It's not that I don't believe that all i.r.f.'s are on an equal footing, it's just that I can't seem to get that to square with the following thought experiment, undoubtedly 'cause I'm getting lost in my own undies here, and I need help getting a handle on this. Suppose you have a universe with just ONE planet in it (so that there's no outside ref points to determine relative motion of any kind). Within that universe there would have to be an "absolute" or fundamentally correct reference frame wouldn't there? I mean, that planet is either spinning, or it's not. Period. If it's spinning, especially if it's spinning really fast, your weight, standing on it's surface, is going to be lower than you weight would be if it were not spinning, right? If you're going to say it's spinning, spinning in relation to what? Using an inertial reference frame that spun with the planet (i.e. the planet is defined as completely motionless) would lead you to an incorrect value for gravitational attraction. The fact that you don't know that the planet is spinning, 'cause you don't have any reference points to compare to, doesn't change the fact that you wind up with the wrong value for "g" does it? I mean wrong really is wrong right? Sooner or later you're going to notice and have to explain coriollis effects. It's not like spinning or not spinning is a subjective condition. You're either spinning, or your not, and those two conditions are distinct with distinct and observable consequences. So how can all inertial ref frames stand on equal footing? I'm choking on my elastic waste band here. HELP!