Simulates motion of two planets. Just click green flag. Set "blue planet moves?" to 1 if you want to allow the blue planet to move under the yellow planet's gravitational pull. Look inside and play with the initial variable settings in the script triggered by the green flag. In particular, try changing the speeds (x and y) of the planets to get different behavior. Or the masses of the planets. Currently, the yellow planet starts with initial velocity so that it more or less orbits the blue planet (if stationary).
