The executable is in orb.jar and the code is in orbcode.jar . Moon colors, sizes, masses, positions, and velocities are applet parameters. Trajectories are plotted with a symmetric multistep method . Here's a page that puts the applet through its paces , and another of neat orbits . There are 20 refreshes per second.
Bigger values mean faster orbits and bigger errors.
Increases accuracy, increases CPU usage. It means all objects interact only with the first moon. Have all the moons leave trails. Zooming or rotating erases the trails. Calculating energy requires velocity, and my integration formulas don't track velocity, so I approximate it from the positions.