One interesting thing that's not in the video: there is a real application beyond the astronomy one, a scenario where you would want to roll one object around a

One interesting thing that's not in the video: there is a real application beyond the astronomy one, a scenario where you would want to roll one object around another object, and care about how many rotations it makes. https://en.wikipedia.org/wiki/Epicyclic_gearing https://en.wikipedia.org/wiki/Epicyclic_gearing These are used for car differentials, bicycle gear hubs, pencil sharpeners, and many other applications.