> The Sun is in the focus of the ellipses made by planets around it. The opposite doesn't hold. Certainly the opposite holds - if you draw the Sun's orbit from

> The Sun is in the focus of the ellipses made by planets around it. The opposite doesn't hold. Certainly the opposite holds - if you draw the Sun's orbit from a planet's frame of reference, you'll draw an elliptical path with the planet at one focus. That's the whole point here, that neither drawing describes observable reality any better than the other. I don't think I'm making any headway here so I'll defer to Einstein: > Can we formulate physical laws so that they are valid for all CS [coordinate systems] ... ? If this can be done ... the struggle, so violent in the early days of science, between the views of Ptolemy and Copernicus would then be quite meaningless. Either CS could be used with equal justification. The two sentences, 'the sun is at rest and the Earth moves', or 'the sun moves and the Earth is at rest', would simply mean two different conventions concerning two different CS. Could we build a real relativistic physics valid in all CS; a physics in which there would be no place for absolute, but only for relative, motion? This is indeed possible! (from The Evolution of Physics)