The Observant Astronomer

The passing scene as observed by an observant Jew, who daylights as an astronomer.

Monday, June 06, 2005

Geocentrism II

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.


Anonymous Boruch said...

WRT you last paragraph, are you saying that philosophically, weak geocentrism is tenable (ie not disprovable by any experiment)?

If so, then surely that is all the Jewish proponents of it (like the Lubavitcher Rebbe, obm) are saying?

And if sol what's all the fuss?

1:35 a.m., June 06, 2005  
Blogger The Observer said...

The fuss comes from those who don't recognize what variety of geocentrism is meant when the concept is invoked. If all that is meant is weak, philosophical geocentrism, then apologetics for the data are irrelevant and shouldn't be brought into the discussion. When some stronger, physical geocentrism is meant, then the arguments begin.

10:37 a.m., June 06, 2005  
Anonymous Nobody said...

Wow, thanks for such a detailed response to my questions.

Please don't read any of my comments as an attempt to support a hard geocentrism, so we can stay on the same page.

"Sliced with Occam's razor, the simplest interpretation of these observations is that it is the Earth that is moving."

Apparently, these do not contradict a stationary earth reference frame, correct?

If so, then bluke's point that "the observable facts that we have seen about the world" contradict describing the sun rotating around the earth daily would be wrong in such a context?

"And philosophical geocentrism is not a scientific position at all."

But since the choice of reference frames is philosophical, wouldn't all reference frame choices be equal, scientifically? Then a geokinetic view would be equally philisophical? If not, why not?

9:04 p.m., June 06, 2005  
Blogger The Observer said...

No, they don't contradict a stationary Earth reference frame, in the weak sense, but it's hard to remember that it is only in the weak sense when so many want to promote it to a strong geocentrism.

The difference between physical and philosophical geocentrism is that the former implies some practical, observational, consquence of choosing one or other of the viewpoints. I.e., what is the nafka meena? Or, in a Popperian sense, for something to be scientific, it needs to have a falsifiable prediction. Weak geocentrism just doesn't seem to have any such prediction, so it can't be considered scientific. To extend the talmudic metaphor, it is like a disagreement in agaditah with no halachic ramifications.

1:56 p.m., June 07, 2005  
Anonymous Nobody said...

"Or, in a Popperian sense, for something to be scientific, it needs to have a falsifiable prediction."

So are you saying that stating that earth revolves around the sun and rotates on its axis is equally lacking in a falsifiable prediction? Or more precisely, picking a reference frame in which that would be true is equally lacking?

The nafka meena here is that there are many who take the position that if you do not believe the earth rotates around the sun you are ignorant, that anyone claiming such a thing is wrong, and that this is conclusively proven.

7:47 p.m., June 08, 2005  
Blogger Rebeljew said...

My impression is that:

1) The argument only works if we are discussing the weak variety.

2) In the case of the weak variety, no apologetics are relevant.

Hence, it is the apologetics themselves make the problem for the proponent.

Is that it?

12:52 p.m., June 12, 2005  
Blogger The Observer said...


A rotating earth does have falsifiable predictions. For example, if the earth is rotating, then, under General Relativity, the rotation of the earth should produce a measureable effect on orbiting satellites. This is the aformentioned Lense-Thirring effect, now observed and in the process of being measured more precisely. Note the classic scientific method here: Hypothesis, experiment, confirmation.

A moving earth is in opposition to strong geocentrism, not weak geocentrism. In order to reproduce the observable consequences of a moving earth, the strong geocentrist is reduced to ad hoc theories and apologetics, as rebeljew writes.

One thing to remember is that the traditional argument, as you've indeed expressed it, is Geocentrism vs. Heliocentrism, but the real opposition, in the strong sense, is fixed earth vs. moving earth. And arguing for a fixed earth is an uphill battle these days.

1:14 p.m., June 15, 2005  
Anonymous Nobody said...

