by Steven Bee
Our solar system began as a great swirling cloud of dust, gas, ice, rocks and metals. As these materials began to “clump” together, they would impact each other & set them spinning (like when you hit a pool ball with the stick with a glancing blow). Our Earth is spinning particularly fast, because it is theorized that early in our formation, another planet the size of Mars hit into us, causing us to spin even faster (incidentally, this impact blew enough material off of our planet to form our moon - or so the current best theory holds).
The reason why the moon does not spin in relation to us, is that it is tidally locked to us. This will happen anytime an object is orbiting very close to another large gravity source. Our moon is tidally locked to us, so the same face is always pointing towards us. Mercury, the closest planet to the sun, is tidally locked to the Sun. Other moons, such as Titan - Saturn’s closest moon, is tidally locked to it.
The reason tidal locking occurs, is because the side closer to the great gravity source experiences a stronger pull of gravity than the farther side. For the moon, the close side experiences a fraction of a percent more gravitational pull to the Earth than the farther side, since the pull of gravity decreases with distance.
This acts as a brake on the rotation of the smaller object (well, on both objects, but it affects the smaller object more because it has less mass to brake). The moon would have spun (in relation to us) originally, but after several hundred million years or a billion or two, it became tidally locked. During this same time, the Earth’s rotation has been experiencing this same braking, slowly becoming tidally locked to the moon. A few billion years ago, the Earth’s day would have been maybe 17 hours instead of the 24 we have today.
Eventually, given enough time, the same face of the Earth would always face the moon as well, making us both tidally locked to each other, and our days will be the same amount of time it takes for the moon to go around us once (currently 27 days - that will veeery slowly increase, as the moon drifts away from us about 4 cm per year). The amount of time this tidal locking would take, however, is likely longer than it will take for our sun to use up its nuclear fuel, become a red giant, and destroy our home (in about 5 billion years or so).
Those are great questions, but let’s take the second one first.
The Moon spins once per revolution around Earth
The Moon does spin (rotate). It rotates once per revolution around the Earth. That is what it means to be gravitationally/tidally locked in its orbit around Earth.
Imagine yourself sitting on a merry-go-round facing the center. Your face is the Moon. The center of the merry-go-round is Earth. As the merry-go-round spins, you are always facing the Earth. The back of your head (the far side of the Moon) is never visible from Earth (center of merry-go-round). If you weren’t rotating, someone at the center of the merry-go-round could see the back of your head when you got to the opposite side of your orbit. This may help—think of your face as that red part on the Moon:
Why do planets and other things spin (rotate)?
Gravity (whether you consider it as a force or a space-time curvature) leads to things with mass drawing toward each other. That attraction would defy all odds to be perfectly equal in attraction for all particles approaching from all angles and interacting with their common center of gravity (barycenter) and with one another. The differences in attraction vectors, no matter how slight, aggregate into angular momentum or spin. –Quora
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