In the comments to Mark’s post about the embarassment being caused to the U.S. by the creationism trial in Dover, a scuffle has broken out over another deep question: does the Earth go around the Sun? See here and here and here.

It’s actually a more subtle question than you might think. The question is not “Was Ptolemy right after all?”, but rather “in the context of modern theories of spacetime, is it even sensible to say `X goes around Y,’ or is that kind of statement necessarily dependent on an (ultimately arbitrary) choice of coordinate system?”

You’ve come to the right place for this one; biologists can have their fun demolishing creationism, but we’re the experts on the whole geocentrism/heliocentrism thing. The answer, of course, does indeed depend on what one means by “move around,” and in particular the comments refer to the notion of a “reference frame.” I can think of at least three different things one might mean by that phrase. First there is the idea of a “global reference frame.” By this we mean, set up some perpendicular axes (some choice of coordinates x, y, and z) locally, right there in the room where you are sitting. Now extend these coordinates globally throughout space, by following straight lines and keeping everything appropriately perpendicular. That would be a global reference frame. (I am implicitly assuming that the coordinates are “Cartesian,” rather than using polar coordinates or some such thing — no reason to contemplate that particular complication.)

The second notion is that of an “inertial refere