Running program twice with same conditions yields different outputs?

[Drolileapourl]

I made a little program to model the orbit of the earth around the sun. It is here:http://jsfiddle.net/NBvPp/Running it with dx=0 and dy=4 yields very different orbits on different runs, even when the planet doesn't get close to the center of the sun.Any ideas why? Is the computer handling roundoff error differently each time or what?