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.

That Dante should believe the spheres to exist as actual entities was inevitable, for his principal authorities, Greek, Arab, and Christian, all taught this. And there is abundant evidence that he did think of them thus. He calls them bodies:—“omnia corpora,”[576] “questi corpi grandi,”[577] “cerchi corporai,”[578] “il maggior corpo,”[579] etc. They are transparent, one sphere not obstructing the light from another;[580] and composed of æther, “questo etera tondo.”[581] They have a certain thickness, and would be visible if near enough, for Dante speaks of the inner margin of the Primum Mobile, and says that it was too far above him for him to have seen it yet, when he was in the Star Sphere.[582]

He gives the order of the eight heavens as in Ptolemy, and adds the ninth, which is only perceived by the diurnal movement of which it is the cause; and it is called by many the Crystalline, that is, the diaphanous or completely transparent Heaven.[583] The order of these spheres has been discovered by observation and reasoning, with the use of the principles of Perspective, Arithmetic, and Geometry. Dante gives as an instance the experience of Aristotle, who with his own eyes saw the moon pass below Mars, and so hide it for some time; and he mentions also how in eclipses it is evident that the moon is below the sun (i.e. nearer to us).[584]

To these nine spheres of the astronomers, the Catholic Church, “which cannot lie,” adds a tenth, the Empyrean Heaven, which means the heaven of flame or light, and is the abode of the blessed spirits and of God Himself.[585] And here Dante is able to give himself the pleasure of adding that Aristotle seems to have been of this opinion also; “Ed anco Aristotile pare ciò sentire, a chi bene lo intende, nel primo Di Cielo e Mondo.”[586] He is no doubt alluding to what Aristotle says about the existence beyond the finite universe.[587] The whole passage is thus translated in the “Temple” Dante:[588] “Nor is there any change of any of those things which are ranged above the outmost rotation, but they are unchangeable and passionless, enjoying the superlative existence, and passing in absolute self-sufficiency their eternal life.”

The ninth sphere rotates on two poles, which are absolutely fixed, “fermi e fissi e non mutabile, secondo alcuno rispetto,”[589] the eight within it have each two poles which are only relatively fixed (for they move with the movement of the Primum Mobile), and every sphere has an equator, every point on which is equally distant from the two poles of rotation, as anyone may see who will turn round an apple, or any other spherical thing. Any point on the equator moves more quickly than any other point on the sphere, and each part moves more quickly the nearer it is to the equator, more slowly as it is far from it and nearer to the pole, since all the circling is done in the same time, and some parts have a greater, and some a less distance to go, according to their distance from the equator. Dante adds that the nearer any part of a heaven is to its equator, the nobler it is compared with its poles, for it has more movement, and more actuality, more life, and more form, and it touches more of the heaven just above, and in consequence is more noble.[590]

Besides these ten heavens there are others which, strictly speaking, should be included in the number: these are the little spheres which are called “epicycles” by astronomers, and they also have equators and poles of rotation, and are fixed on the large spheres, or heavens. Dante does not enter into any particulars about these, but only mentions (as belonging specially to the subject in hand) that the brilliant star of Venus is carried “on the back” of an epicycle, which is itself fixed on the back of the third Heaven.[591] The apparent movement of Venus is therefore compounded of no less than four movements: that of her epicycle, of her whole heaven (the “deferent” Dante does not name, probably as introducing too technical a word), of the west to east motion of the star sphere (precession), and the diurnal. The epicycle of Venus is also called in this treatise, as we have already seen, “quello suo cerchio che la fa parere serotina e mattutina, secondo i due diversi tempi,”[592] and the “terzo epiciclo”[593] is referred to in Par. viii. 3.

A little further on in the same treatise, Dante tells us the periods in which all these nine spheres revolve. It is an interesting passage, and illustrates the poet’s vivid way of bringing home to his own mind and those of his readers the effects of the different celestial motions.[594] The ninth sphere, which is the Crystalline or Primum Mobile, causes by its daily revolution the daily revolution of all the other spheres, which are within it; it is, as Beatrice says, “costui, che tutto quanto rape l’altro universo seco.”[595] Therefore if we imagine it possible for the motion of this Crystalline Heaven to cease, what will be the effect on the motions of the other heavens? Obviously their diurnal movement from east to west will cease, and we shall only see them revolving, each in its own period, from west to east. There will be no alternation of day and night, or rather, there will be one long day of six months, and one long night of the same length, in any given place all over the earth. In the same way the moon will be invisible for half the month; and all the planets will be invisible to us for half their periods; and as Dante believed only the known hemisphere of Earth to be inhabited, he could say that during one half of each period no one would see the planet at all. He enumerates the half period in each case, as follows:—