"A rotating earth does have falsifiable predictions. For example, if the earth is rotating, then, under General Relativity, the rotation of the earth should produce a measureable effect on orbiting satellites."

How does this square with your statement that Weak geocentrism is admissible?

3:38 p.m., June 15, 2005  
Blogger The Observer said...

Remember, weak geocentrism is a GR coordinate transformation to a system where the surface of the earth is not moving. Otherwise, the physics is identical. Much messier mathematically, but identical.

(The classic trick in solving problems in GR is to find the coordinate system in which the problem is particularly simple, solve it in those coordinates, and then transform the solution into some other coordinate system where the solution is more useful. GR does not make value judgements about coordinate systems. You are free to work in the one that you prefer.)

4:56 p.m., June 15, 2005  
Anonymous Nobody said...

"Otherwise, the physics is identical."

This implies to me that the earth could, in GR, not actually be moving, even daily. Am I misunderstanding you?

5:07 p.m., June 15, 2005  
Blogger The Observer said...

Yes, at least in terms of a discussion of weak geocentrism. In that case, as I tried to indicate in the parenthetical comment above, the choice of coordinate system has no physical significance. Strong geocentrists, on the other hand, assert that the coordinate system is significant, and that the true choice is the one where the earth is fixed. On the basis of the observational evidence, this is a very difficult assertion to sustain.

11:50 p.m., June 15, 2005  
Anonymous Nobody said...

"the choice of coordinate system has no physical significance"

The way I understand that is that either way, all observations will be accounted for to the same degree. Or in other words, they are physically different (in one reference frame one thing is stationary, in another a different thing is), but the difference predicts no differentiating observation. Is that what you mean to say?

As I said at the beginning I am not trying to sustain a strong geocentric argument.

9:40 a.m., June 16, 2005  
Blogger The Observer said...

I think you've got my statement backwards. There is no disagreement over which is the accelerated frame, i.e. that the earth is rotating. It is only with intertial systems, i.e. non-accelerating, where there is no way to tell the difference between them. In the present case, doing your calculation with a fixed earth does not imply that the earth is fixed and universe rotating around it.

1:55 p.m., July 13, 2005  
Anonymous Anonymous said...

Has "Rebel Jew" not heard of Mach's Principle?

Foucault's Pendulum, Coriolis Force, Earth bulge, weather patterns etc, can no longer be used as proof of a rotating Earth.

NASA uses Geocentric maths for all its launches!

The simplest interpretation of the zero-velocity result of the Michelson-Morley experiment is that the Earth really is motionless at the center of the universe: "The Earth is located at the center of the universe" (Rambam, Hilchos Yesodei HaTorah 3).

9:52 a.m., July 14, 2005  
Anonymous Anonymous said...

Earthquakes, like the recent one, are often claimed to be a disproof of geocentrism.

The usual explanation for earthquakes is that they are due to to stress build-up along cracks in the Earth. When the stress reaches breaking point, the earthquake happens and the spin of the Earth is affected.

But by the Advanced Potential model and Einstein, cause and effect can be reversed, and earthquakes can be said to be due to stresses and strains within the rotating universe which causes a corresponding strain build-up in the earth.

Once the strain snaps, the Earth slips and the earthquake happens (usually along a weak point like a fault), and the universe adjusts its rotation rate accordingly!

10:21 a.m., July 14, 2005  
Anonymous Nobody said...

"There is no disagreement over which is the accelerated frame, i.e. that the earth is rotating."

You will have to forgive my ignorance here, but I am not understanding the evidence for that statement. How would stellar parallax provide evidence for that statement, for example?

I was intending to bring it up, but only after I understood what you are saying (not quite there yet), however since anonymous brought it up, how does the Michelson-Morley experiment fit in with what you are saying?

11:14 a.m., July 14, 2005  
Blogger The Observer said...

