The fact that China just retrieved rocks from the dark side of the moon confuses me. Does the moon rotate in the same manner as Earth does so we never see the “dark” side of the moon?
In: Planetary Science
it’s not the ‘dark’ side of the moon exactly, it’s the far side of the moon. It does get sunlight, but it always faces away from the Earth.
The moon is rotating at exactly the same speed that the earth and moon are revolving around each other, so the moon never shows us its back.
This is because it’s [“tidally locked.”](https://en.wikipedia.org/wiki/Tidal_locking)
If the moon didn’t rotate, we’d see all of it over the course of a month. It does rotate, but it takes exactly as long to rotate as it does to orbit the earth. You can demonstrate this by putting an “Earth” on a table and taking a (preferably round) “Moon” and moving it around the “Earth” such that the same side always faces it. You’ll see that you had to turn the “Moon” all the way around once to accomplish this.
This happens because the earth and the moon pull on each other with gravity. It’s a *tidal force* that tugs on each body and slows down their rotation (it would happen to the earth too if we gave it enough time). The moon used to rotate faster, but tidal forces slowed it down until it achieved *synchronous rotation.*
This took a couple hundred million years after the moon was formed, which was well before any creature on Earth had evolved eyes…trillions of life-forms have looked at the moon, but none of them saw the other side of it until the middle of the twentieth century.
Synchronous rotation is **common** and all round moons in our solar system are tidally locked to their host planet. Pluto and its moon Charon are tidally locked to each other, and both Mercury and Venus show different forms of synchronicity to the Sun.
Because its orbiting around us it HAS to rotate for it to always have the same side facing us. Its known as being tidally locked, which means it rotates at the exact speed it orbits.
Imagine standing in a room with a friend. The friend walks around you in circles, but is always facing the front of the room.
Do you see the friends face constantly, or do you see all sides of their head?
Because they aren’t rotating, but where they are compared to you is constantly changing, you would be able to see all sides of their head.
Now, instead of constantly facing the front of the room, your friend now constantly turns as they circle you so that they are always facing you.
Now that your friend is turning to face you, you are no longer able to see the back of their head. But this situation requires the friend to constantly turn as well as walk around you. This is what is happening with the moon and only one side ever facing the earth.
Imagine someone holding a shotput and spinning around getting ready to throw it. That person is only ever going to see the side of the shotput that the chain attaches to, the whole shotput is still spinning in circles but the thrower only sees the same part.
Now swap the chain for gravity, the thrower with the Earth and the shotput with the moon.
It does rotate, at the exact same speed it orbits the Earth. This is called being “tidally locked”
Think about how the tides work here on Earth. The ocean closer to the Moon gets pulled up by the Moon’s gravity, causing high tide, and the sides further away have water pulled away, so they experience low tide.
Basically the same thing happened to the early Moon, except instead of oceans, it happened to rock instead of ocean. The friction between these rocks being shifted and squeezed turned the Moon’s rotational energy into heat, which radiates away and the Moon orbits slower. This happened up until the Moon became tidally locked, and now the rock is no longer shifting around because the same side is always facing the Earth, so the same parts of rock are always receiving the same forces, so there’s no shifting.
The Moon does the same thing to the Earth, and it is slowing the Earth’s rotation, but the Earth has so much more inertia that it takes longer for the Earth to slow down.
An example of two mutually tidally locked bodies would be Pluto and Charon, the same sides always face each other.
The moon is tidally locked. It rotates at the exact same rate as it orbits, which is why we always see the one side of it.
Tidal locking is actually extremely common in the universe! Especially when the orbiting body is way smaller than the host. Most of the moons in the solar system are tidally locked, and Pluto and its moon are both locked to each other.
Lots of exoplanets are most likely tidally locked to their stars. Mercury *almost* is; it’s locked in a 3:2 resonance where it spins 3 times in 2 years.
Planets/moons can also very frequently get into resonances with each other. For example, the 3 biggest moons of Jupiter are locked into a 1:2:4 ratio of orbits, and Neptune will always complete 2 orbits in the time it takes Pluto to complete 3.
The moon DOES rotate. It rotates once on its axis in the same time it takes to orbit the earth one time. Thus, the same face of the moon is always towards the earth. If you want to know how it ended up that way, Google is your friend!
They call it the dark side of the moon because it’s radio dark not actually dark. It rotates at the same speed as it orbits so the same side always faces the earth.
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