Having laid out “the sum total of the chief topics one may mention as having to do with the fixed stars, in so far as the phenomena [observed] up to now provide the means of progress in our understanding” Ptolemy now moves on to “the treatment of the five planets” each one getting a book to itself.
But before diving in, Ptolemy first proposes that we “avoid repetition” by “explain[ing] the theory of the [planets] by means of an exposition common [to all five], treating each of the methods [for all planets] together.”
Beginning this discussion, Ptolemy introduces us to the order of the spheres for these planets, the sun, and moon.
The first thing Ptolemy notes is that each of the spheres for the planets and moon have their poles “nearly coinciding with the poles of the inclined, ecliptic circle” which is another way of saying all these circles lie in nearly the same plane.
Ptolemy then cites a general consensus among astronomers that
all the spheres are closer to the earth than that of the fixed stars, and farther from the earth than that of the moon, and that those of the three [outer planets] are farther from the earth that those of the other [two], and the sun, [with] Saturn’s being greatest, Jupiter the next in order towards the earth, and Mars’ below that.
That leaves us with the sun, Mercury, and Venus which evidently had some debate:
But concerning the spheres of Venus and Mercury, we see that they are placed below the sun’s by the more ancient astronomers, but by some of their successors these too are placed above [the sun’s], for the reason that the sun has never been obscured by them [Venus and Mercury] either. To us, however, such a criterion seems to have an element of uncertainty, since it is possible that some planets might indeed be blow the sun, but nevertheless not always be in one of the planes through the sun and our viewpoint, but in another [plane], and hence might not be seen passing in front of it, just as in the case of the moon, when it passes below [the sun] at conjunction, no obscuration results in most cases.
I actually found this comment somewhat surprising in several regards. The first thing that surprised me is that Mercury and Venus do both transit the sun but no observations of this existed for Ptolemy.
In a sense, this isn’t particularly surprising since a transit would be nigh impossible to view without a filter or moderate cloud cover and transits are sufficiently rare that they would go unnoticed, or perhaps more likely, they were observed but not understood due to the relatively small number of people trained in astronomy.
But that Ptolemy didn’t consider the amount by which either of these planets would potentially obscure the sun being largely unnoticeable is, in and of itself, rather surprising to me.
I’m curious as to the first actual recorded transit of these two planets. After all, I’m aware that Charlemagne observed a dark spot on the sun that he incorrectly attributed to a transit of Mercury in $807$ CE. However, it appears that the first recorded transit of Mercury was in $1631$ and Venus in $1639$.
Regardless, Ptolemy declares the problem unsolvable as
there is no other way, either, to make progress in our knowledge of this matter, since none of the stars1 has a noticeable parallax (which is the only phenomenon by which the distances can be derived), the order assumed by the older [astronomers] appears the more plausible.
Why does Ptolemy find this more plausible?
For, by putting the sun in the middle, it is more in accordance with the nature [of the bodies] in thus separating those which reach all possible distances from the sun and those which do not do so, but always move in its vicinity; provided only that it does not remove the latter close enough to the earth that there can result a parallax of any size.
What Ptolemy is referring to here is that Mercury’s maximum elongation is $28º$ and Venus’ is $47º$. However, Mars, Jupiter, and Saturn can have any elongation.
There’s no reason that dividing the planets this way would have explanatory power over this fact. Rather, this division Ptolemy suggests is merely categorical.
However, Ptolemy does offer the caveat that, if Mercury and Venus were brought sufficiently close to the Earth with their epicycles, then parallax should be visible, and thus, any model would need to ensure this weren’t the case to any appreciable degree.
