Sorry for the delay Phil! Geogebra makes it pretty easy to put something like this together. It has the ability to parametrically define coordinates, and then adjust a parameter with a slider. I added some conditions for showing/not showing objects, but that was about it.

It’s good you haven’t run into problems – I have always loved using Tracker. I installed it on my new Macbook Air, and I suddenly lost control of my wifi and ended up having to reset things. I installed it on two student laptops (new Macbook pros) and the computer rebooted and never was able to get past the boot screen. The OS had to be reinstalled. A headache for many people involved, but luckily it was only about a week into school, so the students didn’t lose too much.

I’ve also plan to use Tracker extensively this year on a school supplied MacbookPro, so I wasn’t too thrilled to read your comments about it. Fortunately I haven’t run into any issues (fingers crossed).