Johannes Kepler (1571-1630)

The most radical thing Kepler did was to remove the assumption of circular motions, a motion viewed by the Greeks and Copernicus as sacred. This simple generalization allowed Kepler to make a model which was consistent the the data acquired by Tycho.

Kepler's Laws of Planetary Motion

Law I: Law of Ellipses

Each planet moves about the Sun in an orbit that is an ellipse with the Sun at one focus of the ellipse.

Law II: Law of Equal Areas

A straight line joining a planet and the Sun sweeps out equal areas in equal times.

Law III: Harmonic Law

The square of the sidereal revolution period of a planet is proportional to the cube of the size of its orbit (actually, semi-major axis of its orbit). For the Solar System,

P² (years) = a³ (Astronomical Units).

In tabular form,



Note that the last two entries in the above table are discrepant.

Comments:

• Comparison to data and Copernicus
• Titius-Bode Law
• Galilean Moons of Jupiter