Anonymous: The problem with your argument is that a series of words does not a physical argument make. An invocation of "Mach's Principle", as if it were some sort of absolute truth rather than the conjecture that it is, can not suddenly sweep all the other arguments away.

Astronomers use geocentric math to plan their observations. So what? It is the simplest coordinate system to work in. The map is not the territory. Further, with all respect to the Rambam, an attempt to launch a spacecraft with his physical model will not get you anywhere.

As for the Michelson-Morley experiment, remember that this was an experiment to determine the absolute velocity of the Earth with respect to the "ether" which was supposed to pervade space. But no velocity was measured.

Consider then these propositions:
P1: The ether exists and forms an absolute reference frame.
P2: The earth is moving.

Hypothesis: Given P1 and P2, the earth's absolute velocity can be measured with the MM device.

Result: No velocity is measured.

Inference: Either P1 or P2, or both, are wrong, or we've misunderstood the physics involved.

It turns out that the last is the case, as Lorentz showed, and the new physical theories, Special and General Relativity, fully explain the Michelson-Morley result, and, along the way, do away with the ether.

2:27 p.m., July 14, 2005  
Blogger The Observer said...

Michelson-Morley I discuss above. As to your other question, reread the main post. Something is moving. The balance of evidence and plausibility says it is the Earth, not the rest of the universe in a quite coincidental way. Rewriting the physical equations in a coordinate system where the Earth is fixed doesn't change that.

Look at it this way. If you are on a carosel, I can write the equations for your motion either in a frame where the carousel is fixed and the fairground rotating, or where the carousel is rotating and the fairground fixed. But all will agree that it is the carousel that is spinning. That is why the force that you feel trying to throw you off is labelled a "fictitious" force when the calculation is done in the carousel-fixed frame. It is one that arises from doing the problem in an accelerating frame of reference. One of the main postulates of General Relativity is the Equivalence Principle: That gravity is the same as accelerated movement. I.e. gravity, in a sense, is a fictitious force.

2:39 p.m., July 14, 2005  
Anonymous Anonymous said...

If you accept Relativity then your are obliged to accept that "the geocentric paradigm is at least as good as anyone else's!" (Sir Fred Hoyle).

Any proof of heliocentricity/disproof of geocentricity is simultaneously a disproof of Relativity.

If the results of "Gravity Probe B" disprove Einstein, then the simplest explanation is that the zero-velocity MM result is real, & that the ether exists. Might not the ether be Malchus of Assiyah - "He clothes Himself with light as with a garment" (Tehillim 104).

In Barbour & Bertotti's geocentric model, gravity is viewed as a real, not a fictitious force.

4:47 p.m., July 14, 2005  
Anonymous Nobody said...

"One of the main postulates of General Relativity is the Equivalence Principle: That gravity is the same as accelerated movement. I.e. gravity, in a sense, is a fictitious force."

Right, so my confusion is in how you categorically determine that one is operating and not the other when, unlike the carousel, you don't know or control the origins of the motion.

Let me try to explain my thought with a thought experiment. Following what you already said that physics can be counter-intuitive if not bizzar, let's pretend our intuitive feeling were different. Let's pretend that it was more intuitive to postulate a moving universe around the earth than a moving earth. (I realize you might say that was precisely the situation 400 years ago, but then they thought in absolute space-time. I'm saying pretend it is more intuitive to imagine a rotating universe).

Now, you, as the physicist, would come along and say that we have to abandon our intuition in light of which evidence?

1) The coincidental nature of the movement (it looks like the universe moves for very special effect on the earth).

2) Events on the earth affect the whole rotation of the universe.

Both of those might be the intuitive view postulated in my thought experiment. Or is there something more than intuition driving problems with those views?

So we are left with your 4 observations. I'll pick the most likely candidate because I understand it the least. The Lense-Thirring effect? That effect would not be expected if the universe was doing the rotating?

6:31 p.m., July 14, 2005  
Blogger The Observer said...


