Astronomical Diagrams of Inferno

(For an overview on diagramming astronomy in the Commedia, including introductory diagrams, see here.)

Inferno 1.37-40

The verses which already in Inferno 1 effectively constitute the poem’s first creation discourse also make up the poem’s first astronomical reference.

Temp’ era dal principio del mattino,
e ’l sol montava ’n sù con quelle stelle
ch’eran con lui quando l’amor divino
mosse di prima quelle cose belle
(Inf. 1.37-40)

As will frequently be the case throughout the Inferno, this reference to the stars is an indication of time—both the time of day and the time of year.

It is the morning, “Temp’ era dal principio del mattino,” and the Sun is just rising over the horizon: “e ’l sol montava ’n sù.” The stars are in this first instance presented as spatiotemporal entities; they therefore enjoy a unique metaphysical status in the perceptible world in that they participate in some yet-undefined relationship to time. The association to time remains open to a number of interpretations: minimally, for example, the movement of the stars through space could be correlated as no more than a sign of changing times; maximally, the movements of the stars could be causal—that is, on such an interpretation, the stars are time itself. In this opening passage there is no indication of causality or determinism (neither temporal nor fatalistic), nor is there any information which suggests one interpretation over another. All we find in these verses is the evident association of the cyclical movements of heavenly objects through space with the experience of time upon the Earth. We know that it is morning because we see the sun is rising (alternatively: we expect and do see the sun rising because we know it is morning); we have the expectation that sunrise and morning occur together because of the daily and historically constant correlation between the two. 

In this first astronomical periphrasis, Dante ties the onset of the pilgrim’s voyage—an event that is tantamount to the creation of the poet’s virtual world—to the formation of the universe itself: the sun is said to be rising with those same stars that accompanied it (the ‘lui’ of verse 39 refers back to the ‘sole,’ of verse 38) when divine love (God) first ‘moved the things of beauty’, which is to say ‘created the world.’ This is a rather simple statement that declares the sun has returned, as it would each year, to the place where its movement first began on the day of creation. Scholars of Dantean astronomy often emphasize the metaphoric link introduced here between the creation of the poet’s imagined cosmos and contemporary theology regarding the creation of the ‘real’ cosmos in which the poet (and, if you like, those of us who read his text) lived (and live). Dante—like Moses and like Plato—views the whole of creation as the artifice of a God (or demiurge); this God is accordingly the divine and masterful creator of the world and all that persists within it.

“Quelle stelle” is understood to be a reference to the sign of Aries, the 30-degree segment of the ecliptic which immediately follows the location of the vernal equinox. Following a theological tradition which features thinkers such as Philo of Alexandria and Bede, Dante links the creation of the universe to the beginning of the spring season, marked by the sun’s annual entry into Aries (see Philo. Questions on Exodus. trans. Ralph Marcus. Harvard University Press. 1953. sec. 1.1, pp. 2-6; Bede. The Reckoning of Time. trans. Faith Wallis. Liverpool University Press. 1999. pp. 24-25). Interestingly, linking creation with Spring breaks with astrological tradition (see, for example, Maternus, Firmicus. Matheseos Libri VIII. (Ancient Astrology Theory and Practice). trans. Jean Rhys Brahm. Noyes Press. 1975. sec. 3.1.9; p. 72; Macrobius, Ambrosius Aurelius Theodosius. Commentary on the Dream of Scipio. trans. William Harris Stahl. Columbia University Press. 1990. p. 179). 

Dante is vague regarding the specific location of the sun within the 30-degree belt of Aries, and there is disparity in the theological tradition as well: Philo mentions the vernal equinox specifically, “(Scripture) thinks it proper to reckon the cycle of months from the vernal equinox. Moreover (this month) is said to be the ‘first’ and the ‘beginning;’” while Bede places the beginning of time at the fourth degree of Aries (approximately four days after the equinox): “That is, [the Sun], when it first rose above the Earth, was positioned in that part of the sky which philosophers call the fourth degree of Aries.” In Inferno 1 Dante only tells us that it is the morning, and that his journey begins in early spring.

