Diagramming Dante

Diagramming Dante: Astronomy in the Commedia (2020)

Louis J. Moffa, Jr., Columbia University

The goal of this project is to bring clarity to some of the most seemingly complex passages in the Divina Commedia: those that deal with astronomical and astrological matters. In his 1903 Studies in Dante, Edward Moore wrote, “[i]t is a matter of regret that even students of ability and culture often refuse so much as to attempt to understand Dante’s astronomical references. They assume either that they are not to be understood at all, or at least not without special astronomical and mathematical training” (p. 1). Though there are many useful studies that treat astronomy and astrology in the Commedia, Moore’s comment still rings true. The difficulty that readers perceive in grasping Dante’s astronomical references is due to several factors, all of which serve to distance the contemporary reader of the Commedia from the poet.

For starters, the language of medieval astronomy is intricate, technical, and dense. It requires that a reader define many key terms before attempting to grasp the literal level of any astronomical discourse. Second, Dante saw the cosmos through a Ptolemaic lens, one which located the Earth at the fixed center of a spinning universe. Readers of the Commedia today see the cosmos through a Copernican lens, subscribing to a heliocentric model of the solar system with a rotating Earth. Third, in Dante’s time astronomy and astrology comprised the final liberal art of the quadrivium, making it a commonplace of intellectual and academic life; today instead faculties of thought are much more segregated and most readers of Dante tend not to equivalently study astronomy, physics, or the history of science. Lastly, Dante’s world was not riddled with artificial lights, and he did not tell time by a mechanic clock. As a society we mediate and understand time very differently than he did, and those of us who live near large cities do not see the stars at night. The moving sky is not as present a component of daily life for us as it was for Dante. 

To help cover this distance, I begin with a quick overview of the tenets of Ptolemaic cosmology, and then move to addressing sections of the Commedia that feature astronomical or astrological references. As Richard Kay, among others, has pointed out, the distinction between the terms astronomy and astrology is less concrete for Dante than it is for us (p. 1, notes 1-3). Medieval thinkers did not regard the latter skeptically as a pseudo-science, which is more or less the norm today. For this reason, my analysis of the text engages in both astronomy and astrology (using today’s nomenclature) in order to adequately treat the text’s presentation of the stars.

In the above diagram, Nc and Nt mark the North celestial and North terrestrial poles; Et and Ec mark the terrestrial and celestial equator; and St and Sc mark the South terrestrial and South celestial poles. The poles are points while the equator is a plane.

The Ptolemaic system which Dante inherits begins with a few initial postulates about the Earth:

(1) the center of the Earth is the center of the universe

(2) the Earth is a sphere

(3) the Earth is unmoving 

These postulates were derived from observing the movements of the planets and stars over long periods of time and drawing conclusions from recurrent patterns. The Ptolemaic model persisted with some amendments until the time of Galileo, and though Ptolemaic astronomy has limits and flaws, it was (and still is) a useful theoretical model for tracking and predicting the movement of the stars and planets in the night sky. 

Note that the Sun and the Moon are both considered to be planets in this framework. This system views the planets as “wandering stars,” a category which comprises the seven bodies which appear to exhibit a second, westward motion counter to the eastward motion of everything in the sky. Of course, today we know that everything in the sky appears to move from east to west due to the Earth’s rotation; in Dante’s time and prior, this movement was considered an essential quality of the celestial bodies and spheres. Along with the Sun and Moon, the other wandering stars are the five planets visible to the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn (Neptune, Uranus, and Pluto are not visible without a telescope and as such had not yet been discovered). The term wandering stars is a distinction from what we would call constellations, which do not appear to ‘wander’ but instead only exhibit the shared easterly motion. The constellations are referred to as the “fixed stars,” for they do not wander. [For more on the terminology “wandering stars” and “fixed stars,” see note below.]

The fixed and wandering stars are taken to be located on a series of concentric and diaphanous heavenly spheres. The celestial bodies are ordered in the following manner, according to the size of their apparent rotation about the Earth: (1) Moon; (2) Mercury; (3) Venus; (4) Sun; (5) Mars; (6) Jupiter; (7) Saturn; (8) Heaven of the Fixed Stars. The planets and stars are taken to be located on the surface of the spheres which they inhabit. With the Earth at the center, these spheres are nestled like Russian dolls within the heaven of the fixed stars. Further, the north-south axis of the Earth is in perfect alignment with the north-south axis of the celestial sphere, meaning that the celestial poles stand in alignment to the terrestrial poles–and this is necessarily the case since it is the rotation of the Earth which gives the appearances of rotation to the heavens. Accordingly, the Earth’s equator and the celestial equator are concentric circles and are parallel.