I'm not saying that the evidence is absolutely conclusive, but, in the situation that you propose, it would seem to me that the "Universe is rotating, the Earth is fixed" paradigm would soon collapse under the weight of several other contradictions in addition to the two you mention.

a) The discovery of Jupiter's satellites by Galileo was amongst the more shocking of his discoveries. Here was clear evidence of revolution around an external object, and clearly showed that there was more than one possible center for celestial motions. The discovery over the last decade of over a hundred planets around other stars reinforces this. For your scientist, despite his intuition, the question has to be, why the Earth?

b) A universal rotation, at least under relativity, has certain consequences beyond just causing the sun to rise and set, but this is a very technical point that I'm not sure I fully understand.

c) Go back to my original post. That moving universe has to be a composite of both the diurnal and annual motions, so it isn't just a constant rotation. And, when you look at it more subtly, there is far more to it than just those two motions. There is precession and nutation of the diurnal rotation. The annual motion is linked to the motions of the other planets in the solar system. With sufficiently precise measurements your going to end up asking not how the universe is causing earthquakes, but how come it is also being dragged around by Jupiter. Or is it vice versa.

What most of the geocentric apologists seem to miss is that the current scientific model has a certain coherence extending to phenomena completely divorced from orbital motions. This coherence is conspicuously lacking in the geocentric model, and I think in your hypothetical case this lack of coherence would eventually lead to a scientific revolution.

The Aristotelian, pre-SR model is about as intuitive as you can imagine. And it cannot withstand experimental scrutiny.

1:59 a.m., July 18, 2005  
Anonymous Nobody said...

"For your scientist, despite his intuition, the question has to be, why the Earth?"

Yes, but as a religious Jew, that isn't a question. The earth is a special place in creation.

"This coherence is conspicuously lacking in the geocentric model."

Fair enough. I did completely misunderstand your original post. I thought you meant that strong geocentrism implied that there was evidence for a rotating universe around a stationary earth over and above any other conceptualization, and weak geocentrism meant there could be no evidence conclusively showing one over the other. Rather what you meant was that someone might like to do their math a certain way. Interesting, but scientifically pointless.

It would be nice if people less familiar with the evidence than yourself would realize "the evidence is [not] absolutely conclusive."

Anyway, I have a couple of quotes from other scientists to ask you to put some context on, given your explination of the scientific view point. I really appreciate the time you have spent on this.

The first is Sir Fred Hoyle:

The relation of the two pictures [geocentricity and heliocentricity] is reduced to a mere coordinate transformation and it is the main tenet of the Einstein theory that any two ways of looking at the world which are related to each other by a coordinate transformation are entirely equivalent from a physical point of view ... . Today we cannot say that the Copernican theory is “right” and the Ptolemaic theory “wrong” in any meaningful physical sense.

And George Ellis

“People need to be aware that there is a range of models that could explain the observations,” Ellis argues. “For instance, I can construct you a spherically symmetrical universe with Earth at its center, and you cannot disprove it based on observations.” Ellis has published a paper on this. “You can only exclude it on philosophical grounds. In my view there is absolutely nothing wrong in that. What I want to bring into the open is the fact that we are using philosophical criteria in choosing our models. A lot of cosmology tries to hide that.”

Finally, although not necessarily the statement of a scientist, wikipedia:

If general relativity is true, then there is no way to prove that the Earth is not the immobile center of the universe.

Thanks again for all your time.

3:44 p.m., July 18, 2005  
Blogger The Observer said...

Sir Fred and Ellis know, or knew, a lot more GR than I ever will, but I think their comments are offered in the sense of weak geocentrism as I've presented it. I don't think they meant it in the strong sense.

11:39 a.m., July 19, 2005  
Anonymous Nobody said...

On rereading everything you wrote, I am really only seeing philosophical objections to geocentrism. Perhaps I am missing something?

A reference frame involving a fixed earth would predict the same observations as a different reference frame, correct?

