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Moon apogee

  1. Oct 10, 2011 #1
    This website has an animation that shows the difference between sidereal reference frames and synodic.

    The motion of the moon is circular about the earth, yet apparently the moon has an apogee length that is 50,000 km further away from earth than its perigee length.

    In this animation have they just assumed a circular orbit for simplicity? How would it look if they didn't make this assumption?
  2. jcsd
  3. Oct 11, 2011 #2


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    Very similar. The animation is just illustrating the concept to explain why sidereal and synodic periods are different. I don't think anything is drawn to scale.

    The eccentricity of the Moon's orbit is 0.055. It's nearly circular (50,000 may seem like a large difference, but the semi-major axis of the Moon's orbit is close to 385,000 km). Additionally, perigee moves, taking a little less than 9 years to progress 360 degrees.

    That would affect the actual timing of new moons, full moons, etc, but the variation is small enough that using the average synodic period would still enable you to find the night of the new moon, full moon, etc.

    And being so close to circular, are you sure the animation used a perfectly circular orbit instead of the Moon's actual orbit? (Personally, I'd guess they used a circular orbit, since it wouldn't make sense to use such accuracy on the shape of the orbit when the size of the orbits isn't drawn to scale.)
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