Dante and the Heaven of the Moon: lunar spots and broken vows

In the cosmology Dante inherited from Aristotle and Ptolemy, the Moon is the first and lowest of the heavens revolving around the motionless Earth. With it, in the second canto of the Paradiso, the poet's ascent towards God begins. The Heaven of the Moon occupies cantos II to V and brings together, in exemplary fashion, the two faces of the third canticle: scientific and philosophical discussion and the encounter with souls.

Entering the eternal pearl

Dante and Beatrice enter the lunar body as a ray of light enters water without dividing it. The poet calls it the «eternal pearl» and marvels at how one body can receive another. Before this entry, at the opening of the canto, Dante warns readers following him in little boats to turn back: only a few, nourished on the bread of angels, will be able to follow him onto the open sea.

The problem of the lunar spots

Dante asks Beatrice what the dark marks visible on the Moon are, the ones that on Earth make people tell stories of Cain. Already in the Inferno, in canto XX, he had alluded to the popular belief that the Moon showed Cain with a bundle of thorns, condemned to that celestial exile. Now the problem becomes scientific, and the poet puts forward the explanation he himself had defended in the Convivio: the spots would depend on the different density of lunar matter.

Beatrice refutes the thesis with an elaborate argument and an experiment: three mirrors set at different distances from a light would reflect images of equal brightness, though of different size. The true cause, she explains, is metaphysical. Divine power, descending through the heavens, unites differently with the different celestial bodies, and from this varied mingling arises the difference in brightness.

Piccarda, Constance and the peace of the will

In canto III the first blessed souls appear, so faint that Dante mistakes them for reflections. They are the spirits who failed in their religious vows, not through their own fault but because others forced them. The first to speak is Piccarda Donati, sister of Forese and Corso, taken by force from the convent of the Poor Clares to be married off for political reasons. Beside her is Constance of Hauteville, mother of the emperor Frederick II.

Dante asks Piccarda whether she and the other souls do not long for a higher place in Paradise. Her answer is among the most famous in the Comedy: blessedness consists in conforming fully to the divine will, and so no blessed soul desires anything other than what it has. The line that sums it up is «And in His will is our peace». In cantos IV and V Beatrice explores the theme of vows and their possible commutation.

The Heaven of the Moon reveals the method of the Paradiso: each star corresponds to a condition of the soul, and the science of the time becomes theological language. The Moon, the changeable star par excellence, receives the souls whose will was inconstant, yet they too share fully in joy. Centuries before the great discoveries, her light remains in Dante the sign of a perfect order.