Dante and the early astronomers

by M. A. Orr

Then follows a discussion concerning the substance not only of the moon but of all the heavenly bodies. On hearing that he has reached the boundaries of the immortal world, and has entered “la prima stella,”[669] Dante eagerly enquires of Beatrice what are those dark markings in the body of this planet which are seen from Earth and have given rise to the fable of Cain. She smiles a little, and bids him tell her first what is his own opinion. He advances the theory which he had taught in the Convivio, viz. that the parts of the moon which look dark to us are less dense than the rest, and therefore the rays of the sun when striking them are not stopped and reflected back to us; hence the brightness is less than in other parts of her surface.

“L’ombra ch’ è in essa, ... non è altro che rarità del suo corpo, alla quale non possono terminare i raggi del sole e ripercuotersi così come nell’ altre parti.”[670]

This explanation seems to have been first suggested by Averroës, the Arab philosopher and astronomer of Cordova, who was known in the Middle Ages as “The Commentator,” from his famous commentary of Aristotle. The suggestion occurs in his De Substantia Orbis, and as it follows a quotation from Aristotle (to the effect that the moon is more nearly related to Earth in her nature than to the stars), it was believed by many to be originally due to Aristotle himself.

The strange circumstance that the moon alone, among all the heavenly bodies, appeared to have dark shadows on her bright face, had roused much curiosity already among the Greeks; and Plutarch, in his dialogue “On the Face in the Moon,” mentions the different explanations suggested in his day. Some thought that the polished surface of the moon reflected, like a mirror, parts of the earth; the Stoics said that air was enclosed within her globe of fire and obscured it in certain places, but this view was rejected with contumely by Plutarch.

“It is a slap in the face to the moon when they fill her with smuts and blacks, addressing her in one breath as Artemis and Athena, and in the very same describing a caked compound of murky air and charcoal fire, with no kindling or light of its own, a nondescript body smoking and charred like those thunderbolts which the poets address as lightless and sooty!”[671]

The third hypothesis, which was favoured by most of the speakers in the dialogue, was that the moon was not a star at all, but a kind of more beautiful Earth; and it was even suggested that men might dwell on her who thought their world the only place fit for human beings, looking down with contemptuous pity on our Earth, as a sort of sediment and slime of the Universe, appearing through damps and mists and clouds, a place “unlighted, low, motionless,” entirely incapable of supporting moving breathing warm-blooded animals!

This was not a view which could possibly be held by mediæval Christians, and in a treatise on Aristotle’s De Cœlo, attributed to Albert of Saxony, we find others put forward. The author disproves the old theory of a mirror-like moon reflecting parts of Earth, and also another that the moon draws up cold vapours which are seen like clouds on her surface, and upon which she is nourished—for how should an eternal heavenly body need nourishment?—and finally he expounds the theory of “The Commentator,” with which he agrees. The markings are rare parts of the moon’s substance, which cannot shine so brightly as the denser parts, and this varied surface he compares with alabaster, in which “the dense and not translucent part is very white, while that which is translucent like glass is obscure and tends to blackness. And if,” he adds, “it is asked why the moon is thus dissimilar in her parts, the reply is that this is her nature.”[672]

This theory was perhaps the most popular one with mediæval scholars, though there was some difference of opinion as to whether the dense or the rare parts of the moon were those that showed dark. Ristoro perhaps alludes to it, in the passage where he speaks of the moon’s markings, but in a very confused way, and the contrast of “polished” and “rugged” surface is what he chiefly lays stress upon.

After upholding this theory in the Convivio, Dante rejects it in the Paradiso: perhaps because it was more or less bound up with the idea that the moon was partly of an earthy nature, and this was inconsistent with the other theory of Aristotle, so popular among classical writers, that all within and above the sphere of the moon was eternal and heavenly. He is evidently very anxious to convince us of the falsity of his old belief, for he is not content with a simple assertion, but lets Beatrice reason like a learned doctor, using arguments which would appeal to his readers, drawn now from experiments which might be used in a school, and now from the accepted astrological beliefs of the day.

First she reminds him that the numerous stars scattered over the surface of the eighth sphere differ in brightness, like the different parts of the moon’s surface, but they differ in the quality (colour?) of their light, as well as the quantity, and we know that there are essential differences between them, because their “virtues” are different. It cannot be, therefore, that they differ simply in density or rarity of their substance.