If we compare these with the zodiacal periods given by Alfraganus, we see that Dante has taken those of Saturn, Jupiter, and Mars at the nearest whole number of years and halved them, but with the sun he seems to have used that more exact value with which we have already seen that he was acquainted. For to divide the 365¼ days of Alfraganus by 2 would have given him exactly 182 days 15 hours; but dividing 365 days 5 hours, and neglecting minutes gives 182 days 14 hours. The large oscillations of Venus and Mercury would make their periods of visibility very irregular, but the mean period would be, as Dante says, about the same as that of the sun. He has made a slip with the moon, for of course it is her sidereal period of 27 days 7¾ hours which he should have taken, giving a half period of less than 14 days, whereas he has evidently taken the mean synodic period of 29 days 12¾ hours, and ignored the hours. All the printed editions of Alfraganus give the sidereal period in this passage, except the Paris edition of 1546; but that gives the synodic,[596] so it doubtless appeared on some mediæval MSS. Dante must have had the misfortune to possess or consult one of these.

As for the stars, one third of their sphere would not yet have been seen. This is not difficult to understand if we remember what the movement of the star sphere was supposed to be, and how long it was since man had first begun to look upon it, according to Dante’s chronology. The movement which makes the equinoctial point shift its place among the stars was thought to be a movement of revolution of the whole star sphere, and its rate to be one degree in a century; the age of the world, we learn from Par. xxvi. 118-123, was over sixty centuries when Dante wrote. For Adam said that he had spent 930 years on Earth and 4302 in the Limbo, so that Dante evidently counted 5232 years from the Creation to the Crucifixion, that is (5232-33 =) 5199 to the beginning of our era at the birth of Christ, which is the date given by Orosius; adding 1300 years to this, we find that the age of the world in Dante’s day was more than 6000 years. Therefore the star sphere had revolved, since the beginning of the world, through more than 60 degrees.

The first inhabitants of this earth saw half the starry sky or 180°, and since then it had revolved through 60°, which together is equal to 240°; therefore there would still remain 120°, or one third of a revolution, to be made to complete the whole 360°, and bring every part of the star sphere into view. Just before this passage Dante had expressed the same idea in another way. This movement proper to the star sphere, he says, will never have an end, for the end of a revolution is to return to the same point; and this heaven will never do so, for since the beginning of the world it has turned through a little more than the sixth part of its whole revolution (60° = ⅛ of 360°), and we are already in the last age of the world, and indeed are awaiting the consummation of the celestial motion.[597]

If the premises be granted, the conclusions are correct. As regards sun, moon, and planets, they are correct even from the point of view of modern knowledge. For if the diurnal motion (that is Earth’s rotation) were to cease, while her other motions, and those of the rest of the solar system remained as at present, her motion round the sun would cause it to rise and set once in a year, and the appearance of moon and planets also would be as described. The vastly distant stars would have no appreciable motion, save that caused by precession, for if the earth revolved round the sun without turning on her axis, the same stars would always remain on the meridian at any given place. Precession, however, would not have the result which Dante supposed. He considered it to be a rotation of the star sphere, but we know that it is merely a nodding. Hence at any given place a star at the pole would describe, in the course of centuries, a small circle in the sky, and those at the equator would move in short lines first north and then south, while stars between would move in ellipses which become narrower as they approach the equator. All round the horizon, therefore, there would be slight very slow changes, bringing a few fresh stars into view, and causing others to disappear. The whole amount of the sky actually seen would vary but little, even if the world should last much longer than Dante expected.

Dante evidently took his value of precession from Alfraganus, for he was one of the few Arab astronomers who accepted Ptolemy’s erroneous figure.

In the Paradiso, as the poet mounts with Beatrice from sphere to sphere he mentions the number of each one in its order. The moon is “la prima stella,”[598] Mercury “il secondo regno,”[599] Venus “il terzo ciel,”[600] the spirits met in the sun are “la quarta famiglia,”[601] Mars is “questa quinta soglia,”[602] and “più levato”[603] than the last heaven, Jupiter is the “stella sesta,”[604] Saturn “il settimo splendore.”[605] The starry heaven is alluded to as “la spera ottava.”[606] The Primum Mobile is “il ciel velocissimo,”[607] “il maggior corpo.”[608] The Empyrean, “il ciel ch’ è pura luce,”[609] is called “l’ultima spera,”[610] but this has no limits and no movement, and therefore no poles:—

In Conv. II. iv. 35-39 it is thus described:—

“Questo è il sovrano edificio del mondo, nel quale tutto il mondo s’inchiude; e di fuori dal quale nulla è; ed esso non è in luogo, ma formato fu solo nella Prima Mente, la quale li Greci dicono Protonoe.”[612]

We must remember, however, that the spheres are sometimes counted in the other direction, from the outermost to the innermost or lowest, and therefore this “ultima spera”[613] is elsewhere spoken of as “il primo cielo,”[614] “il primo giro,”[615] “cœlum primum.”[616]