> Isn't that a normal orbit around a common centre of mass? If you limit yourself to looking at the position of one body with respect to another, the orbit wil

> Isn't that a normal orbit around a common centre of mass? If you limit yourself to looking at the position of one body with respect to another, the orbit will be an ellipse. A circular orbit is simply a special-case ellipse with an eccentricity of zero. This is true for any two orbiting masses -- from the perspective of either of them, the other body's orbit will be an ellipse. And yes, it's always with respect to the common center of mass. Interestingly, mostly because of Jupiter, the solar system's common center of mass can sometimes lie outside the sun.