• The origin (x=0,y=0) is in the center of the screen.  Choose x- and y- coordinates in the range +/-100 or the masses will go off screen.
• Keep masses in the range 0.01 to 500.  Keep initial speed in the range 0 to 3.
• Other than that... play!

Download the Orbit Simulator Gravity4!

If you want to alter this program, you can see the QuickBasic source code for Gravity4. You will need a QuickBasic or VisualBasic compiler to work with the code.