What he takes issue with is the following quote from Lewis:
The reader of this book will already know that Earth was, by cosmic standards, a point -- it had no appreciable magnitude. The stars, as the Somnium Scipionis had taught, were larger than it. Isidore in the sixth century knows that the Sun is larger, and the Moon smaller than the Earth (Etymologies, III, xlvii-xlviii), Maimonides in the twelfth maintains that every star is ninety times as big, Roger Bacon in the thirteenth simply that the least star is 'bigger' than she. (Discarded Image, 97)
Van Helden uses this quote to claim that Lewis was "curiously uninformed" about the dimensions of the medieval model of the universe, and states, "Had Lewis actually looked at Roger Bacon's Opus Maius instead of using a secondary source, he would have found the Ptolemaic System in all its detail laid out before him" (Measuring the Universe, 28).
There are a few points to make about this. First, to claim this one quote somehow demonstrates a general unfamiliarity with medieval cosmology on Lewis's part is simply unjustified. We would need more evidence, much more evidence, to warrant such a conclusion. Second, Lewis directly quoted Bacon only a few pages earlier (Discarded Image, 93), so he was obviously familiar with his writings firsthand. Third, the quote in question is only referring to the size of the stars, not to their distances; something made evident by the fact that immediately following the quote, Lewis clearly changes the subject to their distances, writing, "As to estimates of distance..."
With this, I am unable to see that Van Helden's criticism has any merit. Lewis's statement that "Roger Bacon in the thirteenth [century maintains] simply that the least star is 'bigger' than she [i.e. the Earth]", seems relatively innocuous. Perhaps Bacon wrote extensively about the size of the stars, and so Lewis should not have written the word "simply". However Van Helden does not cite any such estimations on Bacon's part, and I don't have a copy readily available to check. At any rate, Lewis's statement does not appear to be a significant misrepresentation. Bacon certainly affirms his main thesis when he writes that "the Earth does not possess any sensible size with respect to the heavens" (Opus Maius, 1:258), something Van Helden himself notes (Measuring the Universe, 36).
(cross-posted at Agent Intellect)
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