• Question: Why do planets orbit in an ellipse with the Sun at one focus? Why not a circle? And as a circle is an ellipse with the foci in the same place, would an actually circular orbit be possible?

    Asked by doppler to Adam, Sheila on 24 Mar 2011 in Categories: .
    • Photo: Adam Tuff

      Adam Tuff answered on 24 Mar 2011:

      Indeed – you could have a circular orbit, there’s nothing preventing that – however the universe isn’t a simple two-body system, it’s a many-body one – things like asteroids, comets passing through, all perturb the orbital paths of planets, ever so slightly, however in most cases they are only small perturbations which change the nature of the circular stable orbit to an eliptical one, which is also stable.

    • Photo: Sheila Kanani

      Sheila Kanani answered on 24 Mar 2011:

      It is due to Keplers laws and gravity. Some moons have more circular orbits than others. But the reason most are eliptical are because most of the time there are more than one thing orbiting the star in question, and everything affects each other.

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