
Kepler’s first and second laws
Fig. 1.12 . Kepler’s first law states that the orbit of a planet about the Sun is an ellipse with the Sun at one focus. The other focus of the ellipse is empty. According to Kepler’s second law, the line joining a planet to the Sun sweeps out equal areas in equal times. This is also known as the law of equal areas. It is represented by the equality of the three shaded areas ABS, CDS and EFS. It takes as long to travel from A to B as from C to D and from E to F. A planet moves most rapidly when it is nearest the Sun (at perihelion); a planet’s slowest motion occurs when it is farthest from the Sun (at aphelion).
Copyright 2010, Professor Kenneth R. Lang, Tufts University

