Kepler Applet for Mars

As the planet P is moving around the ellipse, the end of the velocity vector OP' will move around the gray circle (center M), and FP is always pependicular to to MP'. The mean anomaly M is the angular distance from perihelion which a (fictitious) planet would have if it moved on the circle of radius a with a constant angular velocity and with the same orbital period T as the real planet moving on the ellipse. By definition, M increases linearly (uniformly) with time. phi is the true anomaly.

The angular difference E - phi:

Updated: 2023, Oct 06