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Moon moving away

  1. Sep 24, 2004 #1


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    I read somewhere that the distance earth-moon increases. How close by was the moon in the beginning? And what I do not understand is why. It seems to me that due to tidal movement, energy is lost. Which would result in the moon slowing down, and thus getting closer to earth. Why is the reverse happening?
  2. jcsd
  3. Sep 24, 2004 #2


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    Energy is never completely lost. While some of the energy involved in the Earth-moon tidal interaction is radiated away as heat, some of the Energy the Earth loses when it slows its rotation is transfered to the Moon via that same interaction pushin the moon into a higher orbit.

    Analogy: You have a spinning potter's wheel. Sitting beside it is a man stiing on a wheeled chair. He puts his hands out to slow the wheel. As the wheel slows, the friction between his hands and the wheel will start to drag him around the wheel. The wheel will transfer some of its energy to him.
  4. Sep 24, 2004 #3


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    The Moon creates tides on Earth. There is a high tide below the Moon's position, and another one on the opposite side of the Earth. Because of these buldges, the Earth is not perfectly spherical. The buldges have gravity too. The buldge under the Moon is closer to the Moon than the buldge on the opposite side of Earth, so its gravity is stronger. The rotating Earth carries this bulde forward, so the Moon is always slightly lagging behind the buldge it creates. So the buldge pulls the Moon forward, which raises its orbit. If the Moon orbited in the opposite direction, or the Earth rotated in the opposite direction, the Moon would spiral in. Mars' moon, Phobos is spiraling in because it orbits the planet slightly faster than Mars rotates on its axis. So it is always slightly ahead of the buldge it creates.

    The buldge also gives the Moon's gravity something to grab on to and so it slows the Earth just a little bit.

    When the Moon first formed, it was probably much closer to Earth, ~30,000 km altitude. The tides it raised were HUGE, and the rate at which it receeded was much faster than today. Earth also spun much faster on its axis.

    Eventually, the Moon will slow down the Earth enough that they will be tidally locked to each other. Then the Moon will receed no more. That is if the Sun doesn't turn into a red giant first.
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