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.
“Ben m’ accors’ io ch’ io era più levato, Per l’affocato riso della stella, Che mi parea più roggio che l’usato.”[681]
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.
“E quale è il trasmutare in picciol varco Di tempo, in bianca donna, quando il volto Suo si discarchi di vergogna il carco, Tal fu negli occhi miei, quando fui volto, Per lo candor della temprata stella Sesta, che dentro a sè m’avea ricolto.”[682]
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:—
“E tal nella sembianza sua divenne, Qual diverrebbe Giove, s’egli e Marte Fossero augelli, e cambiassersi penne.”[683]
And when Dante looks down upon the seven planets he sees
“il temperar di Giove Tra il padre e il figlio.”[684]