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@MikeA thanks for the link - there's a ton of ideas there. Thang knows his stuff. After I made the model in post 1, it bugged me that although it worked, I didn't really understand why the red spinning part had to rotate at 2X the speed of the larger part, and even why the cylinders stayed in sync in the slots. I have managed to solve it with a bit of simple geometry if you're interested - turns out the size of the red part doesn't matter -

Nicely done! I felt like I was at a kiosk in a science museum. Very professional and some good information. I also like how you showed the orbital path of the moons with their inherent wobble. Plus the stars rendered very well....no flickering (there could be a whole tutorial on how to render star backgrounds properly...with and without motion blur. It is not as trivial at task as you would think)! So very well executed. The only thing I questioned was the size comparison of Mar's moon's versus our Moon. Now there are pictures at the Nasa.gov site which do match pretty close to what you showed. But Earth's moon is 3475 Km versus Mar's moons at no more than 27 Km. If you were to actually make this to scale, it would look like this: Yeah...some artistic license needed to taken. Dave

If I may, I love Cinema 4D and my intent with my Avatar is not to profess fanatical devotion to Blender. Now, Blender is a good program. It's users do have a lot to be happy about. I did try it when the interface improved with 2.8. But despite those improvements, it is still a bit clunky. Everything is there and for the most part stable, but (IMHO) the UI unnecessarily gets in the way of fully enjoying the program. In short, C4D is a lot more fun to use! So why do I have that Avatar? To serve as a reminder to MAXON's leaders should they troll the site that the hobbyist community does have a pretty valid option other than C4D for our CGI fix. Yes, we could follow the subscription plan, but we are hobbyists....we use C4D for the love of it and not as part of a business model. As for me, should my personal financial circumstances change such that I can no longer afford to stay current with the program, I still want to have something that works and not watch years of work go away when my subscription turns off. You can ONLY keep C4D on with a permanent license and those costs have increased significantly (from $620/year in 2017 for the Studio MSA to ~$950/license upgrade). So my avatar is really part of that old argument of subscription vs. permanent licenses. I am a hobbyist that wants permanent licenses as long as it remains affordable for a hobbyist. Blender as an open source program will always be affordable. Blender gives us options....a reminder for the MAXON employees and CEO who visit the site and not intended to disrespect its members. Dave

Archimedes made a 2 slot trammel that drew ellipses. https://en.wikipedia.org/wiki/Trammel_of_Archimedes I made one in C4D a while ago that had 3 slots Recently I saw this device on YouTube, and thought I'd try it with 2 spinners I imagined I'd need some trig to make it work, but it seems the small object just has to rotate 2X the speed of the large one. Not sure why... I got a bit carried away, adding planetary gears 😀 I think epicyclic gears are used in electric screwdrivers, and Sturmey-Archer 3 speed bike hub gears. Epicyclic gear math can be a bit difficult, but you're welcome to look at my methods trammel 9.c4d here's a regular epi system that has sliders for speed epi.c4d

Version 2 is more planetary ie the planets move -