That NOS Guy wrote:I just may be mistaken here but if the Earth is the center of the universe explain the retrograde motion of I dunno, Mars.
That's an interesting question. Histrorically, Plato had criticised astronomers for failing to explain anything and lacking theory altogether (
Republic: St. II 527d-530c). Eudoxos, a friend of Plato, was the first to come up with a theory that does this, requiring four uniformly rotating spheres per planet: the first to account for motion shared with the stars (in the modern sense, rotation of the Earth), another for the ecliptic, and two more to account for the the observed variations in movement speed. Herakleides, one of Plato's students, suggested that if the Earth rotates, the first circle becomes unncessary, and later Apollonios reduced it further to only two circles--the epicycle model which Ptolemy adopted and revised. An epicycle is actually a kind of
epitrochoid, just defined differently, so you can see how the retrograde motion would be produced by that illustration alone. (If one takes an epitrochoid's main circle and extends it to the center of the second circle, leaving the position of the second circle untouched, and decreases the radius of the second circle so that the point P [in the link] is on it, then the epitrochoid is reproduced exactly by some uniform rotation [instead of rolling] of both circles.)
Darth_Zod wrote:metaphysics has already more or less been debunked as bullshit.
'Debunked'? When? By whom?
Dooey Jo wrote:The universe has no center. Or the center is everywhere. So you could say that wherever you go, you're still in the center of the universe. So technically he's right if he says that the Earth is in the center of the universe. He would also have been correct if he had said the Moon was, or the Sun or M31.
That's certainly true.
Dooey Jo wrote:And it's not wrong to use the Earth as a frame of reference of the Solar system. It's just very stupid and inconvenient. You can do it, but it will mean that you're probably deranged and will claim that the Bible predicted relativity or something... But the motion of the universe gyroscopically stabilizing the Earth? That doesn't make any sense. How the hell is that even supposed to work?
General relativity does not care about any particular choice of coordinates. The (tensorial) equations are exactly the same regardless--which shouldn't be suprising, since tensors were specifically invented to be coordinate-independent. One could define a frame of reference which treats Earth as a stationary, non-rotating center, and it will be just as empirically successful. This is already quite common in Newtonian mechanics; for example, gravitational potential energy enables one to treat a body gravitating with another as stationary (even though it is not), and likewise the centrifugal and Coriolis forces enable the treatment of rotating frames as if they were inertial. The problem is that while rotating frames give rise to forces which can clearly be identified as `fictitious' in both Newtonian mechanics and special relativity, no such identification is possible in a general GTR situation, with a notable exception of an asymptotically flat spacetime (which is just a compact way of saying that at sufficiently great distances, spacetime looks like that of special relativity).
So what? Well, a scientist might either rely on the barycenter to tell what is rotating around what (and since the center of mass for the solar system is by far dominated by the Sun, the issue seems rather clear under this definition), or the response might be that treating the Earth as a special point explains nothing new and only obfuscates matters, and so is incorrect by Occam's razor. A philosopher might agree with either, but some might hold that the whole question is scientifically meaningless, since our most general theory (GTR) does not allow one to empirically decide the issue--it seems to be a question of computational practictality alone. I'm not sure which is more damaging to geocentrism--the view that its central claim makes sense but is wrong or the view that the claim is completely devoid of scientific meaning.
-- Asuka
I'm sorry, that was a terrible pun. I'll go to bed now.
