There are some who call me… Tim?
While I was out reading my daily fill, I stumbled across a neat little program linked from Bad Astronomy. 
This app, designed by PhET Interactive Solutions at the University of Boulder, simulates the orbital physics of a small solar system. While I have spent many hours playing java games before, there is something about setting comets and planets on collision courses or hurling massive objects into the sun that makes me feel a little like Tim. In fact, I keep putting off writing the next sentence to try and make a stable orbit for a star with one planet and two moons. Well that’s done – I think with the rest of the week I’ll make a few oceans, some animals, and a couple of people. Then it’ll be about time for a siesta.
While this app has a color scheme reminiscent of a McDonald’s ball pit, it is amazingly addictive. There is something so predictably unpredictable about multi-body orbital dynamics. Is that comet going to smack the planet, or does it get a pass till next year? What does it look like when two pairs of binary stars cross orbits? This sort of steroidal calculus is a full-nelson on the imagination, so I’ll happily rely on software to do the heavy lifting. My Solar System, as the program is appropriately called, allows you to determine mass, position, vector, and initial velocity. 
You can put up to four celestial bodies in motion at any given time. That sounds sexy because it is sexy.
Definitely check out the preset list before you start starscaping. If you (like me) imagine a moon spinning around its planet like an elliptical circle, paper-machete mobile mock-up from kindergarten, then you have got to see the “sun, planet, moon” preset. The vicious gravity wobble in the trajectory of the moon is really fascinating, and the trail left behind gives an appreciation for the absolute motion of planets in a way I’ve never experienced. Make sure to set the tempo to accurate – gravity simulation is number crunching madness, so the program has to make assumptions if you speed it up (and we all know what they say about assumptions).
I also had a lot of fun with PhET’s other interactive science software. There are a ton of similar java apps on the site, geared for all age ranges. For the kids, there is a sort of silly NES-quality ancestor to Spore that teaches the basic concepts of natural selection. Preteens get a ballistic trajectory app in the vein of the late, great Scorched Earth. The science simulations ramp all the way up in complexity to optical quantum controllers and DNA stretchers. Want a cool java app to learn about glacial sliding? They’ve got that too. Most of the programs, similarly to My Solar System, are addictive in a really sneaky sort of way. It must be those damn scientists trying to brainwash our kids again – yep, I see an app for the greenhouse effect. Torches? Pitchforks?
The Paradigm Revolution 