This passage also introduces the notion of a horizon line, which will grow in relevance as the pilgrim enters the southern hemisphere at the end of Inferno 34. The horizon is perspectival, in other words it is determined by an observer’s relative position on Earth. Imagine the horizon as a plane which emanates from the Earth’s midpoint and is perpendicular to the zenith, or highest point in the sky, at a given location. Here are a few examples to illustrate how the horizon changes with location:

1) When standing on the north pole of the Earth, the zenith, always found directly overhead, will be marked by the north star, Polaris, or the north celestial pole; in this case, the horizon is superimposed upon the equator, as the equator is the midpoint between the two poles. See diagram 1, where the perspective is that of an observer located at the North Pole.  

2) When standing somewhere in between the northern pole and the equator, the zenith will be located at the highest point directly overhead. This point would be marked by the position of the sun at noon on the day of the equinox. The horizon is located at the plane which is perpendicular to the zenith, and will necessarily be found between the two extremes of P = North Pole and P = Equator. See diagram 2, where the perspective is that of an observer located in the northern hemisphere, at an arbitrary latitude south of the North Pole and north of the equator of the Earth.

3) When standing anywhere along the equator, the zenith, always found directly overhead, will be located on the line of the celestial equator; in this case, the horizon line will necessarily intersect the northern and southern poles of the Earth. See diagram 3, where the perspective is that of an observer located on the equator of the Earth.

All three diagrams are representations of the position of the Sun as described in Inferno 1.37-41, that is, on the day of the vernal equinox. In diagram 1, the black line is both the equator and the horizon and the red line is the Ecliptic; in diagrams 2 and 3, the black line is the equator, the blue line is the horizon, and the red line is the ecliptic. This planetary snapshot represents the time indicated at the beginning of Inferno 1.

Inferno 2.76-78

O donna di virtù, sola per cui
l’umana spezie eccede ogne contento
di quel ciel c’ha minor li cerchi sui,
(Inf. 2.76-78)

For the majority of Inferno 2 (vv. 43-126), Virgil tells the pilgrim a story about his encounter with Beatrice, providing a pre-history for the outset of the poem in Inferno 1. It is in the midst of this story that Virgil introduces the poem’s next astronomical periphrasis, “quel ciel c’ha minor li cerchi sui” (Inf. 2.78), and, in doing so, begins building out in earnest the cosmological framework that will bolster the setting of the final cantica. The cosmos will also form a critical part of the poem’s philosophic system.

In this project I am focused on the information that Dante is providing for us in the places where he actually provides it, referencing source material when necessary in order to provide context for the cosmic systems the poet uses to structure his imagined world. This world does not simply appear. Rather, the poet builds out his world in time using a number of tools, astronomy chief among them. I therefore do not want to anticipate in this project any of the poet’s inventions merely because they have become so rooted in our imaginary as readers of the Commedia: the spectacular craft of the poet’s world-building is often wrought by his astronomical periphrases and discourses, and to anticipate this is to diminish the poetic artifice. Consequently, it is for this reason that my diagrams do not yet feature Mount Purgatory. For a more elaborated explanation of the reasons behind this choice, see my Note on Method of Presentation.

This astronomical periphrasis, which stands in for the Moon, is by no means emphasized in the canto: it immediately follows Beatrice’s climactic speech (vv. 58-76), it is presented only in an appositive to “donna di virtù,” and, after uttering it, Virgil quickly moves to accept his charge and ask Beatrice an unrelated question (vv. 79-84). Nonetheless, these verses already inform us about much of the poet’s cosmos, and, accordingly, much of the narrative to come.

Dante’s cosmos consists of manifold spherical domains which increase in size from the Moon up to the Primum Mobile. Dante first articulated this idea of the cosmos in the Convivio. While it is not always the case that readers will find consistency between these two texts, there is some congruence between the astronomy of the Convivio and that of the Commedia. In Convivio 2.3.7-8 Dante outlines his thoughts on the ordering of the heavens:

Ed è l’ordine del sito questo, che lo primo che numerano è quello dove è la Luna; lo secondo è quello dov’è Mercurio; lo terzo è quello dov’è Venere; lo quarto è quello dove è lo Sole; lo quinto è quello di Marte; lo sesto è quello di Giove; lo settimo è quello di Saturno; l’ottavo è quello de le Stelle; lo nono è quello che non è sensibile se non per questo movimento che è detto di sopra lo quale chiamano molti Cristallino, cioè diafano, o vero tutto trasparente. 8. Veramente, fuori di tutti questi, li cattolici pongono lo cielo Empireo, che è a dire cielo di fiamma o vero luminoso; e pongono esso essere immobile per avere in sé, secondo ciascuna parte, ciò che la sua materia vuole.

