I was over at ChabadTalk, that old bastion of geocentrism, among other things. While there, someone (not me) made the comment that the Rambam was mistaken about science. Another poster asked for some elaboration, which I provided:
The planets aren't attached to spheres, for starters. Even the most ardent geocentrist would have to admit that the Rambam was wrong on that.
In addition, even if you're a geocentrist, the orbits of the planets are ellipses, not spheres, as the Rambam describes.
The Rambam states that the Earth is 40 times larger than the moon - in reality, the Earth is eighty times as massive as the moon. He also states that the Sun is about 170 times the size of the Earth -- that, too, is wrong, by quite a margin -- the Sun is about 333,000 times more massive than the Earth.
The Rambam states that there are no stars larger than the Sun. That's true to the observable eye, but is clearly false -- there are many stars that are much larger than the Sun.
He also states that there is no "star" smaller than Mercury. Well, that's true on the face of it -- a body that small cannot start nuclear fission. However, if you're going to posit that the Rambam used the term "kochav" to mean any celestial body (as you would have to, unless you are positing that Mercury and the other planets are stars too), then that statement is false, as there are plenty of celestial bodies smaller than Mercury.
All these statements of the Rambam can be found in Hil. Yesodei HaTorah chap. 3.
One poster decided to answer my challenge regarding the weight of the moon with what has to be the single most mind-numbingly stupid thing that I've heard in all the science/Torah debates (bolding mine):
The moon isn't made of gas, as far as modern scientists know (the bunch of crackpots that they are).
Of course, we have to realize that 1) nature changes (so maybe in times of Rambam the moon was takeh made of gas), 2) modern science does not rely on certainties but only on probabilities, so it is only 99.999% probable that moon isn't made of gas; on the other hand, everything that Rambam wrote was guided by h"p, so he can't be wrong, even if he himself said (for kiruv purposes surely) that sages of Torah may be wrong in the matters of science. Which makes it 100% true that moon is made of gas.
The moon is made of gas?? Forget the fact that we've sent out probes to the moon. Forget the fact that twelve people have actually walked on the surface of the moon. Forget the fact that moon rocks have been bought back to earth. What the poster doesn't seem to realize is that you can simply go out at night and look at the darn thing in the sky and see that it's made of rock. Gas doesn't cause the craters that you can see on the moon on any clear night.
Lest you think that the stupidity ends there, I followed up by asking the following question:
So, how do you then explain away all the data indicating that the moon isn't made of gas???
The response I got from another poster (warning: brain-numbing response ahead!):
well, that's your problem. The Torah is absolute(ly) true.
I'm just completely flabbergasted. Seriously, what we need in our yeshivos is a good re-education as to what exactly is Torah and what is science and where the two intersect. The Rambam's statements regarding astronomy (despite the fact that he placed them in Hilchos Yisodei HaTorah) are not actually fundamentals of Torah. There is no Torah source before him that states these facts - they simply represent the science of the day as he knew it. It's not like he looked up in a Gemara where it says that there is no celestial body smaller than Mercury - it was something that he either observed on his own or learned from other astronomers. The simple facts that he presents can easily be disproved, and yet, people are so blinded by the mistaken notion that everything the Rambam ever put in writing is Torah (and therefore irrefutable - even by the evidence of our own eyes) that these people are forced to come to the mindboggiling conclusion that the moon is either (a) made of gas or (b) somehow changed from gas to rock sometime since the Rambam.
I'd say that the parents of these people should ask for their money back from the yeshivos they went to, but, sadly, they probably got exactly what they wanted.