We have seen that Dante believed the circling of the nine moving spheres and their epicycles to be the immediate cause of the movements which he saw in stars and planets. If we ask what were his ideas with regard to their sizes and distances the question is easily answered.

It is a curious fact that although our ideas about the size of anything we see depend upon the distance at which it appears to be, and no one can say how far away the sun looks, most of us nevertheless have a quite definite idea as to how large it looks, about the size of a dinner-plate, a cart-wheel, &c. Cleomedes, in the days of Augustus, quotes Lucretius and the Epicureans as believing that the sun is no larger than it looks, that is a foot in diameter; and he rightly remarks, amongst other arguments to the contrary, that if it were only that size it would be invisible unless nearer than the tops of the mountains, yet we see islands and hills projected against it when rising, which shows that its distance is greater. Dante twice quotes the same fallacy, as an instance of the folly of trusting to the impression of our senses, when not corrected by reason, and he tells us what he believes the true size of the sun to be.

In Conv. IV.[657] he says:—

“Thus we know that to most people the sun appears to be a foot wide in diameter; and this is so utterly false that according to the investigation and discovery made by human reason with her attendant arts, the diameter of the sun’s body is five times that of the earth’s, and a half besides. For whereas the earth has a diameter of six thousand five hundred miles, the diameter of the sun, which when measured by sense-impressions seems to be a foot in extent, is thirty-five thousand, seven hundred and fifty miles.” (See also Ep. x. 42-46).

When speaking of the planet Venus he says she is “far off from us, being distant even when she is nearest to us one hundred and sixty-seven times the distance of the distance of the centre of the earth from us, which is three thousand two hundred and fifty miles.”[658]

Mercury is spoken of in the Paradiso as a small star, “Questa picciola stella,”[659] and in Conv. II.[660] Dante says:—“Mercury is the smallest star in the sky; for the length of its diameter is not more than 232 miles, according to Alfraganus, who says that it is one twenty-eighth of the diameter of the earth,[661] which is six thousand five hundred miles.”

It is only in the last passage that our poet quotes his authority, but in the others also the diameter or the semi-diameter of the earth is used as the unit of measure, and it is the same as that given by Alfraganus; so are the sizes of Mercury and the Sun, and the least distance of Venus is the value given by Alfraganus for the greatest distance of Mercury, which he asserted to be equal. It seems quite clear, therefore, that the figures given in the tables on pp. 189-90 represent Dante’s belief as to the dimensions of the Universe. These passages also are another proof that Dante had not studied Ptolemy’s own work, for Ptolemy had said that the distances of Venus and Mercury could not be accurately known.[662]

We do not expect exact measures of distance in Dante’s descriptions of his soaring flight through the spheres, but he so constantly speaks of the marvellous speed with which he passes from star to star that we are not required to ignore the stupendous distances between. The one indication he does give us of the space which separates him from earth is full of poetry and symbolic meaning: I refer to Folco’s words in the heaven of Venus:—

This is the last of the three heavens in which Dante has met spirits whose lives did not reach the perfection of the saints in higher heavens, but were marred by broken vows, earthly ambition, earthly love; and here he is reminded that he has not yet soared high enough to be beyond reach of the cone of shadow thrown by the earth. This is another allegory resting on what was to the poet an astronomical fact. For Earth’s shadow was said by Ptolemy and Alfraganus to extend to 268 times the distance of her semi-diameter, and the distance of Venus from Earth varied, according to Alfraganus, between 167 and 1120 times this unit, while the next planet, the sun, at his least distance was 1120 units distant (the same as Venus’ greatest). Therefore the spheres of Venus and the two nearer planets Mercury and the Moon, were all within reach of Earth’s shadow, but the sun and the rest were far beyond it.[664] Venus and Mercury themselves never could be actually touched by the shadow like the moon, however, because they keep too near the sun for Earth to interpose herself between them and the source of light.

(Though the length of the shadow thrown by Earth is nearly accurate, see pp. 191-92, the planets are a great deal further than Alfraganus supposed; for the least distance of Venus is nearly 6000 times Earth’s semi-diameter, and the least distance of Mercury 12,000).

Except for this we have only the passage in the Paradiso already referred to, where Dante, standing among the stars of Gemini, found the sphere above him too distant to be seen, but turning his gaze downwards was able to survey all the planets and the earth at the centre of the World. He knew very well that at a distance from Earth 20,000 times her own semi-diameter it would be impossible to see her as a disc,[665] far less to distinguish oceans and continents, hills and river-mouths. It would in fact be like looking at an object one foot in diameter at a distance of two miles. But as we have before observed, his power of vision here was more than human: at every step in this marvellous journey it had grown clearer and stronger, and in the fourth heaven he had been able to look on spirits which shone in the sun more brilliantly than the sun itself, although on earth no eye can look upon the sun.[666]

In the Empyrean, distinctions of space, and impediments to vision vanished altogether.[667]

With regard to the physical nature of the heavenly bodies, Dante and his contemporaries had no means whatever of investigation, and could only profess ignorance or accept one or other of the guesses of their predecessors. Some of the Greek philosophers, as we saw, guessed that the planets were worlds somewhat like the earth, others thought they were composed of fire or of air. But the general belief in the thirteenth century was that planets and spheres alike were composed of a kind of celestial matter, called by Aristotle and the Greeks “æther,” by the Arabs “al-acir,” an immortal substance which had neither heaviness nor lightness, and was altogether distinct from any of the four elements existing below the sphere of the moon. Beatrice speaks of “this sphered ether,” “questo etera tondo.” Par. xxii. 132.