[The order of their position is as follows. The first in number is the one in which the Moon resides; the second is the one in which Mercury resides; the third is the one in which Venus resides; the fourth is the one in which the Sun resides; the fifth is that of Mars; the sixth is that of Jupiter; the seventh is that of Saturn; the eighth is that of the Stars; the ninth is the one which is not perceptible to the senses except for the movement mentioned above, and which many call the Crystalline (that is to say, the diaphanous or completely transparent) Heaven. Moreover, outside all of these the Catholics place the Empyrean Heaven, which is to say, the “heaven of flame,” or “luminous heaven”; and they hold it to be motionless because it has in itself, with respect to each of its parts, that which its matter desires (Lansing).]

This is precisely the ordering of the heavens that we will later find in Paradiso. Very much in line with medieval belief, Dante holds that each of the seven visible planets “resides” on the surface of massive spheres which constitute their heavens. The so-called eighth sphere is the heaven of the Fixed Stars, which houses all the constellations and stands at the reaches of the perceptible cosmos. The Primum Mobile, which is not physically perceptible except for a movement previously described (“non è sensibile se non per questo movimento che è detto sopra”), then follows the eighth sphere. The movement to which Dante refers here, which he discusses in Convivio 2.3.3-6, is what astronomers have termed “precession of the equinoxes,” a phenomenon whereby the constellations appear to slip across the sky from West to East at an incredibly slow pace. The discovery of this movement is what led to the logical supposition of the Primum Mobile, so as to not assign two distinct and complicated motions to one single heaven (the eighth). And finally, outside of the Primum Mobile is the Empyrean.

So, when Dante says, “quel ciel c’ha minor li cerchi sui,” he is referring to the heavenly domain with the smallest celestial circles, that is, the Heaven of the Moon. “Ogne contento,” then, of the heaven of the Moon, would refer to everything that exists within the sphere of the Moon, which is, ultimately, the Earth. All that is called “sublunar” or “within the heaven of the Moon,” is simply another way of stating “all that which is on the Earth.” Dante uses a similar locution in Inferno 7.64, when he says “[…] tutto l’oro ch’è sotto la luna,” by which he means “all the gold that is on the Earth.”

Inferno 7.97-99


Or discendiamo omai a maggior pieta;
già ogne stella cade che saliva
quand’ io mi mossi, e ’l troppo star si vieta.
(Inf. 7.97-99)

Virgil declares in Inferno 7.98-99 that “già ogne stella cade che saliva / quand’ io mi mossi,” indicating that the time has come for the pilgrim and his guide to move on to the subsequent circle of Hell. This phrase, which seems innocuous enough, presents two issues: one epistemological, one interpretive. The first issue, which we ultimately will set aside, is how exactly it is that Virgil can know that the stars, which were rising when he first moved, are now falling. As we learned in Inferno 3.22-23, it is impossible to see the stars in Hell: “Quivi sospiri, pianti e alti guai / risonavan per l’aere sanza stelle” (my emphasis). Virgil’s evident ability to track the movement of the stars implies that there is some means by which the souls in Hell can intuit the movement of the heavens even without having any physical perception of it. Dante does not ever explain how it is that this is possible.

The second issue, which is interpretive by nature, regards the terms “cade” and “saliva.” How we choose to understand these terms relies on how we contextualize Virgil’s “mi mossi” within the narrative. Astronomically, there are two ways in which the terms “cade” and “saliva” can be read: one from a horizontal perspective and one from a global perspective. In the diagram above, “horizontal interpretation” and “global interpretation” are terms I use for the sake of clarity, as I have not seen any standard nomenclature used regarding these interpretations. Buti and Bertagni, for example, who argue for the global interpretation, simply say “Cade e saliva non possono riferirsi ad un orizzonte (in questo caso all’orizzonte di Gerusalemme […]) e indicare il ‘sorgere’ e il ‘tramontare’ delle stelle” (page 79). Edward Moore, one the other hand, who sides with the horizontal interpretation, does not mention the “global interpretation” at all (page 42).

