# Why are all the orbits elliptical and not circular?

1. May 1, 2013

2. May 1, 2013

### Staff: Mentor

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.

3. May 1, 2013

### Bobbywhy

4. May 16, 2013

### tfr000

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.

5. May 17, 2013

### Chronos

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.

6. May 17, 2013

### HallsofIvy

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

7. May 17, 2013

### tiny-tim

Hi Monsterboy!

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!

(but it has nothing to do with relativity)