I'm starting to add some details to my world of Ubora. One of the things I'm looking to add is info on the planetary dynamics. Right now I'm trying to reconcile orbits for three moons. One large and two small. I want them to be similar to ours but not too close. Does anyone know of any easy to use (Free) software where I can input and test to see if the orbits are stable and see what the phases of the moons are (if any)? Or should I just say screw it and make it up and physics be damned! I would like to have something I can point to and say "Yes this is correct and see I can even show you calenders with the moon rises and phases."