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

  • before seen: the far side of the moon.

  • We always see the same old side of the moon because the moon rotates exactly once on its

  • axis each time it orbits Earth. If it weren’t spinning at all, we’d get at least one 360

  • degree view of its surface with each lap. If it were spinning twice as fast, we’d

  • also see the moon’s entire surface more than once per orbit. But instead, our moon’s

  • motionslike the spin and orbit of most other moons in our solar systemare, remarkably,

  • in perfect sync.

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

  • asteroid impact, and its initial spin and dizzying 10-hour orbit were almost certainly

  • not in sync with each otherthough we don’t know which was faster.

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

  • bulges facing Earth. Those bulges quickly swung out of alignment, thanks to the moon's

  • asynchronous spin and orbit, but Earth’s gravity continually squeezed them back again.

  • What’s more, this gravitational tugging would have influenced the moon’s rotation

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

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

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

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

  • spin enough that one rotation of the moon corresponded to one trip around the earth,

  • leaving one side forever locked facing away.

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

  • orbit gives us peeks beyond its average eastern and western horizons, and its tilted axis

  • causesmoon-seasonsrevealing more of the lunar north or south poles. But those

  • glimpses only add up to an extra 9%, leaving 41% of the moon hidden from earth. Satellites,

  • starting with Luna 3, have allowed us to map the rest, but it’s safe to say that our

  • relationship with the moon is still pretty one-sided.

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

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