On the horizontal interpretation (left diagram), according to which one takes into account the visible sky stretching from Eastern to Western horizon, critics have interpreted “saliva” to indicate a period of 6 hours during which the stars rise from the Eastern horizon to their highest point in the sky, the zenith, which thus implies that “cade” indicates another period of 6 hours, during which the stars are falling from the zenith to the Western horizon. The horizontal interpretation accounts for a subsection of the total motion of the stars, namely, that section which is visible in the sky—i.e. above the horizon—at a given observer’s location in the northern hemisphere. The fundamental issue with the horizontal interpretation is that, from most observational loci on Earth, there is an apparent asymmetry regarding the stars’ path over the horizon. Some stars, like the Sun during the equinox, do appear to rise for roughly 6 hours and fall for another 6 hours regardless of one’s location on the Earth. But some of the fixed stars located closer to the northern celestial pole, for example, never actually fall below the horizon, while some other stars closer to the southern celestial pole are only ever above the horizon for a few hours each day. In sum, not every star in the sky crosses the horizon, rises for 6 hours, falls for another 6 hours, and then crosses the horizon again: depending on the position of the stargazer, some stars will rise for 8 hours and fall for 8 hours, spending only 8 hours a day below the horizon; some stars cross the horizon and rise for 4 hours, fall for another 4 hours, and then dip below the horizon for the remaining 16 hours of the day. In fact, some stars never dip below the horizon at all, and instead rise for 12 hours and fall for another 12, hanging perpetually in the sky, while others never cross the horizon at all and are therefore never visible in the northern hemisphere. For this reason, the horizontal interpretation hinges on an ideal revolving sky that features the uniform rising and setting of every star, something which cannot match reality but in one instance: namely, when the rotational axis (the axis connecting the northern and southern poles of the Earth) is on the same plane as the horizon line. In other words, the horizontal interpretation only ever holds in reality if we are standing on the Equator itself, where the line of the horizon is coplanar with respect to the rotational axis of the Earth. For any observers who are not at the Equator, then, it would not make sense to say “saliva” and “cade” and intend sorgeva and tramonta, since the stars are rising and setting at asymmetric intervals, ranging from just a few minutes to 12 hours one way.

If we consider what I call the global interpretation (right diagram), by which one accounts for the entirety of the motion of the stars from their absolute nadir (lowest point) to their absolute zenith (highest point) with respect to the entire celestial sphere, then every star can indeed be said to rise (salire) for 12 hours, and fall (cadere) for another 12. The global interpretation is, strictly speaking, the most technically correct in astronomical terms, yet we will see that it is much more difficult to fit it to the narrative due to the attendant “mi mossi” of Inferno 7.99.

The horizontal interpretation implies that Virgil’s “mi mossi” is in reference to an event 6 hours in the past, while the global interpretation ties “mi mossi” to an event 12 hours in the past. Those who choose the horizontal interpretation claim that “mi mossi” refers neatly back to Inferno 1.136 and Inferno 2.1, where Dante states clearly “Allor si mosse, e io li tenni dietro” (my emphasis) and then “Lo giorno se n’andava, e l’aere bruno […].” Thus, the horizontal interpretation holds that Virgil is referring to the moment when he and Dante first started their journey, understood to be around 6 PM, just as the day was departing (Inferno 2.1), elegantly linked by the repeated verb. The horizontal interpretation amounts to the indication that it’s now just after midnight, 6 hours having passed since Dante and Virgil first left the selva oscura at the close of Inferno 1.

