I only wanted to comment briefly on the claim that photographs of star trails are conclusive evidence of the Earth's rotation about its axis ("Circum-Axial Leaf Trails", APOD May 19, 2007).
I believe the authors of this explanation have ignored the fact that star-trail photographs could also be interpreted (by themselves) as evidence of the rotation of the stars around the Earth (and that is how they would have been interpreted, during the many centuries in which the geocentric model of the universe was claimed to be correct). I think it is one of the principles of simple galilean relativity of motion that the mere observation of the movement of a body cannot be taken as conclusive evidence of the movement either of the observer or of the body observed as moving.Explanation: Are photographs of star trails really evidence of the Earth's rotation about its axis? Yes they are, and science journalist Trudy E. Bell discovered that there is a simple way to demonstrate this, if you have the stomach for it.
Actually, the observer standing on the MGR (Merry-Go-Round) platform would have to appeal to evidence of another sort to conclude that it is him and the MGR which are moving, and not the trees (and the entire world) surrounding him --perhaps he could say that he feels an apparent centrifuge force which would not be present if it were not for the fact that he is rotating with the MGR. [By the way, Isaac Newton articulated a similar (and very famous) argument to prove the existence of absolute motion in his Principia.]
But not even this new argument of the MGR-observer is sufficient to prove that it is him, and not the entire world around him, which is rotating. As every scientist or historian familiar with the history of relativity knows, Ernst Mach presented a counter-argument to Newton's in his brilliant (albeit sometimes obscure) Science of Mechanics: if the entire universe were rotating around the MGR, it would be possible to suppose the existence of a "radial gravitational field", produced by the rotation of the entire universe around the MGR, which would be liable as an explanation of the same feeling of apparent centrifugal force. Thus, in Mach's opinion, it is possible to "relativize" not only the concept of movement, but also the concept of force. As Mach himself wrote:
It can be argued that this contribution of Mach had a great influence on the formulation of the special theory of relativity by A. Einstein."Every mass and every velocity, and consequently every force, is relative. There is no decision concerning the absolute and the relative which we could possibly find, which we are forced to adopt, or of which we could obtain an advantage of intellectual or of any other kind".
Of course, it is separate (and legitimate) question to ask what kind of evidence can we put forward to demonstrate that it is the Earth, and not the entire Universe around it, which is rotating (different than star-trail photographs). We certainly have *very* good evidence for this claim nowadays, in part due to the fact that we now have access to extra-terrestrial points of view.
---I hope that you find my comments sound and interesting.
If anything, I believe that APOD should strive to maintain scientific rigour in every case, even if it means sacrificing the demonstrative appeal (but not the beauty!) of a photograph such as the one on APOD May 19.
Sincerely yours/
Javier Guillot
Universidad Nacional de Colombia
