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General Relativity and the Moon's orbit.

  1. Apr 16, 2008 #1

    I have a question regarding the Moon's orbit and general relativity.

    I think I understand the concept of space time, that an object placed in the fabric of space time distorts the fabric of space time. I also understand how "Classicly" the moon stays in orbit.

    Going back to GR and the fabric of space time. From my understanding the moon will orbit the Earth allong the path of least resistance, but if the fabric of space time surrounds the Earth then is there not an infinite number of orbital paths for the moon to follow?

    I expect my understanding may need some revision but if someone has an answer it would be much appreciated.


  2. jcsd
  3. Apr 16, 2008 #2


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    Welcome to PF!

    Hi Darren! Welcome to PF! :smile:

    Yes … but once you choose the direction and the speed, there is only one orbital path through each point. :smile:

    (same basic principle as in Newtonian mechanics, of course!)
  4. Apr 16, 2008 #3


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    In that way (and many other ways), an orbital path is no different from a strait line. If you throw a rock in deep space, there are an infinite number of paths along which you could throw it. But once you have thrown it, and given it a specific speed-and-direction, there is only one path that it will take.

    However, in other ways an orbit is somewhat different. There is a certain range of conditions, especially speed and distance from the host Bonnie, that will result in an orbital path. Anything that falls outside this range will not orbit. For example, too fast or too far away, and he "orbit" becomes an open curve (an object that simply bends its path a little as it "flies by"). Too low or too slow, and the object falls into the main body.

    As you know, some planets have multiple moons, which represent multiple solutions to various combinations of speed and altitude. Several planets even have rings representing a mathematical solution to where a body could have formed, but didn't.
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