If so, then your objection to a physical geocentrism is that the way in which everything moves in such a reference frame seems so weird. "This coherence is conspicuously lacking ..." but the coherence you speak of seems to be one of why is the earth special.

Also, while Ellis' statement may be taken to mean the earth at the proximate center, not the absolute center, I don't see how Hoyle's statement can be just about math.

10:24 a.m., July 22, 2005  
Blogger The Observer said...


Let's consider this from another angle.

Let G be the proposition that the Earth is absolutely fixed and unmoving, i.e. strong geocentrism. And, in opposition, let M be the proposition that the Earth can move.

Let D represent the various observations suggesting accelerated motion.

Finally, we will consider all this evidence from the standpoint of S, modern science.

Then, it is fairly straightforward to show that:

P(G|D,S) P(D|G,S) P(G|S)
------------ = ------------ ----------
P(M|D,S) P(D|M,S) P(M|S)

where P(X|A,B) means "The probability of X given A and B".

Now, P(G|S)/P(M|S) is a small number, since P(G|S) is well nigh unto zero.
Even assuming G, hypothetically, it is pretty unlikely that we would observe D, whereas, assuming M, some such observations are quite likely. So P(D|G,S)/ P(D|M,S)is also small. Consequently P(G|S)/P(M|S) is very very small. In other words, the odds on G with respect to M, given the observations D, are infinitesimally small, from the viewpoint of modern science.

Your typical advocate of strong geocentrism doesn't actually hold by S, however. He's starting from the viewpoint of F, the dogmatic belief that the Earth is fixed. For such a person P(G|F) = 1 and P(M|F) = 0 and no amount of data is going to sway him. So, P(G|D,F)/P(M|D,F) in indefinitally large.

The problem is that most of the time the parties to the argument are trying to calculate

P(G|D,F) P(D|G,F) P(G|F) P(D|S)
------------ = ------------ --------- -----------
P(M|D,S) P(D|M,S) P(M|S) P(D|F)

where the third factor on the right side comes because the two parties are operating from different sets of assumptions. The first factor, in the absence of apologetics is close to zero. The second is of order unity. The third is close to 1/0, since the dogmatist does not expect to see any evidence for accelerated motion.
The result is that P(G|D,F) /P(M|D,S) is indefinite, so it isn't surprising that nothing much comes of such conversations.

Weak geocentrism, call it W, on the other hand, is the proposition that you can look at D from either the perspective of a fixed or moving Earth equally validly within General Relativity, i.e. S. The person who holds F would like to take W to mean G, but it doesn't.

1:24 a.m., July 24, 2005  
Anonymous Nobody said...

"Even assuming G, hypothetically, it is pretty unlikely that we would observe D, whereas, assuming M, some such observations are quite likely."

That gets back to my original question, which was:

"A reference frame involving a fixed earth would predict the same observations as a different reference frame, correct?"

That would seem to be what Hoyle is saying. Do you disagree with that? Are these observations not predicted by GR, but rather only fitted post-observation, into either reference frame?

Also how do you quantify the "pretty unlikely" if not by philosophical reference?

10:47 a.m., July 29, 2005  
Anonymous Anonymous said...

GR does not say that there is no center to the universe. It says that we cannot determine that center!

7:00 p.m., August 22, 2005  
Anonymous avakesh said...

I think that the first 2 and the fourth proofs can be dismissed based on the Ptolomeian assumption that while the sphere of the stars rotates around the earth, the sun rotates around a point near but not inside the earth. If we make the same assumption for the stellar sphere, these phenomenae can potentially be explained since stars would move laterally in relation to the earth. It would not take a greater adjustment of the Ptolomeian system than the observations forced ancient astronomers to make, namely the epicycles.

Philosophically, the center being in the Earth makes the earth the lowes point, where really Hell should be and the Heavens the highest. By locating the center outside of the earth, we would have a system where the earth is between Heaven and Hell.

12:50 p.m., April 12, 2009  

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