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Thread: Why is the Moon moving farther away from the Earth?

  1. #1 Why is the Moon moving farther away from the Earth? 
    Forum Cosmic Wizard
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    Lunar distance (astronomy) - Wikipedia, the free encyclopedia

    The Moon is spiraling away from Earth at an average rate of 3.8 cm (1.5 in) per year, as detected by the Lunar Laser Ranging Experiment.[2][3][4] The recession rate is considered anomalously high


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  3. #2  
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    i'll have a go, but hopefully someone with a bit more expertise will correct any mistakes i make.

    the moon causes tides on earth as you are aware. these tidal bulges are not directly below where the moon it. they are slightly behind. this makes these spots have more gravitational attraction due to more mass. because it is slightly behind the moon this causes to moon to slow down a little. therefore it goes into a higher orbit.


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    Quote Originally Posted by Chrispen Evan View Post
    i'll have a go, but hopefully someone with a bit more expertise will correct any mistakes i make.

    the moon causes tides on earth as you are aware. these tidal bulges are not directly below where the moon it. they are slightly behind. this makes these spots have more gravitational attraction due to more mass. because it is slightly behind the moon this causes to moon to slow down a little. therefore it goes into a higher orbit.
    Wrong way around - ahead and makes the Moon gain momentum - ie moves away from the Earth.
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    Ah, OK I think I am getting it now.
    Tidal acceleration - Wikipedia, the free encyclopedia

    It does seem a bit odd though.
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    Quote Originally Posted by dan hunter View Post
    Ah, OK I think I am getting it now.
    Tidal acceleration - Wikipedia, the free encyclopedia

    It does seem a bit odd though.
    The idea of speeding the Moon up eventually slows it down took me ages to get my head around.
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    thanks for that. my excuse is that i'm a pom in australia and haven't got used to everything being arse about.

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    Forum Cosmic Wizard icewendigo's Avatar
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    The odds than any moon or planet is in perfect orbit to the nanometer is quite small, which means one can expect most bodies in orbits are either spiralling away or spiralling inwards. The moon happens to be spiralling outwards.
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    Quote Originally Posted by icewendigo View Post
    The odds than any moon or planet is in perfect orbit to the nanometer is quite small, which means one can expect most bodies in orbits are either spiralling away or spiralling inwards. The moon happens to be spiralling outwards.
    I'm not quite sure what you mean by a "perfect orbit". It seems that you think that if an object is not at an exact distance and speed that it's orbit is unstable. This is not the case. Orbits will not spiral in or out unless something is actively causing them to. Put something in a circular orbit around another object. Give it a nudge, either slowing it down or speeding it up and assuming that there is nothing else effecting its orbit, it will just settle into new orbit. The new orbit will be elliptical, but stable. To make it spiral in or out you have to provide a constant force to it. Something like a drag or tidal acceleration.

    Granted, you are never going to find a satellite that is not subject to some outside influences that will affect its orbit, but this is different from it spiraling in or out just because it isn't in some "sweet spot".
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    Clarification: If you can calculate an orbit in nanometers you might find an orbit is spiralling out at 0.0012 nanometers per cycle. In which case its orbit a gazillion years later would not be the exact same as it was now, even without external forces.
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    If you can calculate an orbit in nanometers you might find an orbit is spiralling out at 0.0012 nanometers per cycle
    If that's happening, then there's an energy source causing the outward spiral. But absent an energy source (such as tidal forces) orbits are fixed and eternal.
    Its the way nature is!
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    Then my understanding of physics is lacking and my amazement at the level of precision of orbits is great..
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    You speak of the precision of orbits as though it was an artifact of some kind. Orbits are precise because they are gravitationally balanced. Absent a change in gravity, or mass, or the addition of some kind of energy, an orbit is stable forever.
    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
    Philosophically more pleasing, more psychologically easy
    Prof Richard Feynman (1979) .....

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    wow, so what happens if an amazingly dogmatically absolute perfect orbit of Jupiter is pushed by 0.0001 nanometer the exterior AND inceases orbital velocity by 1km per hour? Does it stay in an orbit that is incredibly hard to distinguish from the precise perfection ad infinitum and forever, or does it say in an obit that appears good enough but gradually causes it to expand outwards over eons and eons (hence a spiral)? What are the odds that a planet is in exact perfect orbit to within less than 0.0000000000000001 nanometer? But then what are the odds that all planets, moons, asteroids in various orbits are ALL within less than 0.0000001 of an infinitly forever stable obit. Im impressed. We had a good toss of the dice in our solar system I tell you.
    Last edited by icewendigo; April 17th, 2014 at 03:30 PM.
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    Quote Originally Posted by icewendigo View Post
    Then my understanding of physics is lacking and my amazement at the level of precision of orbits is great..
    It's a simple matter of conservation or energy. The orbital energy of a satellite is equal to the sum of its kinetic and gravitational potential energies. Without and addition or subtraction of energy, this is constant.

    The orbital energy can also be found by



    M and m are the masses involved and a is the average orbital distance of the orbit. It doesn't matter whether if it is a circular orbit of radius a, or an elliptical one where the instantaneous orbital distance varies from less than a to greater than a.

    If the satellite were spiraling away, then a, the average distance of the orbit, would be increasing, meaning an increase in orbital energy. Thus without a loss or gain of energy, the average orbital distance must stay constant.
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    What are the odds that a planet is in perfect orbit to within less than 0.0000000000000001 nanometer?
    What do you mean by 'perfect orbit'? All orbits are perfect, as they follow the laws of gravity. If it's moving outward or inward, there's another environmental factor at work.
    Its the way nature is!
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  18. #17  
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    Quote Originally Posted by icewendigo View Post
    wow, so what happens is an amazingly godly perfect orbit of Jupiter is pushed by 0.0001 nanometer the exterior? Does it stay in an orbit that is incredibly hard to distinguish from the precise perfection ad infinitum, or does it say in an obit that appears good enough but gradually causes it to expand outwards over eons and eons (hence a spiral)? What are the odds that a planet is in perfect orbit to within less than 0.0000000000000001 nanometer?
    I already covered that. It just enters a new orbit, one with a different eccentricity than its previous one. There is not just one combination of speed and distance that results in an orbit. If you pick any distance from the Sun, there are an infinite range of velocities an object passing through that point can have and still maintain an orbit. They will just be different orbits. (there are also an infinite number of different orbits that pass through that point at the same exact velocity and in the same orbital plane. They will all have the same orbital energy, but different eccentricities).
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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