Dante and the early astronomers

by M. A. Orr

1. THE SPHERES.

Dante’s astronomical phenomena, though they are just the same that are familiar (or ought to be familiar) to us, have led us into some strange by-ways, because they are so intimately connected with his works of imagination. His astronomical theories, by which these phenomena were explained, will also take us into paths which are now seldom trodden by astronomers or others, because mediæval astronomy was mingled with magic, with metaphysics, and with religion, in a way which seems very strange to us.

The second treatise of the Convivio has for its text the Ode which is quoted in Paradiso viii. 37, “Voi che intendendo il terzo ciel movete.”[565] When speaking of the literal meaning of this Ode, Dante takes the occasion to discourse on the system of astronomy which he had learned, and describes how all the apparent movements of the heavens are explained by postulating the existence of spheres and epicycles, entering with special detail into the system of Venus, the third heaven; and later on, when expounding the allegorical meaning of the Ode, he draws an elaborate comparison between the spheres and the different branches of science and philosophy, in which he incidentally gives his readers a good deal of information about the heavens.

From this interesting treatise, therefore, as well as from many scattered passages in all his works, we learn what Dante thought were the real movements of the heavenly bodies, their dimensions, their nature, and the forces which move them; and it forms a valuable commentary on the Paradiso, in reading which he supposes us to understand the elements of the Ptolemaic system, and the ideas of the philosophers and the Fathers of the Church concerning the organization of the Universe.

Of these ultimate truths concerning the heavens, Dante says, very little can be known, but the little which is within the reach of human reason, gives more delight (as Aristotle says) than the abundance and the certainty of other things easier to be understood.[566] This thought finds an echo in the Paradiso, where the poet reminds us of the beautiful order to be found throughout the universe, and invites us to contemplate the skill of the architect as shown in the movements of the planets.[567]

Dante knew that great men in ancient days had speculated on the possibility that it was Earth, and not the skies, that moved in the diurnal period, but the balance of authority was against them, and they had been confuted by the Master of those who KNOW,[568] that glorious philosopher to whom Nature most fully revealed her secrets,[569] Aristotle. In Treatise III. of the Convivio he gives a clear and concise account of the theory of Philolaus, and of what he understood to be the theory of Plato.[570] In the former he seems to have been struck with the deduction that, if Fire were at the centre of the Universe, the real motion of fire on Earth, which to us appears always to ascend, would in reality be a descent towards its own place at the centre.

Both these hypotheses he had evidently learned from Aristotle’s De Cælo,[571] and when thus presented they seem entirely fanciful; so although Dante had reverence for both Pythagoras and Plato it is not surprising that he rejected the systems of both. He adds that it is not his intention to recount here the arguments by which Aristotle confutes them and establishes the truth: it is enough for his readers to know, on so great an authority, that the earth is fixed, and does not revolve, and is at the centre of the World.

Dante has to confess reluctantly that Aristotle made two mistakes in his astronomy: he said that there were only eight spheres, those of the seven planets, and that of the stars, “e che di fuori da esso non fosse altro alcuno;”[572] also he placed the heaven of the sun immediately above that of the moon, that is to say second in order, counting from Earth outward. Aristotle’s loyal admirer hastens to add that these serious mistakes “questa sua sentenza così erronea”[573] are explicable and excusable because, as Aristotle himself shows in the Metaphysics, when he treated of astrology he was merely following the opinions of others. That is to say, he took his facts from others, and they cannot all be depended upon, but his philosophical deductions are his own, and are never in error. In the above question of the possible movements of Earth, we saw ([pp. 101-2]) that Aristotle’s arguments were almost entirely drawn from metaphysics.

There have been many opinions regarding the number and the positions of the spheres, Dante says, but the truth about them has at length been found. The scheme which he expounds is that of Ptolemy, modified by the Arabs and adapted to the doctrines of the Church by Christian writers. In the Vita Nuova he almost seems to imply that the nine moving spheres form an article of faith. “Secondo Tolommeo e secondo la Cristiana verità, nove siano li cieli che si muovono.”[574]

Quoting from Albertus Magnus, or Averroës, he apparently credits Ptolemy with the discovery of precession, for he says that it was Ptolemy who added a ninth sphere to the eight of Aristotle, because he perceived that otherwise the outer sphere would have a double movement, and so he felt obliged to assume a sphere beyond the star sphere which should have only the one simple motion of the diurnal east to west revolution. The period of this revolution (the sidereal day) Dante gives as 23 hours and ¹⁴/₁₅ths of an hour, “grossamente assegnando,”[575] i.e. 23 hours 56 minutes, which is the correct value ignoring the 4 seconds. We know that it was not Ptolemy in fact who thought it necessary to add a ninth sphere; he was not sufficiently interested in spheres, since to him they were only mathematical abstractions: but the Arabs of Baghdad noted that he had added another movement, the discovery of Hipparchus; and they, accepting the spheres as material instruments of celestial motions, felt obliged to assume the existence of the Primum Mobile or 9th sphere.