Greeks and Romans and waxing and waning of the moon

[bonzer]

@njtt

As for the brightness of planets, obviously that depends on a lot more than your geometrical model of the solar system, which is not going to tell you, for instance, why Jupiter is brighter than Mars or why Venus is brighter than either. I suppose it might tell you something about long term variations in the brightnesses of individual planets. I do not know if this was something that actually influenced Copernicus.

Offhand, I think it may be true that the argument about the variation in the brightnesses of the outer planets as evidence for heliocentrism is not something Copernicus himself ever expressed, but there is the following passage in Rheticus’s Narratio prima, as part of his third of six arguments in favour:

Although Saturn and Jupiter, as they appear to us at their morning and evening risings, readily yield the same conclusion, it is particularly and especially supported by the variability of Mars when it rises. For Mars, having a very dim light, does not deceive the eye as much as Venus or Jupiter, and the variations of its size is related to its distance from the earth. Whereas at its evening rising Mars seems to equal Jupiter in size, so that it is differentiated only by its fiery splendor, when it rises in the morning just before the sun, it can scarcely be distinguished from stars of the second magnitude. Consequently at its evening rising it approaches closest to the earth, while at its morning rising it is furthest away; surely this cannot in any way occur on the theory of an epicycle. Clearly then, in order to restore the motions of Mars and the other planets, a different place must be assigned to the earth.

(Edward Rosen’s translation in Three Copernican Treatises, 1939; Dover, 1959, p137. For those who don’t know, the Narratio prima was Rheticus publishing a “trailer” for the forthcoming Copernicus.) It can certainly be argued that the Narratio prima didn’t influence the writing of De revolutionibus, but it’d be perverse to argue that Copernicus never read it and hence wasn’t aware of this particular argument. All in all, the argument was part-and-parcel of Copernicanism right from the start.

I’d also worry about playing down the Aristarchus method too much. Until the late-17th century, it tended to be what most everybody relied on, in one form or another, to establish the scale of the solar system.