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Kepler's Laws of Planetary Motion

1.

ELLIPTICAL ORBITS: The orbits of the planets are^10.6 ellipses^10.7 with the Sun at one of the foci.
 

2.

CONSTANT AREAL VELOCITY: The area swept out per unit time by a line joining the Sun to the planet in question is constant throughout the orbit.^10.8
 

3.

SCALING OF PERIODS: The square of the period  T  of the orbit is proportional to the cube of the length of the semimajor axis (or, in the case of a circular orbit, the radius  r) of the orbit:

$$T^2 \propto r^3$$