth,

1:25leaving one side forever locked facing away.

1:28We do end up seeing slightly more than that one side, because the moon’s elliptical

1:31orbit gives us peeks beyond its average eastern and western horizons, and its tilted axis

1:36causes “moon-seasons” revealing more of the lunar north or south poles.

1:41But those glimpses only add up to an extra 9%, leaving 41% of the moon hidden from earth.

1:46Satellites, starting with Luna 3, have allowed us to map the rest, but it’s safe to say

1:51that our relationship with the moon is still pretty one-sided.