Why Do We Always See the Same Side of
the Moon?
Pure physics can explain
this one.
By Adam Hadhazy in
Discover Magazine
Q: Why does the moon
always present the same face to us? I find it impossible to believe that this
could happen by chance. — Michael Connelly, Toronto
A: Nope, not by chance — it’s pure
physics.
For starters, the moon is not stuck in
place with one side facing us. Our lunar companion rotates while it orbits
Earth. It’s just that the amount of time it takes the moon to complete a
revolution on its axis is the same it takes to circle our planet — about 27
days. As a result, the same lunar hemisphere always faces Earth.
How’d this come to be? In a word:
gravity. The moon’s gravity slightly warps our planet’s shape and gives us
tides. Likewise, Earth tugs at the moon, creating a rocky, high-tide “bulge”
facing us. That bulge ended up working like a brake, slowing the moon’s spin
down to the current rate, so the lunar high tide permanently faces us.
When that happened,
about 4 billion years ago, the moon became “tidally locked,” and it has
presented us the same visage ever since.
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