On the other hand, those who side with the global interpretation have a much harder time determining where to place “mi mossi” given that, ultimately, the information value of the global interpretation has to be more or less the same as the horizontal interpretation: it’s still going to have to be just after midnight in Inferno 7.97-99. Critics who prefer the global interpretation find themselves somewhat stuck since the next indication of “fabula” or narrative time will come in Inferno 11.112-115, where we will see that it is still before sunrise. Since the Sun will rise around 6 AM given the proximity of the equinox, “mi mossi” cannot refer to Inferno 1.136 on the global interpretation: a duration of 12 hours from that moment would effectively postdate Inferno 7 to Inferno 11. So, in the end, the only substantive difference between the two interpretations is the assertion that, on the global interpretation, “ogne stella cade che saliva” means, on a purely technical level, that 12 hours have passed since some indeterminate moment when Virgil first moved from Limbo, rather than 6 hours since Inferno 1.136.

Buti and Bertagni’s Commento astronomico della Divina Commedia is an incredibly astute survey of astronomy in the Commedia. In this particular instance, however, it’s clear that the desire to defend a rigid, scientific technicality for the poet’s craft has led to rather strange conclusions. Buti and Bertagni claim that “mi mossi” does still refer to Inferno 1.136, but that it is midday at the close of the first canto and there are an inexplicable “6 ore ‘vuote’” (page 81) between Inferno 1.136 and Inferno 2.1 (one imagines Virgil and Dante must have spent those six hours chatting about poetry through the Tuscan afternoon before the events of Inferno 2 commence). Others who side with the global interpretation claim instead that “mi mossi” is a reference to an event that precedes Inferno 1: namely, what is mentioned in Inferno 2.118, “E venni a te così com’ ella volse,” the moment when Virgil first moved from Limbo at Beatrice’s behest. This latter solution is certainly preferable to leaving 6 hours unaccounted for between Inferno 1.136 and Inferno 2.1, but there is no clear indication of when it was that Virgilio spoke to Beatrice and departed from Limbo. It quickly begins to feel like one is needlessly splitting hairs by forcing the global interpretation when the outcome, as regards Inferno 7, is precisely the same in either case. Further, as previously stated, Virgil cannot actually see the stars. As we have seen, the horizontal interpretation hinges on the location of the stargazer: in order to completely rule it out, we would have to know where on the Earth the observation of the stars’ rising and falling is made. As the epistemological issue will remain unanswered, so too will the interpretive question remain unsettled.

Given that the information value of the two interpretations yields identical results, siding with one or the other is ultimately immaterial. But the spirited and centuries-long desire to reconcile these verses to a particular temporal map of the journey does underscore the extent to which astronomy is continuing to serve Dante as a buttress to the poem’s mimetic effectiveness. The rising and setting stars are brought into the poem as a narratological sleight of hand: in highlighting the movement of the stars (which Virgil of course cannot even see), Dante manages to make the poem appear as if grounded in real time. And Dante does not only try to present this particular moment of the poem as rooted in real time, but he further links it to another scene in the fiction and anchors them both to historical reality. The upshot of this time-reference is that the poet is trying to convince us, once more, that this fiction is built of real, historical truth. In doing so, the mention of rising and falling stars and the narrative past spurs the attentive reader to try to weave these time-references together in such a way as to locate the progression of the journey and in real, historical time. The stars, which are at bottom indications of fabula time, look like indications of real, historical truth-time.

If we want to try our hand at locating the poem in historical time, up to this point our clues to the date and time of the poem are the following.

(1) This journey takes place in the middle of Dante’s life (Inferno 1.1), therefore between 1265 and 1321. The year 1300 stands out as the obvious choice, particularly in the context of Convivio 4.23.9-10:

Là dove sia lo punto sommo di questo arco [della vita], per quella disaguaglianza che detta è di sopra, è forte da sapere; ma nelli più, io credo, tra il trentesimo e ‘l quarantesimo anno; e io credo che nelli perfettamente naturati esso ne sia nel trentacinquesimo anno. E muovemi questa ragione: che ottimamente naturato fue lo nostro Salvatore Cristo, lo quale vollemorire nel trentaquattresimo anno della sua etade; ché non era convenevole la divinitade stare in cosa [in] discrescere […].

