One may be forgiven for thinking that by “solid” and “hard” he really means a really hard surface. After all, his proofs include the “tarkia” verse from Iyov which he mistranslates to say that the sky is “hard” (I will elaborate on this in another post), and Chazal’s application of that to the original formation of the “rakia.” His proofs also include his comment on the Hirhurim blog (“What do you think the spheres are made of? They are certainly solid, as the stars and planets are embedded within them!”), where someone invoked the fact that space ships went to the moon and beyond unhindered by anything in the way, as means of proof against the Rambam’s belief in the existence of celestial spheres.
One can also be forgiven for accepting the “Rationalist’s” claims that the belief in spheres that were hard, solid material, such as crystal or glass, was adopted by Chazal from the ancient Babylonians. However, recognized experts (including one Rabbi Slifkin considers the best) opine that the ancients themselves did not necessarily consider the spheres to be solid substances, or even anything beyond mathematical abstractions. (A Not-So-Solid Proof About The Spheres) When this was pointed out, Rabbi Slifkin thereupon clarified his position. He reported that,
My point was NOT that Chazal, or Rambam, held the spheres to be solid in the scientific sense of solid as opposed to liquid or gas. Rather, it was that the sphere is something with substance i.e. it is not the atmosphere, or outer space.
After I then challenged this absurd claim that the atmosphere has no substance, the “Rationalist” was forced to introduce a further clarification:
The only reason why I am qualifying my use of the word "solid" as not a scientific term is that one view in Chazal was that the rakia is made out of water, which congealed. "Congealed water" is not ice, and is not a scientifically-acknowledged phenomenon, so it can be misleading to describe it as a "solid." But it is substantive, by which I mean it is firm. Unlike the atmosphere, which while being substantive in the modern scientific sense of being made of molecules, is not firm and is not what Chazal were describing when they spoke of the rakia, which the sun travels on both sides of. (January 6, 2011 11:24 PM)
So you see, when Rabbi Slifkin described the atmosphere as non-substantive, he did not mean it in the modern scientific sense of being comprised of molecules. He meant it in a non-scientific sense—something not “firm.” But the difference is that whereas the atmosphere cannot be called “firm” in any sense, the celestial sphere was definitely considered firm in some sense. And anyway, they thought it was opaque and that the sun travels on both sides of it.
This is why, you see, the assertion that everyone believed in a solid celestial sphere is not contradicted by the fact that Rabbi Slifkin’s own expert, among others, is not certain at all that the ancients considered the celestial spheres to be solids, or even physical realities at all.
Does one get a sense of someone confused and floundering over his position, twisting and turning his own words to unsuccessfully avoid self-contradiction and absurdity?
In any case, dear readers, be not misled. Rabbi Slifkin in the end does not really think Chazal believed in really hard spheres which, only because of their hard solidity, could envelope and grip the stars and planets, would be shattered by rocket ships, and whose existence has been thereby disproven.
Never mind that he had written, and resumed to assert, repeatedly, “it was obvious to them all that the basic nature of the firmament is something hard and flat” (The Firming and The Flattening of the Rakia).
Yes, they thought it was hard. The facts are otherwise? Well, I didn’t mean hard. I mean substantive. No, wait. I mean firm. Look at the Gemora Pesachim which indicates they were thought to be opaque, and don’t forget about the Iyov verse and how Chazal applied it. So you see, Chazal thought the rakia was something hard.
But this is not an aberrant occurrence. It’s no less equivocal than saying the Blind Watchmaker Theory—which by definition means there is no divine guidance, manifest or hidden, behind the process of life development—is compatible with the Torah. It’s no less disingenuous than mistranslating “חזק” as “hard” rather than “strong,” or mistranslating Rabbi Shimon bar Yochai’s statement to hide the fact there was no mesorah about the existence of celestial spheres. Rabbi Slifkin and his admirers will ignore or declare irrelevant these errors that destroy his “proofs” and conclusions. What counts is the conclusion, regardless of how one gets there, that the mesorah of Creation must be rejected in face of cherished belief in academia’s methodologies, presumptions and consensus.
