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Thread: The Moon

  1. #1 The Moon 
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    Why doesn't our moon's face ever change? How unusual is it to have a satellite in the vacuum of space not to even drift slightly. I can't get my head round how it can stay so still for so long!!


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    Anti-Crank AlexG's Avatar
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    It's gravitationally locked. In a vacuum, what would cause any drift?


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    Forum Radioactive Isotope cosmictraveler's Avatar
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    The moon is slowly moving away from Earth and one day thousands of years from now the Earths gravity won't be able to hold it steady so eventually it will start to either oscillate or rotate.
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    Quote Originally Posted by Jónar View Post
    Why doesn't our moon's face ever change? How unusual is it to have a satellite in the vacuum of space not to even drift slightly. I can't get my head round how it can stay so still for so long!!
    The Moons rotation speed matches its orbital speed. It is rotating.
    This is called Tidal Locking. There are many explanations and definitions of tidal locking on this here interweb.
    The rest of the O.P. appears to be based on misconceptions.
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    Quote Originally Posted by Neverfly View Post
    Quote Originally Posted by Jónar View Post
    Why doesn't our moon's face ever change? How unusual is it to have a satellite in the vacuum of space not to even drift slightly. I can't get my head round how it can stay so still for so long!!
    The Moons rotation speed matches its orbital speed. It is rotating.
    This is called Tidal Locking. There are many explanations and definitions of tidal locking on this here interweb.
    The rest of the O.P. appears to be based on misconceptions.
    this. and to add, tidal locking is seen in quite a few (maybe most?) of the moons in the solar system, so it's not really unusual.
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    Quote Originally Posted by mat5592 View Post
    this. and to add, tidal locking is seen in quite a few (maybe most?) of the moons in the solar system, so it's not really unusual.
    I've directly observed this on Pizmo Beach in California- where many moons get locked by the tide and stay there all day long.
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    Quote Originally Posted by Jónar View Post
    Why doesn't our moon's face ever change? How unusual is it to have a satellite in the vacuum of space not to even drift slightly.
    Technically, the face does change a little.
    As already mentioned by others, the Moon rotates with the same period as it orbits the Earth. This is because of something called tidal friction. Basically, the near side of the Moon feels more gravity from the Earth than the far side. This pulls the moon out of shape slightly. When the Moon spun at a different rate than it orbited, this bulging moved over the Moon's surface. This created friction which changed the Moon's rotation until over time, its rotation matched its orbit.

    However, even today this match is not perfect. The Moon's orbit is not perfectly circular, sometimes it is further and sometimes it it closer. This also changes the speed of its orbit at different points. When it is closer it orbits faster that when further. The rotation rate doesn't change however. So at times the rotation gets ahead of the orbit and at others behind. So, as seen from Earth. the Moon appears to rock from side to side a bit.

    Also, the axis of the Moon's rotation is not parallel to the axis of its orbit. So at parts of its orbit it South pole leans slight toward the Earth and at others, the North pole. This makes the moon look like it is nodding up and down as seen from the Earth.

    These apparent motions are called libration.

    Here's a YouTube video showing the libration.

    Lunar Libration - YouTube
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