In Ptolemaic worldview, each element of the physical world existed below the earth, on the earth or up to the orbit of the moon. The perigee of the moon, the front edge of the galgal, began a spiritual realm where the celestial bodies began. Both the body and the track that drives its motion were intellectual beings. The apogee of the body ended the galgal and began the next. The Rambam allows no space between the galgalim. Thus, the apogee of the moon is the perigee of Mercury. The apogee of Mercury is the perigee of Venus, followed by the sun and so forth out to Saturn. Then, was the star galgal which went slowly and then the empty retrograde galgal that drove the sky. The outer planets were not yet known. It is important to remember that gravity was not part of the equation yet, so there was no obvious problem with Venus being so close to the sun nor with galgalim driving celestial bodies in motion.
The moon was approximated at 1/40th the size of earth and the sun at 187 times. Of course these were based on estimates of distance that were completely wrong. While the Rambam does not bring any distances, the Talmud says that from one Rakia to the next was 500 years. At 30 miles a day (40 parsahs), we can estimate that each galgal was about 5.5 million miles. The Rambam does not bring this in Mishna Torah, though the first Chabad Rebbe brings it in the Tanya as a literal example.
Questions abound in this. For one, there is no way to resolve the moon and sun to their sizes using the same scale, volume, radius etc. Nor can we resolve that the sun is the largest and Mercury is the smallest, by volume, radius, apparent magnitude, actual magnitude etc. We know the exact distance to the moon, Venus, Mars, Jupiter, and Saturn as we could not have gotten spacecraft there successfully without that knowledge. We realize that gravity, rather than galgalim drive the planets. We do not understand how the inanimate rocks and gases of space nor the imaginary galgalim can be intelligent.
The short answers:
1) Chabad has explained that the geocentric universe is not a problem given the theory of relativity of motion. Hence, we cannot resolve whether the sun orbits the earth or vise versa.
Fair enough. But that only applies to a two body system. Given the Rambam's model, Venus and Mercury must always be closer to earth than the sun, since he holds that the galgalim never cross. But, in the heliocentric model, it must happen frequently that these two bodies are on the far side of the sun. Remember, we have landed a probe on Venus; we know where it is. We know without doubt that it does indeed go behind the sun. (I hope we don't get an argument on that.)
2) Chabad has explained that we do not know to what the 187 factor of the size of the sun refers. There may be a layer of the sun that fits this measurement.
Fair enough. Ignorance is rarely acceptable as proof, but, remarkably, Ptolemy has the same measurement, presumably for the entire sun. There is no measurement for which the sun is 187 times the size of earth and that for the same measurement, the moon is 1/40th. One would think that the 187 factor was relevant to something obvious and observable. That is all that they had.
3) To the above, they answer that Ptolemy copied the work of the Bnei Yisachar, the Jewish sages who were experts on astronomy.
Really??? Then why does the Rambam reference the "Greek Sages" in this section of Mishna Torah? And if it was secret, as the Rambam implies, how did Ptolemy get it but the Rambam did not. Also, in Pesachim 94b, why do the Jewish sages have ideas that are markedly different than the Ptolemaic model? And let's not lose sight of the fact that Ptolemy was WRONG!!!
4) Some of the facts can be resolved with known phenomena. For instance, gravity may be the same as galgalim.
Perhaps, but I cannot glue a head, ears and trunk together and create a living elephant. The view as a whole is completely inconsistent with reality. It is completely consistent with the science of the Rambam's time.
I could certainly go on and on, but I will not torment you, friend. The answer is clear and obvious. The only conclusion is that the Rambam did not get this from a little bird at Mt. Sinai, as he said himself. To this, they will offer that the Chabad Rebbe knew as I do that the physical facts and the Rambam's own statements were problematic, but he insisted on this stance anyway, due to the fact that it is stated as halacha, which must be true. Assuming that the source is the Chabad Rebbe - I'll take them at their word - I'd have to put the question to him. Though he has been dead for over 10 years, some Chabad Chasidim claim that they can still communicate with him and he with them. So now we even have a test for this notion as well.
(Note: I have left out references for brevity and the fact that they chop up the point to the casual reader. The astronomy facts, however, are not controversial and are well known, as are the Rambam's statements.)