Johannes Kepler (1571-1630)

The most radical thing that 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 with the data as acquired by Tycho.

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 their orbit (actually, semi-major axis of their orbit). For the Solar System,

P**2 (years) = a**3 (A.U.).

In tabular form,

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

Comments:

• Comparison to data and Copernicus

• Variation in Speed Around Orbit

• Orbital Speeds

• Galilean Moons of Jupiter

• Titius-Bode Law