In
this post in the Jewish Worker, Nobody requests clarification of the observational arguments against geocentrism in the light of late 58th century science. It will be useful to distinguish two further pairs of geocentrisms beyond the diurnal vs. annual distinction of my original post on the subject. Let us consider the oppositions of
strong and
weak geocentrism and
physical and
philosophical geocentrism.
By strong geocentrism, I refer to the concept that the Earth is indeed fixed, and all the observed heavenly motions are the true physical motions, and are not reflections of the motions of a moving Earth. This concept requires that there be something that the Earth is fixed with respect to. Consequently, strong geocentrism is inconsistent with relativity, where no such absolute frame of reference is possible. Ironically, while the Cosmic Microwave Background does appear to give an absolute reference frame even within General Relativity, the Earth is moving with respect to it.
To be consistent with relativity, we need to turn to weak geocentrism. In this view, what is claimed is not that the Earth is absolutely fixed, but rather that the preferred reference frame is the one where the Earth is unmoving. Note that the reference frames under comparison are accelerating ones, and consequently are not equivalent in the sense that steadily-moving, i.e. inertial, frames are.
Now let us consider four observations that suggest that it is the earth that is moving, and not the rest of the universe. In three of the four cases, apologetic arguments can and have been made that the observation does not present a proof for a moving Earth. (The fourth result is too new for the apologists to have gotten around to yet.) The problem with the apologetics is that they are mutually incoherent, and do not lead to an overall, consistent description of the universe. A series of special pleadings do not make for a universally useful physical theory.
1)
Stellar parallax When viewed from different positions in space, the apparent position of a nearer star on the sky moves with respect to those further away. This is most visible as a consequence of the Earth's annual motion about the center-of-mass of the solar system, but, with sufficiently sensitive instruments, parallax can be observed when comparing the apparent positions of a nearby object in the evening and in the morning. This
diurnal parallax is certainly visible for solar system objects with modern instruments, although the effect for Mars was likely just beyond the reach of the greatest of the naked-eye observers, Tycho Brahe.
2)
Stellar aberration This is a change in the apparent position of stars due to the finite speed of the light reaching us from the stars. Depending on the direction of motion of the observer, the light detected will come from slightly different directions, much like a moving umbrella must lean forward to keep the person under it dry. This has diurnal, annual, and steady components. (This one is admittedly hard to visualize, but does depend on a moving Earth.) For both parallax and aberration, either it is the Earth that is moving, or the rest of the universe is set up so that it
looks like it is the Earth that is doing the moving.
3)
Foucault's pendulum Set up a freely hanging pendulum swinging back and forth in a certain plane. Over the course of the day, the apparent plane of the pendulum will appear to rotate with respect to markings on the Earth around the pendulum. There is no force on the pendulum to change its plane of motion, rather the Earth is rotating underneath it.
4)
Lense-Thirring effect This is the new one. In General Relativity a rotating mass "drags" the fabric of space time as it rotates. This is a subtle effect, but has now been measured for the Earth.
Sliced with Occam's razor, the simplest interpretation of these observations is that it is the Earth that is moving. In addition to
ad hoc apologies to explain these observations, we also have to posit that most of the universe is moving vastly faster than the speed of light to maintain its daily rotation around the earth. That they suffer instantaneous and incredibly forceful accelerations whenever an earthquake changes the length of the day.
While Mach's Principle is often invoked to support the geocentric position, it seems to me that this is to misunderstand the principle itself, while elevating to confirmed status an idea that remains controversial.
In terms of physical geocentrism, such observations lead one to reject strong geocentrism for its physical incoherence. Weak geocentrism, while admissible, is not much practical use, since all calculations are required to include extraneous terms to account for the fixedness of the Earth. The justification for this position is not physical, but rather philosophical; some reason beyond physics for choosing such a reference frame over all others. And philosophical geocentrism is not a scientific position at all.
