Dear All, When observing just the Earth from the perspective of the moon, there would not be any discernible difference between the geocentric and heliocentric models, however, other objects in the universe would behave very differently in the two systems. I used to firmly believe in dynamical equivalence regarding the cosmos, the notion that derives from Mach's principle and asserts that from any vantage point in the universe it would not be possible to ascertain whether the Earth rotated or the starry heavens revolved about the Earth. Certainly this used to be my primary defence for the geocentric model when being attacked by heliocentrists. Now though, I believe that there is a substantial distinction between the two models, because they are physically two very different systems. There are three main models of the universe: Geostatic. Geocentric. Heliocentric. The first model is where the World stands motionless at the centre of the creation, and the entire universe revolves diurnally, the most beautiful and biblical. The second is where the Earth does rotate, but is situated at the centre of the universe. This model is actually a special instance of the acentric model where one has simply just assumed that the Earth is at the centre, and therefore the sun must orbit the Earth once a year. Finally, the blasphemous heliocentric model is where the Earth is merely an insignificant piece of rock that rotates and orbits the sun annually. Confusion can arise with Mach's principal, because most people believe that all three models are the same. Whilst it is true that 2 and 3 are dynamically equivalent, number 1 is not. Why? Take a look at http://www.midclyth.supanet.com/page14.htm for a detailed description. Kind Regards, Steven Jones. --------------------------------- ALL-NEW Yahoo! Messenger - all new features - even more fun!