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Why doesn't the moon fall on earth?

  1. Sep 15, 2005 #1
    I am curious as to why the moon doesn't fall into the earth. Did a few searches and found:


    But that doesn't explain one thing.

    Moons orbit is not an exact circle. Its "almost" circular. So when the moon is closest towards earth, shouldn't earths gravitational force on moon be stronger resulting in a spiral down-fall and eventally moon crashing into the earth?

    And a similar question: why doesn't the earth fall into the sun? Earths orbit is even more oblique. So shouldn't the sun attract earth with greater force when it is nearer on days of equinoxes and eventually lead to a spiral down fall too?
  2. jcsd
  3. Sep 15, 2005 #2

    James R

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    When the moon is closest to the Earth, it's also travelling "sideways" at its fastest rate. This is generally true for any elliptical orbit. An extreme case is that of a comet, such as Halley's comet. When it is close to the Sun, it travels at such a high speed that it takes less than a year to go a fair portion of the distance around, but when it's far away it takes another 75 years to return again. Why? Because it slows down as it gets further from the sun.

    You're right that the attractive force of gravity is greater when the moon is closer to the Earth, but the extra speed compensates for the extra pull, and the moon still "misses" the Earth as it falls around it.
  4. Sep 15, 2005 #3
    Ah, that makes sense.

    And then the question arises: why does the speed of an object increase when its closer in its elliptical orbit?
  5. Sep 15, 2005 #4


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    Conservation of energy is one way to explain it easily. As the planet comes close to the sun it's gravitational potential energy is lowest so it's kinetic energy is highest.
  6. Sep 15, 2005 #5


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    Simply put, when an the moon is moving towards the earth, it speeds up because of the gravitational pull that is pulling it closer to the earth. Its exactly the same as when you jump off a diving board and accelerate as you fall. Imagine, though, if you jumped off a really tall diving board with enough forward velocity that you'd miss the earth when you fell toward it. That's an eliptical orbit.
  7. Oct 18, 2010 #6
    the spped that it travels counter acts the gravity on it. So it doesnt come flying to the earth.
  8. Oct 18, 2010 #7


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    In a way it does, all the time.

    Even if each year it is a little bit more distant.
  9. Oct 18, 2010 #8
    I was asked the same question on October 4, 1957, when I told my soccer coach that the Russians had just launched Sputnik, the world's first artificial satellite.

    Bob S
  10. Oct 19, 2010 #9
    When you're on earth and you fall down from something, you will always hit the ground because that's all there is "down".

    However, it's not the same when you get into astronomical distances. Imagine you you go way up into space, as far as the moon is, and you look "down" towards the earth. This is what you see:


    In fact you see even more sky and less earth, as that picture is cropped. So now the earth is just a small part of the sky. If you "fall" towards the earth, it's not so easy to hit anymore. Chances are you will end up missing it even though it's pulling you towards it. If you do miss it, you'll still a have a huge speed due to the earth's gravity, and you'll end up orbiting the thing.
  11. Oct 19, 2010 #10
    Let us not forget that both the earth and the moon are pulling on each other. Why are we not asking about the earth falling onto the moon?

    Furthermore, it's been decades since we landed on the moon. Why do we still consider outer space to be the in the direction of "up"? Is "up" intrinsically tied in with our definition of gravity?

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