Dante and the early astronomers

by M. A. Orr

It cannot be in the heaven of the moon, for in the moon herself resides whatever influence comes from her sphere;[630] and she goes as far south of the equator as she does north: therefore she would raise land beyond the equator as well as on this side, but this is not so.

Then follows a passage of which commentators can make nothing as it stands:—

“Nec valet dicere quod illa declinatio non potuit esse propter magis appropinquare terræ per excentricitatem; quia si hæc virtus elevandi fuisset in luna, (quum agentia propinquiora virtuosius operentur) magis elevasset ibi quam hic.”[631]

Mr. Wicksteed suggests reading “elevatio” instead of “declinatio,” and apparently deletes the “non,”[632] and Dr. Moore seems to read the same way,[633] for both agree in interpreting as follows:—the author is stating an argument that the elevation of land (in only one hemisphere) might be due to the moon’s coming nearer to Earth in one part of her orbit, owing to its eccentricity; to which he replies that if that were the cause the elevation would be greater in the south than here in the north, because the moon is nearer to the earth when she goes south.

This is supposed to imply a belief that the moon is always in perigee[634] when south of the equator, whereas the fact that her perigee revolves all round the zodiac was as well known to Ptolemy and Alfraganus as it is to modern astronomers. The interpretation therefore convicts the author of the Quæstio of so serious and inexplicable a blunder that these commentators find themselves obliged to consider it as evidence either that it was not written by Dante, or that his knowledge of astronomy was much less extensive than has been supposed. It is suggested that there may have been a popular fallacy in his time that the moon was like the sun in this respect—although to know that the sun has a perigee, and that it is situated in the south, requires some acquaintance with astronomy! However, Dr. Moore has searched Alfraganus and other works in vain to find a suggestion of this kind.

Fig. 52. The moon’s epicycle and deferent.

(The dotted line represents the southern half of the deferent).

Yet an explanation and a source of this statement is not far to seek, and instead of proving ignorance it indicates that the author of the Quæstio knew his Alfraganus well. The passage is evidently slightly corrupt, but the clue to its meaning lies in the word “eccentricity.” In two other places of the Quæstio the moon’s eccentric “orbis” is alluded to,[635] but if we translate this as “orbit” we shall be introducing modern ideas which meant nothing to Dante or his contemporaries. According to Alfraganus and Ptolemy the moon’s orb or sphere[636] was eccentric, but this was not the main cause of her varying distance from Earth. It was the revolution of her epicycle round its centre C, which caused her to move continually from apogee M to perigee P; and as the whole epicycle was meanwhile revolving on the deferent round C, and these two periods were not quite equal, the moon was as often in the perigee of her epicycle when it was south as when it was north. But besides this, the deferent, being eccentric to Earth, had also a perigee and apogee; and Alfraganus says that the perigee of the eccentric is always in the south. “Saturnus, Jupiter, atque Mars eccentricorum suorum absidas summas et imas habent declinatas a zodiaco, illas ad boream, hasce ad austrum, secundum eandem semper deflexus mensuram: quedadmodum res in Luna obtinet.”[637]