The last decree concerning the issue, though, roscoe, was in favor of geocentrism. It would not seriously matter to the Holy See now for there to be a decree on the issue again, of course. Both the heliocentric and geocentric (Tycho Brahe) models have everything in the same position, except with slightly different orbits of the Earth and Sun in relation to each other. So the Gregorian calendar could still work with the geocentric model. You could very much use anything else as a reference point like the planet Jupiter placed as the center and do calculations according to it. Again, this is not all de fide definita dogma, but theologically the Earth is the center of the universe at least anyway.