[It is difficult to determine where the highest point of this arc lies, because of the inequality mentioned above, but in most lives I believe it is attained between the thirtieth and fortieth year, and I believe that in those whose nature is perfect it is attained in the thirty-fifth year. My belief is compelled by the argument that our Savior Christ had a perfect nature and desired to die in the thirty-fourth year of his life, because it would not have been fitting for a divinity to enter into such a decline as this. (Lansing)]

Ultimately, we will have to wait until Inferno 21 for the precise declaration of the date of the voyage.

(2) It is springtime, most likely the day of the equinox (Inferno 1.37-40).

(3) It has just passed midnight on the first day of the journey (Inferno 7.97-99). The journey began in the morning in the selva oscura (Inferno 1.37), and it was already nightfall by the opening of the second canto (Inferno 2.1).

Interpretation of Inferno 7.97-99 by Authoritative Commentators:

Horizontal Interpretation (see note to Inferno 7.98) Global Interpretation (see note to Inferno 7.98)
Jacopo Alighieri, Jacopo della Lana, L’Ottimo Commento, Boccaccio, Benvenuto da Imola, Francesco da Buti, Anonimo Fiorentino, Cristoforo Landino, Alessandro Vellutello, Lodovico Castelvetro, G.A. Scartazzini, Tommaso Casini and S.A. Barbi, G.A. Scartazzini and G. Vandelli, Luigi Petrobono, Attilio Momigliano, Charles Singleton, Anna Maria Chiavacci Leonardi, and Robert Hollander. See also Edward Moore’s 1887 Time-References in the Divina Commedia (page 43). Natalino Sapegno, Daniele Mattalia, Siro A. Chimenz, Giorgio Padoan, and Umberto Bosco and Giovanni Reggio. See also Buti and Bertagni’s 1966 Commento astronomico della Divina Commedia (pages 79-81)

N.B: All commentaries accessed via

Inferno 10.79-81

Ma non cinquanta volte fia raccesa
la faccia de la donna che qui regge,
che tu saprai quanto quell’ arte pesa.
(Inf. 10.79-81)

In Inferno 10 Farinata degli Uberti provides the pilgrim the following prophecy: within fifty full moons, you will know the difficulty of returning from exile. Despite its importance, the prophecy’s historical account of Dante’s experience of exile lies outside the scope of this astronomical study. My focus here is on the indication of time Farinata uses: fifty lunar cycles. The Moon is implicated by the periphrasis, “fia raccesa / la facia de la donna che qui regge,” the reference being to Proserpina, a goddess traditionally associated with the moon. The question is how long do fifty full moons take?

Unlike the Sun, the Moon of course does not shine with its own light. The phase of the Moon is determined by its position relative to the Sun and to the Earth. A new Moon appears in the sky when the Moon stands between the Sun and the Earth, as the entirety of the Sun’s light falls on the side of the Moon that faces away from the Earth; likewise, when the Moon is on the opposite side and it is the Earth instead that stands between the Sun and the Moon, a full Moon appears, as the entirety of the Sun’s light falls on the side of the Moon that faces the Earth. In the Ptolemaic system, the Sun and Moon are both understood to be satellites of the Earth, implicating that a full Moon occurs when the Sun and Moon are in opposition (180-degree difference on the Ecliptic), while a new Moon occurs when the Sun and Moon are in conjunction (0-degree difference on the Ecliptic). The crescent, quarter, and gibbous Moon are each intermediate phases which occur between the new and full Moon: the crescent occurs when less than half the Moon is lit; the quarter occurs when precisely half the Moon is lit; the gibbous occurs when the Moon is more than half lit, but not yet fully lit. In the diagram above, adapted from Buti and Bertagni’s diagram for the phases of the Moon (page 44), the outer circle represents a total view of the Moon while the inner circle shows the Moon’s appearance from the perspective of Earth and indicates the lunar phases.

We can ascertain the length of a single lunation either with a somewhat lengthy observation, or by means of a quick observation and some simple calculations. The former method is rather intuitive: we could just count the days from one full Moon to the next, which would turn out to be a period of approximately twenty-nine days, and then multiply by 50. So, 29 [days from one full moon to the next] x 50 [full moons] = 1450 days / 365 [days per year] = roughly four years (3.9726, to be exact). This is the length of time indicated by Farinata’s prophecy.

