Instructions:
Click to launch an orbiter. Change the initial speed and direction to create different orbits.
Explanation:
Kepler's Second Law of orbiting motion says that a line joining a planet and the Sun sweeps out equal areas during equal intervals of time. In this simulation, the area is shaded in blue. Intuitively, we can understand this because when the planet is near the sun, it will be going faster than when it is further away, so it will travel farther along the orbit in a given time, but the distance to the sun is less. The opposite is true when the orbiter is far away.
Link to stand-alone sim: Here is a link to view the sim on its own, with no distractions.