I need some clarification here. We know that the earth rotates about it's own axis and also around the Sun. Now my questions are: 1. In the geocentric model of Ptolemy the earth was at the center of a circle while the planets (and sun) moved along epicycles and the centers of those epicycles moved along the circle which had earth at the center. Copernicus viewed the motion of planets as though we were standing on the Sun; which made the planetary orbits much simple. Now my question is, when we say that the Earth moves round the sun (and reject the Ptolemy model) do we do that for mare simplicity and convenience? 2. The Earth rotates on its own axis. This rotation is measured relative to some reference frame, right? Now which reference frame did we use to measure earth's rotation speed? 3. Form a certain reference frame (for example one attached to earth) Earth is not rotating. But forces like Coriolis force as detected by a Foucault pendulum suggests that Earth must be rotating. Is Earth's rotation an absolute motion? I thought it is not. But please explain. Thanks for your help.