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 motions – like the spin and orbit of most other moons in
our solar system – are, 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 other – though 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
会用引力牵拉调整它的公轨 减缓自转 如果月球
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
causes “moon-seasons” revealing 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.