why we only see one side of the moon?
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The moon is tidally locked with the earth. This means it rotates once per orbit around the earth meaning the same face is always pointed toward the earth.
Because the moon rotates at the same speed it revolves. [This GIF](https://spaceplace.nasa.gov/review/all-about-the-moon/moon_rotation.en.gif) makes it pretty clear, I think.
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It is like the earth is a dad and the moon is his good daughter who he has grabbed on his arms and it’s spinning with her, like in movies.
If I walk in a circle around you but do so while facing you the entire time, you are only going to see my face and not the back of my head. Same situation with the moon.
The Moon is tidally locked to the Earth. It takes the same amount of time for the Moon to make one revolution (a lunar day) as it takes to make one orbit of the Earth.
The Moon’s gravity pulls on the oceans as the Earth spins under the orbit of the Moon. The tug of the moon is strongest where the Moon is closest, and weakest on the opposite side of the planet. At those two points there will be high tide, and 90° from those points will be low tide. As the Earth spins, the action of dragging all that water around slows the rotation of the Earth ever so slightly.
In the same manner the Earth pulls on the Moon, but the Earth is much larger and has actually pulled the rock of the Moon into a slight egg shape. In the distant past when the Moon rotated faster, dragging that egg shape around the Moon caused it to spin slower until the rotation of the Moon exactly matched its orbital period.
It’s like when you stand on a merry-go-round at your local park, if you’re the moon and you consider the earth to be at the centre of the merry-go-round.
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