Kepler's Laws

The Polish monk Copernicus (1473-1543) first proposed that the earth, rather than being the centre of the universe as was generally thought, revolved round the sun, as did other planets. This heliocentric (sun-centred) model was greatly developed by the German astronomer Kepler (1571-1630) who, following on prolonged study of accurate observations made by his tutor the Danish astronomer Tycho Brahe over a period of 20 years, arrived at a very complete description of planetary motion. Kepler announced his first two laws in 1609 and the third in 1619.

1. Each planet moves in an ellipse which has the sun at one focus.

2. The line joining the sun to the moving planet sweeps out equal areas in equal times.

3. The squares of the times of revolution (T) of the planets about the sun are proportional to the cubes of their mean distances (r) from it: r^3/T^2 is a constant.

Kepler's three laws enabled planetary positions, both past and future, to be determined accurately without the complex array of geometrical constructions used previously which were due to the Greeks. His work was also important because by stating his empirical laws (i.e. laws based on observation, not on theory) in mathematical terms he helped to establish the equation as a form of scientific shorthand.