Hi, I'm more of a maths man but I'm quite keen on astronomy, although without any formal training. I hope I've put this question in the right place and isn't too dumb! What defines the rotation of a planet, ie its DAY? Surely you need some sort of point of reference to measure how much it rotates. (Never mind how to define exactly its axis of rotation!) For gas planets, such as Jupiter, Uranus and Neptune, what are we looking at for our points of reference? Doesn't the gas just flow all over the place? For planets like Mercury, Venus, Earth and Mars, we obviously have solids to observe as reference points; but technically, they aren't really fixed at all either are they? Over the course of the history of the Earth, for example, our continents have moved significantly. I'm probably wrong, but the "DAY" of a planet actually means nothing. Please correct me if I am!