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Moon's orbit

  1. Jun 26, 2007 #1
  2. jcsd
  3. Jun 26, 2007 #2


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    What was funny about that link? It looked good to me. We can't help you unless we know what is holding you back. Can you explain what part of this you are confused about?
  4. Jun 27, 2007 #3
    what is not funny
    woman say if you throw a rock from a building is the same if you throw a rock from a Himalaya? And the moon is 'thrown' at earth and it misses her all the time. - first part I find hilarious, and for 2nd I have to say hey what is up with you

    what is bugging me
    - there is an whole universe around moon and earth, and the moon finds always the same orbit
  5. Jun 27, 2007 #4


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    The Moon's orbit is determined by the laws of physics, this is a good place to learn about them.

  6. Jun 27, 2007 #5

    Jonathan Scott

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    As with many other aspects of physics, this is a matter of "stable equilibrium", in that if the current situation were disturbed the result would tend to restore the previous situation.

    One (extremely simplified) way to think about it is that the gravity of the earth curves the moon's path in towards it by a particular curvature. If the moon were too low, this curvature would not be tight enough to keep it following around the earth, so the moon would get higher. On the other hand, if it were too high, the curvature would make the moon turn in towards the earth more, making it lower. On balance, after all of this has evened out, it therefore lands up going round in approximately a circle relative to the earth. As there's nothing in space to slow it down, it just keeps doing that.
  7. Jun 27, 2007 #6


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    "Always?" No - only once. A stable orbit is a stable orbit.
  8. Jun 28, 2007 #7
    ok, but Why is moon in stable equilibrium?
    and How it got there in the first place?

    you say this as moon has will and can go up or down as it pleases

    does moon has not his own gravity?

    why it does not orbit from pole to pole or around equator?

    You say that universe has no effect then please be kind and call my mother and explain this to her she is spending too much money on astrology calls, and all my life got really fked up cause of those books, her number is 098/9705146, ok?

    If a butter-fly in Brasil can make an storm on the other part of the world, what it can do to the moon?
    Last edited: Jun 28, 2007
  9. Jun 28, 2007 #8
    So far the accepted theory is that when the Earth was somewhat young, it was hit by another large object which sent a lot of material spewing out into space. What formed after that was a temporary ring that circled the entire earth... somewhat like Saturn's ring system. Google 'accretion'. After enough time passed, all the little pieces eventually started to clump together into a BIG piece... the Moon.

    You can almost say that the Moon was born into equilibrium with respect to the Earth.

    It very well does but, the gravity isn't that great compared to the Earths. Therefore, the Earth dominates over the Moon when it comes to gravity. That's why the Moon orbits the Earth and not the other way around. The Earth is more massive.

    Again, it has to do with the Moons original formation. If the Earth was rotating along a plane that stretched from pole to pole, the Moon would be orbiting closer to that plane.

    That's a waaay too involved scenario to even begin analyzing. Then again, so is any other scenario concerning Chaos Theory. Prediction only goes as far the number of variables that are known.
  10. Jun 28, 2007 #9
    Despite what some people might tell you in general relativity all orbits decay, some decay extremely slow, but they do decay.
    Last edited: Jun 28, 2007
  11. Jun 28, 2007 #10
    Very true. At he same time you can also state that "true" or "pure" equilibrium does not exist.

    It's all in your sampling rate.
  12. Jun 28, 2007 #11


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    I think sinisha would benefit from understanding how LIKELY it is that a body will orbit another.

    Say the universe is empty except for the sun and a tennis ball.

    The Sun will attract the tennis ball by gravity. It may take a long time, but eventually the tennis ball WILL end up orbiting the Sun. It is highly unlikely that it will NOT. The noly way it can NOT orbit the sun is if it crashes into it. And that's really, REALLY hard to do.

    See, as the tennis ball falls toward the sun, it will not fall perfectly straight down. The tennis ball (as with any random object in space) is going to have SOME sideways velocity. It may be tiny, it may be large, but unless it's ZERO, it will be enough to have the tennis bakll miss the sun. By doing so, the tennis ball is AUTOMATICALLY in an orbit. It may be a very large one, but an orbit it will be.

    If it has a large sideways velocity, it will be a in a wide orbit. If it has a small sideways velocity, it will be in a tight orbit. If the Moon had a LOT of velocity, it would be in a big orbit, if it had a little, it would tightly orbit the Earth. And here's the kicker: it weould be very hard (and unlikely) for the Moon to have NO sideways velocity with respect to the Easrth, and that's just what it would need to have to actually crash into it. Space is BIG, and planets are small, and the Earth is a very small target to hit.

    It's hard to describe in words but I'm trying to get at the idea that bodies orbiting other bodies is the DEFAULT state - you almost can't NOT be orbiting another body.
  13. Jul 2, 2007 #12
    But universe is not empty space.

    You also say that if we have an hangar on no gravity space, and there we put an 1 tone of Hg (and it forms sphere) and we put 1g Fe anywhere in the hangar and it will eventualy make an orbit around sphere?
  14. Jul 2, 2007 #13


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    Unless two objects like that start off near perfectly stationary with respect to each other, they will end up in orbit of each other. That's why it is said that an orbit is like falling towards something and missing. It only requires a small tangential component of velocity to miss.
  15. Jul 2, 2007 #14


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    Remember the Earth is round. Picture your are hovering in a spaceship (rockets blasting down so you don't fall) at the same distance the moon is from the earth.

    Drop a rock out your window and it will fall straight down hitting the earth right below you.

    Throw it out and it will arc down landing somewhere ahead of you.

    Throw it harder and it will land farther away.

    Throw it hard enough and it will miss the Earth entirely and circle around hitting the back of your spaceship. (It follows an elliptical orbit reaching its closest point opposite the earth from you).

    Throw it even harder and it will actually follow a circular orbit.

    Throw it harder still and it will orbit in a bigger ellipse with your position as its closest point.

    Throw it even harder still (above the escape velocity at that height) and it will follow a hyperbolic trajectory never to return.

    The moon is following an almost circular trajectory around the Earth. The reason it is nearly a circle is that tidal interactions with the earth and sun tend to decrease the eccentricity of its orbit. This is more complex than can be "simply explained" especially if you are just trying to grasp the simple idea of an orbit.

    What may confuse you is your perception of orders of magnitude. Relative to its size the moon appears to be moving quite slowly taking a month to circle the Earth. But figure the distance it travels in that month and work out the speed and you will see it is moving very very fast relative to planes, trains, and automobiles, (and bullets). Specifically it is moving at about 2165 mph.

    So by the time the moon has enough time to fall toward the earth at this same speed it has "moved over to the side" and is still moving tangentially around the earth in its orbit.
  16. Jul 3, 2007 #15
    thanks for answers

    I still dont get it
    maby Im just stupid

    Jules Verne wrought From the Earth to the Moon and Journey to the Center of the Earth - wich is the same
  17. Jul 3, 2007 #16


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    If you could be more descriptive about what you don't understand and why, we may be able to help...
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