How can you tell that the earth is rotating about an axis, and that the stars aren’t instead just moving around the earth? If you watch this fun video, you will get an intuitive feel for the idea of your frame of reference. And it is fun.
The Foucault pendulum started at the beginning, and explained at the end of this video can demonstrate the rotation of the earth. I made an attempt at creating my own Foucault pendulum, but it requires more planning to get right. If your pendulum is short, you have many confounding errors that hide the behavior we are looking for. I’ll work on this, and write about it in a future blog post.