Notes to Lecture 3

The three Kepler's Laws :

1.

the orbits of the planets are not circles but ellipses, with the sun positioned at one of the foci. (ellipse : the curve containing all the points whose sum of the distances from two fixed points, the foci, is constant. The circle is a special ellipse, whose two foci are in the same location).

2.

an imaginary line joining the planet with the sun sweeps equal areas in equal times. Ergo....the speed of a planet in its orbit is not constant, it moves faster when it is closer to the sun.

3.

for all planets, the square of the period of revolution around the sun is proportional to the cube of its (average) distance from the sun, $T^{2} = const\times R^{3}$, i.e. T²/R³ = const