Though slightly more involved, the latter method mentioned above is more precise, and it is closer to how a medieval astronomer would have done the calculation. It will yield the same result. On any given night, we could step outside at midnight and mark the location of the Moon in the sky, relative to the nearest star. Recall that the Moon is one of the “wandering stars” while the constellations are considered “fixed stars;” it is this fixity that will make it possible for us to calculate the length of a lunation in a single day. With this in mind, we step outside on the following night and mark the new location of the Moon relative to that same star. While everything in the sky—including the fixed stars—spins about the celestial polar axis each day from East to West, only the wandering stars move about the Ecliptic from West to East (again: hence, “wandering” stars). Over the course of 24 hours we would come to find that the Moon appears to have traveled a distance of approximately 13-degrees about the 360-degree Ecliptic (see Buti e Bertagni, page 43; see also Brunetto Latini: “Ora è leggier cosa a sapere in che segno rimane lo Sole. E poi che l’uomo sa ciò, e’ può leggermente sapere ov’è la Luna, chè ella si dilunga ciascun dì dal Sole 13 gradi […],” book II, cap XLIX). We could make this same observation for the Sun by noting the stars rising just before sunrise or setting just after sunset, and note that the Sun, too, is traveling eastward along the Ecliptic at a pace of about 1-degree per day.

Now, with this information in mind we can calculate the length of the full lunar cycle. As the diagram above highlights, the Moon is full when it is located 180-degrees opposite the Sun, which we can take as our starting point. From here, for the Moon to be full again, it will need to travel at least another 360-degrees: 360 [degrees] / 13 [degrees per day] = approximately 27.5 days. If the Sun were to be stationary in the sky, then the lunar cycle would be 27.5 days. Since the Sun is not stationary, and instead appears to be moving at a rate of 1-degree per day, the Moon will have to surpass the previous location of the full Moon by the distance traveled by the Sun in that same time. In the time it takes the Moon to round the entire Ecliptic, the Sun, traveling 1-degree per day, will have traveled another 27.5-degrees; at 13-degrees per day, the Moon will come to reach again its position precisely opposite the Sun in a little over two days’ time from that point, as it makes up this extra 27.5-degrees in about two days. We arrive at the same conclusion: the lunar cycle is a period of approximately 29.5 days, again indicating that fifty full Moons will occur in a period of roughly four years.

Inferno 11.112-114

Ma seguimi oramai, che ’l gir mi piace;

ché i Pesci guizzan su per l’orizzonta,
e ’l Carro tutto sovra ’l Coro giace,
(Inf. 11.112-114)

Readers of Dante in the 21st century face a host of challenges, some linguistic and others cultural. Perhaps it is strange to consider that one among these challenges is a direct result of light pollution. Short of traveling to a designated dark sky zone, it is near impossible to fathom the night sky as it once appeared, unencumbered by the glow of artificial nighttime lighting. Of course, this was the norm for Dante and his contemporaries, who enjoyed the nightly show of roaming, constellated skies in perfect darkness. Today, however, it is reported that 83% of the global population live beneath light polluted skies (—our “notte privata / d’ogne pianeto” caused, perhaps paradoxically, not by the “buio d’inferno,” but by shining lights.

Virgil provides a snapshot of the night sky in Inferno 11.112-115, the fourth normative indication of time in the poem (following Inferno 1.37-41; Inferno 2.1; Inferno 7.97-99). The information of these verses is that Pisces is just now glittering above the horizon, while the Wagon (Ursa Major) lies over Caurus (the north-western wind). To determine the time indicated by this utterance, we must recall that the Sun is in Aires, the first sign of the Zodiac. Pisces, on the other hand, is the twelfth sign of the Zodiac, making it the sign that crosses the horizon just before Aries. With Pisces just crossing the horizon, we can already determine that the Sun has not yet risen.

