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Why do we see the same part of the moon’s surface?

Why do we see the same part of the moon’s surface? I cannot imagine that it is by mere chance that the rotations of the earth and the moon are perfectly synchronised. However, i cannot figure out what causes this unchanging connection.
Robert Roach from Cheshire (Aged 55+)

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2 Responses

  1. Its not a coincidence, the rotation of the moon has been slowed down until it is locked into making one revolution each time it orbits the Earth. The reason is tides.

    You are familiar with tides in the oceans twice per day. What is less obvious is that the land also rises and falls with the tides (although not very much compared to the ocean tides because water in the oceans can move far more easily). What is even less obvious is that there are tides in the solid rock of the moon too, but because the Earth is so much larger than the moon, the Earth’s effect on the moon is much larger than the moon’s effect on the Earth.

    These tides lose energy from the rotation of the moon slowing it down until it became locked relative to the Earth. We see the same thing happening on many other bodies in the solar system.

    Another curious fact about this is that although energy can be ‘lost’ from the Earth-moon system by turning it to heat, another quantity called angular momentum is ‘conserved’ – meaning the total ‘rotation’ can’t be changed easily if we look at the Earth and moon together.

    So if the moon’s spin on its axis slows down, the angular momentum (if you like, a measure of how the Earth and moon rotate together) has to stay the same and the only way this can happen is if the moon gets further away from the Earth. The usual analogy is to think of an ice skater spinning rapidly with her arms tight in to her body – when she extends her arms out, she slows down.

    The moon (back billions of years ago!) used to be much closer to the Earth, it would have been a most impressive sight but has got much further away. It is receding at almost 4cm every year from us and this has been measured by firing lasers at reflectors left on the moon’s surface by the Apollo astronauts.

    Another way this can be confirmed is that the tides are also slowing down the Earth and we can predict where solar eclipses would have been on Earth in historical times. It turns out that these predictions – if you don’t take the slight slowing down of the Earth into account – are wrong and predict solar eclipses hundreds of years ago to be observed in places that they were not. Historical records tell us where eclipses were observed of course and the correction because of the slight slowing down of the Earth exactly matches what we expect from the amount of slowing that the Apollo laser reflector experiment tells us.

    An incredible marriage of ancient Chinese and Korean records with modern firing of lasers at the moon!

  2. Firstly, congratulations on spotting this. Most people notice the changing pattern of light and dark but fail to realise the pattern of craters stays the same.

    The reason we always see the same side is simple; One half of the moon is slightly denser than the other. Imagine a wheel that’s suspended and free to spin, but there’s a small weight attached to one side. As it spins it will tend to settle with the slightly heavier side pointing towards the source of the gravitational field (i.e. Earth). This is exactly what has happened with the moon.

    Hope this helps.

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