The Shape of the Moon’s Orbit
This is a short and readable exploration of the question “What is the shape of the Moon’s orbit around the Sun?”
The Moon makes circles (ellipses, really) around the Earth while the Earth makes circles around the Sun, so what does the path of the Moon look like?
It’s a fun problem to think about, and I encourage you to do just that before you click through and see the discussion.
This simply posed problem touches on polar and parametric curves, 3-dimensional geometry, and of course, astronomy. And naturally there are lots of extensions, like “What if something were orbiting the Moon?”