As previously mentioned, the wandering stars appear to travel counter to the eastward rotation of the universe. This movement occurs along one the belt of the ecliptic. Each of the wandering stars travels this path, and takes a different amount of time to do so. The Moon is the fastest, and Saturn is the slowest. The ecliptic is concentric to the Earth’s equator and the celestial equator, but it is not parallel to them. The ecliptic is on an angle of approximately 23.4 degrees with respect to the equators. Dante describes the ecliptic as “torta” in both the Commedia and the Convivio, in reference to this distortion with respect to the equator (See Paradiso 10.16; Convivio 3.5.13). Since the ecliptic is on an angle with respect to the equator, the planets appear to move between the hemispheres. When the Sun appears to be in the northern hemisphere, the northern hemisphere experiences its spring and summer. Likewise, when it shifts into the southern hemisphere, the northern hemisphere experiences its fall and winter. The ecliptic intersects the equator at two points: the “first point of Aries” and the “first point of Libra.”

Due to a phenomenon known as “precession of the equinoxes” (first discovered by the Greek astronomer Hipparchus), the constellations appear to slip gradually across the sky. Today we know this to be caused by the Earth’s wobbling as it rotates; the result of this phenomenon is that the section of sky through which the planets appear to wander has shifted a bit since the ancient inception of astronomy and astrology. Though it is no more the case, it used to be so aligned that the path of the wandering stars covered the belt of the Zodiac, or the twelve constellations: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, Pisces. Despite precession, astrological tradition still acknowledges the ‘signs’ of the constellations as fixed positions in the sky: the constellation Aries, for example, is no longer located at the the vernal equinox, but the sign of Aries is still recognized as the 30-degree section of the ecliptic whose beginning is marked by the first day of spring. 

This is one way of imagining the wandering stars moving about the Earth.

Note that ecliptic is a circle, meaning it contains 360 degrees and as previously mentioned comprises the 12 signs. Each constellation accordingly comprises 30 degrees of the circle. The constellations of Aries, Cancer, Libra, and Capricorn mark particular points along this circle because, when the sun appears to enter into one of them, we note the change of a season: in the northern hemisphere Aries is the start of spring, Cancer of summer, Libra of fall, and Capricorn of winter.

As previously mentioned, the “first point of Aries” is one of two points where the ecliptic and equator intersect, these two intersecting points being the equinoxes. For the northern hemisphere, when the sun crosses the first point of Aries, night and day are equal (hence “equi-nox”). As the sun moves further into the northern hemisphere, the days grow longer until the summer solstice, the day on which the sun appears to stand still (hence “sol-stit”) before beginning its descent back to the southern hemisphere, during which time the days grow shorter. After the solstice, the sun descends to the “first point of Libra,” or the autumnal equinox, when, again, day and night are equal. On this day the Sun reaches the southern hemisphere again. It will descend further into the southern hemisphere until the winter solstice, after which it will begin to ascend again and return to the first point of Aries, starting the cycle anew. Dante explains this exact process in detail in Convivio 3.5. 

From the perspective of the southern hemisphere, the first point of Aries represents the autumnal equinox, and the seasons are precisely opposite.

This is another way of imagining the movement of the Sun along the ecliptic.


Six of the constellations are located in the northern hemisphere: Aries, Taurus, Gemini, Cancer, Leo, and Virgo; and six in the southern hemisphere: Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces.

Cancer is the northernmost constellation, at the height of the Ecliptic, approximately 23.4 degrees north of the line of the equator. Capricorn, which stands 180 degrees opposite to Cancer, is the southernmost, approximately 23.4 degrees south of the equator. If an observer were to sit on the Equator and mark the position of the Sun once a day, at the same time every day for a year, they would see the sun gradually rise 23.4 degrees from the vernal equinox until the summer solstice; descend 23.4 degrees from the summer solstice to the autumnal equinox; descend another 23.4 degrees until the winter solstice; and ascend again, returning to the vernal equinox. If the observer captured a picture of the Sun at the same time every day for a year and put them all together, it would form an analemma, or a figure-8 in the sky.

A note on the terminology “fixed stars” versus “wandering stars” from Plato’s Timaeus, Isidore’s Etymologia, and Dante’s Convivio

The distinction of “wandering” versus “fixed” stars is common terminology used by astrologers and astronomers in the medieval period. Accordingly, Dante will use the terms pianeta and stelle fisse in reference to these entities in the cosmos. In English, the best rendering of these two terms is “wandering stars” and “fixed stars,” respectively, since the English “planet,” to modern readers of Dante, will implicitly exclude the Sun and the Moon.

Fundamentally, the term “wandering star” is a reference to the unique motion of the seven objects in our solar system which move along the Ecliptic: the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. These are understood in contrast with the constellations, which remain fixed in the background on the celestial sphere as the planets “wander.” The distinction originates with the Greek philosophers, who called the stars that moved along the Ecliptic πλανήτης or “wandering ones,” which, in Latin, later became planetae.

In Plato’s Timaeus, which Dante will reference in Paradiso 4, these two distinct kinds of movement are addressed at some length as Timaeus recounts the story of the creation of the universe.

