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Why are all the orbits elliptical and not circular?

  1. May 1, 2013 #1
    In high school whenever i asked questions about planets and their orbits around their star or the orbits of satellites around their planets ,i always got the answers with the terms centrifugal ,centripetal force and an example of a stone tied to a thread and how it behaves when you hold the thread at the other end and by swinging it, you make the stone orbit your fist but this circular not elliptical but all things that i mentioned about move in elliptical orbits ,even electrons revolve around the nucleus in elliptical orbits right ? i never got the answer for my "elliptical question" .can i get an answer please? maybe my teachers thought that it was too complicated for me to understand because i didn't know about relativity or something?
  2. jcsd
  3. May 1, 2013 #2


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    Electrons don't orbit in the classical sense.

    A circle is a special case of an ellipse (with zero eccentricity) so even a circular orbit is elliptical. So the real question should be why is eccentricity never zero? Well that would require an absolutely perfect starting situation and no interaction with other objects to disturb that perfection later.
  4. May 1, 2013 #3


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  5. May 16, 2013 #4
    Circular orbital motion is a unique set of circumstances - exactly the right speed, distance from the central body, angle of the "flight path". Change any of these, and you no longer have the right combination. Imagine that, rather than swinging it around in a circle, you let the stone dangle - a pendulum - you can get it to swing back and forth in a straight line, or around in a circle, but neither is easy. It's very easy to get it to swing in random more or less elliptical loops. Ellipses are the typical, easy, normal condition - circles are atypical, difficult, unusual.
  6. May 17, 2013 #5


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    Circular orbits are virtually impossible save for the case of two body systems. Even then they are subject to distortions due to density anisotropies - not to mention gravitational influences by remote bodies.
  7. May 17, 2013 #6


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    If you were to phrase this "why do not all orbits have eccentricity exactly 0.000...", I think it would be easier to see.
  8. May 17, 2013 #7


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    Hi Monsterboy! :smile:

    Central orbits (ie where the force is always towards a fixed centre) have to obey conservation of angular momentum, which gives you Kepler's equal-areas-in-equal-times law.

    If the central force is inverse-square, then we can prove that the orbit must be an ellipse, and yes, your high school teacher was probably right in thinking that you wouldn't understand the maths! :wink:

    (but it has nothing to do with relativity)
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