KEPLER'S 2nd LAW

The line joining the Sun and planet sweeps out equal areas in equal times, so the planet moves faster when it is nearer the Sun. Thus, a planet executes elliptical motion with constantly changing angular speed as it moves about its orbit. The point of nearest approach of the planet to the Sun is termed perihelion; the point of greatest separation is termed aphelion. Hence, by Kepler's second law, the planet moves fastest when it is near perihelion and slowest when it is near aphelion.

 
  This applet allows you to change the speed of a planet at some point during its orbit and see how the ellipticity changes. The length of the pink line indicates the speed of the planet. Watch how it slingshots around the Sun if the orbit is very eccentric. Challenge: try to make an orbit as circular as possible. It might be harder than you think!