 When taken to task about this, Rabbi Slifkin diverts attention to another claim, that Chazal universally, according to all rishonim, had a mesorah that there is an opaque dome over the earth behind which the sun travels at night (see note 3). This will be dealt with in a separate post.
 “There was an unequivocal mesorah that there is a … a dome above the earth, made of some sort of substantial matter (i.e. not air or space)…. This was the universal, uncontested, view of Chazal, based on Pesukim such as that in Iyov 37:18: "Can you spread out the heavens with Him, hard as a mirror of cast metal?" …(Wednesday, January 26, 2011, What the Firmament Really Is).
 In addition to the above note’s citation (and other available citations): “… [I]t was obvious to them all that the basic nature of the firmament is something hard and flat; after all, there are numerous explicit pesukim describing the nature of the firmament, as well as other pesukim which shed light upon the basic etymology of the word” (The Firming and Flattening of the Firmament, Ratioanlist Judaism Blog, Sunday, November 28, 2010).
 (Natan Slifkin on September 7, 2010 at 2:50 pm Hirhurim blog, “Rabbis and Traveling to the Moon” comment section). Evidently, the “Rationalist” reasons that massive, heavy bodies such as stars can be contained by a body only if that body is a very hard solid. This is poor evidence, because (a) the “Chachmei Yisrael” of Pesachim 94b argued that the stars were not embedded within the spheres, but simply glided along them; and (b) more importantly, as in my reply loc. cit, it is more likely that Chazal held, as the rishonim did, that the spheres were ethereal, so their grip on the (less ethereal) stars was not thought to be that of a simple corporeal solid material.
 To this I responded, “The Rambam does not say the bodies of the spheres are bodies so solid that they would perceptively slow down rockets or would make an impact on them or vice versa. What he says indicates otherwise:
רמב”ם הלכות יסודי התורה פרק ג הלכה ג
כל הגלגלים אינן לא קלים ולא כבדים ואין להם לא עין אדום ולא עין שחור ולא שאר עינות וזה שאנו רואין אותם כעין התכלת למראית העין בלבד הוא לפי גובה האויר וכן אין להם לא טעם ולא ריח לפי שאין אלו המאורעין מצויין אלא בגופות שלמטה מהן.
“All the spheres are neither light in weight nor heavy, and they have no color...We only see them as bluish because of the height of the air...."
(Rabbi Slifkin, whose admirers regard as most civil in his interactions with those who disagree with him, went on to reply to my remarks, “It was standard Ptolemaic cosmology. Learn up about it, and you will understand what they thought the spheres are” (Natan Slifkin on September 7, 2010 at 2:50 pm ).
 See also: Wikipedia, “Celestial Spheres”: “Through an extensive examination of a wide range of scholastic texts, Edward Grant has demonstrated that scholastic philosophers generally considered the celestial spheres to be solid in the sense of three-dimensional or continuous, but most did not consider them solid in the sense of hard. The consensus was that the celestial spheres were made of some kind of continuous fluid (Grant, Planets, Stars, and Orbs, pp. 328–30). The very person the “Rationalist” names the expert on ancient cosmology contradicts his claim that “in the ancient world everyone believed that the sky is solid” in the sense of hard, rather than a fluid, or vaporous substance.”
 One wonders why is it only the “one view in Chazal …that the rakia is made out of water, which congealed,” that leads one to realize that we are not dealing with “a scientifically-acknowledged phenomenon, so it can be misleading to describe it as a ‘solid.’”? Is the other view, that the rakia was formed from “water” and “fire” any more a scientifically-acknowledged phenomenon? According to both views of the rakia’s origin, then, one should conclude that “it can be misleading to describe it as a solid.”