While a solar day is the amount of time it takes for the Earth to rotate such that the Sun appears to travel from the meridian (the highest point in the sky; noon) on one day to the meridian on the next, a sidereal day is the amount of time it takes for the Earth to rotate such that any of the fixed stars appears to travel from meridian to meridian. Recall that everything rises on the eastern horizon. Since, in the Ptolemaic understanding of the cosmos, the Sun is a wandering star that moves from West to East at a rate of 1-degree per day, the solar day—which we know to be 24 hours long—is just slightly longer than the sidereal day, which is only 23 hours and 56 minutes long. The 1-degree eastward displacement of the Sun each day with respect to the fixed stars is what results in this slight 4-minute difference between the solar and sidereal day.

Given that it takes 23 hours and 56 minutes for the fixed stars to appear to make one full rotation and complete a sidereal day, a new sign of the Zodiac must appear on the eastern horizon just under every two hours. Therefore, with Pisces just now darting over the horizon, we are just about two hours from sunrise. The location of Pisces alone is enough to indicate the general time at this point in the journey, though we are also given information about Ursa Major (“’l Carro”). The diagram above is an approximate depiction of all the stars which appear in the sky above Jerusalem in the Spring, moments before sunrise. The two which Virgil chooses to highlight are marked: (1) Pisces darting over the horizon, evidenced in two circles on the left of the diagram and (2) the carro—“wain” or “wagon,” i.e. Ursa Major—located in the northwestern quadrant. The light purple line that passes through the diagram as a partial circle denotes the Ecliptic, which of course is not a visible line in the sky, but a theoretical one imagined in order to simplify mathematical astronomy.

Boccaccio in his Esposizioni sopra la commedia di Dante (accessed via echoes the precise language used by Isidore to describe the northwestern wind, Caurus:

Boccaccio: “l’ottavo [vento] chiamano ‘coro’ o vero ‘maestro’, il quale è tra ponente e tramontana: e chiamasi coro per ciò che compie il cerchio, il quale viene ad essere in modo di coro, cioè di quella spezie di ballo il quale è chiamato ‘corea’.” (ad locum)

Isidore: Corus est qui ab occidente aestivo fiat. Et vocatus Corus quod ipse ventorum circulum claudat, et quasi chorum faciat. [Corus is the one that blows from the west in the summer, and it is called Corus because it closes the circle of winds, and makes them like a ring-dance (chorus) (Barney, et al. translation)] (book XIII, xi)

If we were able to see the sky adorned with the lights of the firmament, we could sit and watch at night as these stars spin from East to West around the lodestar Polaris, that is, the northern pole of the celestial axis. Indeed, there can be no doubt that, from time to time, Dante himself would have sat to admire the nightly spectacle.

Louis J. Moffa, Jr. (2020)


Alighieri, Dante. Convivio. ed. Gianfranco Fioravanti. Mondadori, 2019.

Alighieri, Dante. Il Convivio (The Banquet). trans. Richard H. Lansing. Garland Library of Medieval Literature, 1990.

Boccaccio, Giovanni. Esposizioni sopra la Commedia. ed. Giorgio Padoan. Milano, Mondadori, 1964. 11.83, pp. 557-558

Buti, Giovanni, and Renzo Bertagni. Commento astronomico della Divina Commedia: rassegna analitica con una parte generale sistematica e una appendice critica. Sandron, 1966.

Isidore of Seville, Saint. Isidori Hispalensis Episcopi Etymologiarum Sive Originum Libri XX. ed. W. M. (Wallace Martin) Lindsay. Clarendon Press, 1911.

Isidore of Seville, Saint. The Etymologies of Isidore of Seville. ed. and trans. Stephen A. Barney, W. J. Lewis, J. A. Bach, and Oliver Berghof. Cambridge University Press, 2006.

Latini, Brunetto. Il Tesoro di Brunetto Latini. Luigi Gaiter, P. Chabaille, and Bono Giamboni. G. Romagnoli, 1877

Minos, Scott. “International Dark Sky Week.” Energy.Gov, 28 Mar. 2022.

Moore, Edward. Time-References in the Commedia and their Bearing on the Assumed Date and Duration of the Vision. Nutt, 1877.

Plato. “Timaeus” in Complete Works. eds. John M. Cooper and D. S. Hutchinson, Hackett Pub, 1997. pp. 1224–1291

Recommended Citation: Moffa, Jr., Louis J. “Astronomical Diagrams of Inferno.” Digital Dante. Columbia University Libraries, 2020.

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