Next, he [the creator of the universe] sliced this entire compound in two along its length, joined the two halves together center to center like an X, and bent them back in a circle, attaching each half to itself end to end and to the ends of the other half at the point opposite to the one where they had been joined together. He then included them in that motion which revolves in the same place without variation, and began to make the one the outer, and the other the inner circle. And he decreed that the outer movement should be the movement of the Same, while the inner one should be that of the Different. […] and he made the revolution of the Same, i.e. the uniform, the dominant one in that he left this one alone undivided, while he divided the inner one six times, to make seven unequal circles. (36c-d)

Here, Timaeus’s terminology, “the movement of the Same,” accords to the rotation of the heavens about the celestial axis, or the way that the Sun and the other stars appear to rise and set over the line of the horizon on a daily basis. On the other hand, “[the movement] of the Different,” is a reference to the varied movement of all the wandering stars along the Ecliptic. Indeed, the description of the fashioning of a shape that resembles an X with the ends bent back and attached in circles is precisely the manner in which astrologers and astronomers depict the lines of the equator and the Ecliptic in the Ptolemaic system (see diagram for Inferno 1 for a visual representation of this).

Timaeus goes on:

Such was the reason, then, such the god’s design for the coming to be of time, that he brought into being the Sun, the Moon, and five other stars, for the begetting of time. These are called “wanderers,” (πλανητά) and they came to be in order to set the limits to and stand guard over the numbers of time. When god had finished making a body for each of them, he placed them into the orbits traced by the period of the Different—seven bodies in seven orbits. (38c-d)

The language in the Timaeus addresses the dual movement of the heavens as that of sameness and difference: “the movement of the Same” is the diurnal motion about the celestial axis, which is shared by everything in the sky, while “[the movement] of the Different” is the movement of the seven wandering stars along the line of the Ecliptic. As Timaeus serves to underscore, linguistically speaking, the Greek term πλανήτης is a reference to the unique movement observed in the motion of these particular seven stars in the sky (cf. Liddel, πλανάω, “to make to wander, lead wandering about” or, when passive, “to wander, to roam about, stray”).

Dante did not read Greek, however. To trace how the terminology reaches Dante, we can consult Isidore, who, in Book III of the Etymologiae, expands on the concept:

De Sphaerae Caelestis Situ.

Philosophi autem mundi septem caeolos, id est planetas, globorum consono motu introduxerunt, quorum orbibus conexa memorant omnia, quos sibi innexos et velut insertos versari retro et [e] contrario ceteris motu ferri arbitrantur. (xxxii)

[The philosophers have proposed seven heavens belonging to the universe, that is, seven planets, from the coordinated motion of their spheres. They hold that everything is connected to the orbital paths of these planets, and they think that the planets are interconnected and in a way inserted within one another, and that they turn backwards and are carried by a motion that is opposite to the other heavenly bodies. (Barney, et al. translation)]

De Stellis Planetis.

Quaedam stellae ideo planetae dicuntur, id est errantes, quia per totum mundum vario motu discurrunt. Unde pro eo, quod errant, retrograde dicuntur, vel anomala efficiuntur, id est, quando particulas addunt et detrahunt. (lxviii)

[Certain stars are called planets, that is, ‘wandering ones,’ because they range through the entire cosmos with a varying motion. It is because of their wandering that they are called retrograde, or are rendered irregular when they add or subtract orbital degrees. (Barney, et al. translation)]

The Latin planetas, as Isidore underscores, is a term derived from the Greek philosophic tradition.Thus, the “wandering” exhibited by the seven “planets” is the characteristic that distinguishes them from all the other stars in the sky, which sit still, or “fixed,” on the backdrop of the celestial sphere. In summation: every light in the sky shares in what Timaeus calls “the movement of the Same,” but only the planetae exhibit the movement of “the Different.” This distinction in the motion of the stars is what will ultimately lead Dante, among others, to posit the manifold spheres that comprise the physical cosmos and develop the cosmological system that grounds the entirety of Paradiso.

Dante in the Convivio will refer to the constellations as le stelle fisse (cf. 2.3.3 and 2.14.7). Accordingly, he will at times use the term pianeta, meaning a star which wanders (2.13.7 and 2.13.28), when treating the “planets,” while at other times he will refer to them simply as “stella”—as in “la stella di Venere” (2.2.1); “la stella di Marte” (2.13.22); or “Giove è stella di temperata complessione” (2.13.25). Throughout the Commedia, as we will see, he employs the same terminology: stelle fisse for the fixed stars, and pianeta or stella for the wandering stars. Dante does not, however, use the term “stella errante” or any such terminology. I use this terminology in English to make it clear that the Sun and the Moon are included in the medieval discourse on planets, despite the fact that today we do not consider them to be such.


Alighieri, Dante. Convivio. edited by Gianfranco Fioravanti. Mondadori, 2019.

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

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

Kay, Richard. Dante’s Christian Astrology. University of Pennsylvania Press, 1994.

Liddell, Henry George. A Greek-English Lexicon. Clarendon Press, 1901.Moore, Edward. Studies in Dante. Third Series. Miscellaneous Essays. Clarendon Press, 1903.

Plato. “Timaeus” in Complete Works. edited by John M. Cooper and D.S. Hutchinson. Hackett Publishing Company, 1997.

Recommended Citation: Moffa, Jr., Louis J. “Diagramming Dante: Astronomy in the Commedia.” Digital Dante. Columbia University Libraries, 2020. https://digitaldante.columbia.edu/moffa-astronomy/.

return to top Return to top
Published in partnership with Columbia University Libraries