Hello, I have a few questions about rotation and relative motion. Suppose we transport the proverbial spinning ice skater used to demonstrate conservation of angular momentum to beginning physics students to a universe with only her and two planets. She is now spinning in deep space. Presumably she can measure her rotation and angular momentum relative to the two planets. However, now suppose we can somehow remove these planets, leaving the ice skater spinning by herself... spinning relative to what? Does the concept of "rotation" mean anything when there's nothing around you to rotate relative to? However, let's suppose she was to draw in her arms and decrease her radius. What happens? What is her angular momentum? Let's suppose a star is 10 light years away, and when I look at it, I spin round with a period of one second. From the star's reference frame, I'm spinning round once a second. No problem there. But from my reference frame, doesn't the star complete a 31.4...etc. light-year long circuit around me in one second? But wouldn't that mean the star is travelling almost a billion times faster than the speed of light? How can both my reference frame and the star's then be equally valid?