Dear Philip, Dynamical equivalence of two models concerning the universe is where, although the models behave differently and have different kinematic workings, because they supposedly predict the same observations though, it is not possible to determine which is true due to relativity. Most geocentrists argue that you cannot tell any observable difference between a heliocentric model and a geocentric one, thus they conform to dynamical equivalence which is essentially Mach's principle. As regards the Earth rotating and residing in the centre of the universe, then the sun must orbit the Earth once a year because, the Earth rotates once every twenty three hours and fifty six minutes west to east, which therefore creates the illusion that the stars are going round east to west in the same time. Finally the sun must be accounted for, since the sun goes around in twenty four hours the sun must orbit the Earth once a year, which is why in the heliocentric model the Earth must orbit the sun. This then also explains the seasons and the path of the sun through the ecliptic, which is essentially the path that the sun takes during the course of a year against the background of stars. Kind Regards, Steven Jones. --------------------------------- ALL-NEW Yahoo! Messenger - all new features - even more fun!