Dante’s description of this celestial substance when he first enters the ethereal world is one of the finest instances of his faithfulness to the teachings of astronomy as he had learned it, combined with poetical imagination, and at the same time his power of using material facts (as he conceived them) to present an allegory of the deepest religious mysteries. If we merely listen to the magic of the words, an impression is conveyed of something mysteriously beautiful and dazzling, unlike anything known on earth; but if we look into the meaning, we find that the globe of the moon has just those strange, contrasting qualities, just the colour and the form which a sphere of ether may be imagined to have. It is white and rounded like a pearl, “polished” as Plato said of the universal orb, it is thick and shining, soft as cloud but hard as diamond; and it offers no more resistance to Dante as he enters into it than does water to a ray of light.[668]

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.

Then she shows that in no circumstances is the theory of varying density in different parts of the moon capable of explaining the observed variety of brightness. For if the parts which look dark are less dense than others, either they must be so right through the planet, or there must be an alternation of dense and rare matter in those parts, like fat and lean in a material body, one behind the other like pages in a book. (The double metaphor is characteristic of Dante.) Now if the first be true, the sunlight would shine through the moon is those parts, during an eclipse;[673] and if the second, the sun’s rays must penetrate to the lower-lying denser part, and be reflected thence just as from the dense parts of the moon’s surface. A possible objection that in that case the reflecting surface would be further away, and therefore appear fainter, may be disproved by an experiment. Take two mirrors and place them at an equal distance from you; place a third between them but further away, face the three mirrors, and place a light behind you so that it may be reflected from them all: the distant one will appear smaller, but no less bright than the two near ones. This reasoning is perfectly sound.

Then follows the explanation for which all this is a preparation. It involves an exposition of ultimate causes which we need not enter into here: for the present we need only say that difference of brightness is not caused by differences of much or little of the same substance. There are intrinsic differences between star and star, and between different parts of the moon,[674] but all are manifestations of divine intelligence; just as the different members and faculties of a human body are manifestations of a human soul.

The diversity of the celestial matter is also touched upon in the Convivio, where it is said that the epicycle of Venus is “not of the same essence as that which carries it [the deferent or large sphere], although it is more nearly of one nature with it than with the rest.”[675] Differences of colour are indicated in the glowing masses of the planets, which are entered by Dante and Beatrice. As with the moon, they do not merely alight upon the surface, but penetrate the planetary ether. “Quel ch’ era dentro al sol,”[676] “Nel profondo Marte,”[677] “La temprata stella sesta, che dentro a sè m’avea ricolto.”[678] Mercury is called a pearl—“la presente margarita”[679]—while Mars is “questo fuoco,”[680] and the astronomer poet says he knew the planet by its burning smile, for it seemed to him even more ruddy than its wont.

And when, without being conscious of any movement, he found that the red light shining on Beatrice had turned to purest white, like a blush fading from a fair face, he knew that the sixth star (Jupiter) had received him within itself.

This contrast between the colours of Jupiter and Mars is referred to more than once. In Par. xxvii., the flaming spirit of St. Peter, flushed with righteous indignation, becomes brilliant as the planet Jupiter, and red as Mars:—

And when Dante looks down upon the seven planets he sees

To understand which we must turn to the oft-quoted fourteenth chapter of the second treatise in the Convivio. Here we find that Jupiter “moves between two heavens” which “are antagonistic to its excellent temperateness, that is to say the heavens of Mars and that of Saturn,”[685] and Dante quotes Ptolemy as saying that Jupiter is a star of temperate constitution, between the cold of Saturn and the heat of Mars.

It seems that these ideas of cold and heat, as applied to the planets, were taken literally, not merely as poetical descriptions of colour, nor of astrological significance; for Dante says explicitly in the same chapter of the Convivio that Mars is hot like fire, that this is the cause of his colour,[686] and that he dries up and burns things.[687]

Similarly he appears to have believed that Saturn and the moon were literally of a cold nature. “Quel pianeta che conforta il gelo,”[688] in Canzone xv. is doubtless Saturn; and the chill of the hour before dawn is described as due not only to loss of heat by the earth but sometimes also to the influence of the cold moon or of Saturn.[689]

Besides her shadowy markings, the moon had another peculiarity which distinguished her from all other heavenly bodies, for she was the only planet then known to vary in apparent size and shape “according to the way in which the sun looks upon her.”[690] She was therefore a very dark body, though it was believed that she had some light of her own, and Dante argues in favour of this that she does not become totally invisible during an eclipse.[691] This was a very natural conclusion: it would not readily occur to anyone that the reddish light of the eclipsed moon is sunlight refracted (i.e. bent out of its direct course) in passing through the earth’s atmosphere.

The sun, although the astrologers made him not very much larger than the brightest fixed stars and the largest planets, was much brighter and hotter than any, and it was universally agreed that all obtained their light from him. Even if, like the Moon, they had some light of their own, and did not only reflect like mirrors, this inherent light had originally come from the sun and was but absorbed sunlight.[692] We meet constantly with this idea in Dante’s works. In one of his Odes the sun is said to “donar luce alle stelle;”[693] in the Convivio he says, “Il sole ... di sensibile luce sè prima, e poi tutti i corpi celestiali ed elementali allumina.”[694] The Morning Star is described as deriving her beauty from the sun;[695] and after the sun has set he shines forth again in the light of all the stars.[696]

So also says Brunetto Latini, “Sans faille li solaus est fondemenz de toutès lumieres et de toute chalor.”