Planets move in elliptical orbits
about the sun with the sun at one focus.
The area per unit time swept out by a radius vector from the sun
to a planet is constant.
The square of a planet's period is proportional to the cube of
the major axis of the planet's orbit.
Kepler's equation relates the mean
anomaly, M, and eccentricity, e, to the
eccentric anomaly, E. The eccentric anomaly is the actual angle
between the satellite's position and perigee.
M = e * sin(E)