**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 phi - M:

The angular difference E - phi: