Ancient and Medieval Astronomy

Edward Worth’s collection holds two works on ancient astronomy: Aratus Solensis’s Aratou Soleōs Phainomena kai Diosēmeia : Theōnos scholia. Leontiou Mēchanikou Peri arateias sphairas (Paris, 1559) and Denis Petau’s Uranologion (Paris 1630), a large folio compilatory volume of works by ancient authors such as Hipparchus, Ptolemy and Geminus.

Aratus Solensis, ca. 315 BC – 240 BC, was a Greek poet rather than an astronomer. Ordered by his patron, King Antigonus Gonatus of Macedonia, to immortalise the astronomical work of Eudoxus of Cnidus, ca. 408-355 BC, Aratus composed his Phaenomena. The poem is divided into three parts, the most important being his poetical description of the constellations, which forms the first section; followed by a discussion of the rising and setting of the constellations in the second part. This illustration of the constellation is taken from the Parisian 1559 edition by Guillaume Morel but Worth had more than one edition of this work: a 1540 Parisian edition by Joachim Périon and the text was included in a number of compilatory editions of ancient Greek authors in this collection.

Aratus Solensis, Aratou Soleōs Phainomena kai Diosēmeia (Paris, 1559), foldout plate.

Though purportedly based on the work of Eudoxus, the poem includes clues which indicate that its source was a much older mapping of the stars, one which pre-dated by far Eudoxus’s era. The fact that Ptolemy uses similar names for the constellations allows a comparison between the Ptolemaic map of the sky and the implied poetical one of Aratus. Zhitomirsky’s close examination of the poem (1999), in particular Aratus’s more concrete comments on the position of certain constellations on the celestial equator, suggests that it is based on a primary source, probably oral in nature, which originated sometime around the beginning of 2000 BC. The unknown astronomers were evidently based far to the north, at the latitude of 36o N, rather than near the well-known civilizations of ancient Egypt and Sumeria.

As the title page demonstrates, Denis Petau’s Uranologion: sive, Systema variorum authorum qui de sphæra, ac sideribus, eorumque motibus græcè commentati sunt omnia vel græcè ac latinè nunc primùm edita, vel antè non edita; cura & studio Dionysii Petavii Aurelianensis; accesserunt variarum dissertationum libri octo, ad authores illos intelligendos imprimis utiles, eodem authore (Paris, 1630) brings together works by famous ancient astronomers such as Hipparchus, c. 190 BC – c. 120 BC, Geminus, 1st century BC, and Ptolemy 90 – 168 AD.

Denis Petau, Uranologion (Paris, 1630), title page.

Hipparchus is perhaps best known today for his explanation of the precession of the equinoxes. By calculating the longitude of Spica against previously recorded observations of astronomers such as Timocharis and Aristullus, Hipparchus was able to prove that Spica’s longitude had increased by nearly two degrees. He argued that this motion was a slight progression of the stars eastward with regard to the ecliptic and not the retrogression of the equinoctial points. Ironically, though Hipparchus is now regarded as the most influential ancient astronomer for his works on such topics as the orbit of the Moon, solar and lunar eclipses, his lost star catalogue and his discussions of the size and distances of the Sun and Moon (which are known to us from references in other ancient authors), the only text of his to survive is his ‘Commentary on the Phaenomena of Eudoxus and Aratus’. As the title page of Petau’s Uranologion makes clear, Aratus’s Phaenomena was the focus of not only Hipparchus’s text but also that of Geminus of Rhodes.

Both Geminus and Ptolemy owed much to the observatory work of Hipparchus. Geminus produced a textbook introduction to astronomy, the Isagoge, which was heavily based on Hipparchan findings and so too was Claudius Ptolemy’s seminal work, the Almagest, a far more complex and ultimately more influential work. Much of the lost star catalogue of Hipparchus can be pieced together by studying the Almagest which relied heavily on Hipparchus’s observations. As Ohruhlik (1978) points out, of 28 observations of solstices and equinoxes in Book III of the Almagest, 24 were from Hipparchus with only 4 mentioned by Ptolemy as being his own.

Johannes Hevelius, Selenographia, p. 161 (Diagram of Ptolemaic system)

In the Almagest Ptolemy examined the position of planets but rarely discussed the issue of planetary distances (a topic he reserved for his Planetary Hypotheses). As we see here, Ptolemy’s scheme is geocentric, the earth being solidly at the centre of the universe (for at this stage the solar system was regarded as the entire universe). Ascending from the earth the order was as follows: Moon, Mercury, Venus, Mars, Jupiter, Saturn and fixed stars. Placing the Sun was slightly more difficult: as Ptolemy states in his Planetary Hypotheses, no known transit of the Sun by the five planets had been observed – data which would have proved their position below the Sun. Relying on a nesting hypothesis of planets, Ptolemy concluded that the Sun must come between Venus and Mars. The issue of the order of the planets, particularly the ‘inferior planets’ of Mercury and Venus, continued to be a vexed question, with some medieval commentators suggesting that both Mercury and Venus lay beneath the sun, others that they were located above the Sun, and a third cohort suggesting that Mercury was beneath and Venus was above the Sun. A tendency to conflate sunspots as transits of Mercury and Venus ensured that the position of the Sun remained un-assailed.

Perhaps the most important reason why Ptolemy’s astronomical (and indeed his astrological) works remained so dominant throughout the Middle Ages was because of their superficially symbiotic relationship with Aristotelian physics. In a sense Ptolemy provided mathematical explanations for Aristotle’s cosmology, the latter divided throughout Aristotle’s De Caelo, Physica, Metaphysica and Meteorologia. And they certainly agreed on a number of basic points:

  • The solidity of the planetary orbs – each planet was carried around by a sphere or orb in which it was embedded. Within this sphere were a complex nested set of sub-spheres which helped explain planetary motions.
  • The seven planetary orbs were surrounded by an eighth sphere, that of the fixed stars.
  • The four elements of earth, air, fire and water comprised the terrestrial sphere and were by their nature subject to change; unlike the fifth element, the ether in the celestial zone, located beyond the Moon. In this schema only in the sub-lunary realm could change take place.
  • But there were tensions between the two views, tensions which had been apparent in the Middle Ages: Aristotle and Ptolemy differed on the number of sub-spheres: Aristotle suggesting 55, Ptolemy far fewer (the lowest figure the latter gives is 29). This was as nothing to a more serious divergence: Aristotle argued that the orbs on which planets moved were all concentric, whereas Ptolemy’s geometric model had introduced a complex system of eccentric circles and epicyclic movements which were, by their nature, not concentric.

Medieval commentators sought to marry the two systems together by promulgating what Grant (1996) calls the ‘Three orb compromise’: only the outer orb, the ‘orbis totalis’ was concentric – allowing inner or ‘partial’ orbs to accommodate Ptolemy’s mathematical models. Medieval writers were not afraid to augment the corpus and added further spheres beyond that of the fixed stars: a ninth ‘crystalline’ sphere, a tenth sphere of the ‘primum mobile’, and the eleventh sphere of the empyrean heaven.

Despite the fact that the Almagest had been translated from Arabic into Latin by Gerard of Cremona in 1175 and subsequently appears on several curriculum lists for medieval universities (for example Oxford in 1431), it was clear that these curricular injunctions were more aspirational than practical, for the Almagest, with its complicated series of eccentric and epicyclic circles was far too complex to serve as a textbook for students at medieval universities. Astronomy, as part of the quadrivium, was studied as part of the B.A. degree and the youthful age of the students, coupled with the fact that many of them would never before have studied the subject, called for a simple introductory text. This was provided by Johannes de Sacrobosco, a thirteenth-century professor at the University of Paris.

Johannes de Sacrobosco

All that is known of Sacrobosco, d. c. 1236, is that he taught at the University of Paris in the thirteenth century. He may have been of either English, Irish or Scottish birth – certainly the appellation ‘Sacrobosco’ or ‘Holywood’ can be found in all three countries – but most authors have followed Robertus Anglicus in deeming him an Englishman. We don’t know when he was born or when he died – save that he was evidently considered to be of importance to the University of Paris since his tomb was placed in the monastery of Saint-Mathurin.

Sacrobosco’s 16 page work on the Sphere proved to be one of the most long-lived textbooks ever written. It survives in hundreds of manuscripts all across Europe and was the first astronomical work to be printed (in Ferrara in 1472). A host of editions followed, including 35 Venetian editions alone, and in the period 1501 to 1600 there were at least two hundred separate editions throughout Europe. It is for this reason that Lynn Thorndike’s (1949) states that it was ‘the clearest, most elementary and most used textbook in astronomy and cosmography from the thirteenth to the seventeenth century.’

It was divided into four chapters: the first concentrates on the concept of sphericity of heavenly bodies, their circular motion and the stationary nature of the earth at the centre of the universe; Chapter two investigates the geometrical underpinning of the cosmology: the ecliptic, the equatorial poles and the tropics; the third chapter explores the movement of some heavenly bodies and in particular the movement of the sun along the ecliptic; and the final chapter examines solar and lunar eclipses – following a whistle-stop tour of Ptolemaic astronomy. Sacrobosco was not trying to produce a monograph on astronomy but a textbook which could serve as a short and understandable introduction to more complex texts. It was a work pedagogical in intent and owed its continuing success not to any originality of thought but to its concise and clear nature. Like the Almagest it said little about planetary motion, a deficiency addressed by an unknown author who produced a Theorica Planetarum which, along with the De Sphaera of Sacrobosco, formed the corpus astronomicum of the later Middle Ages.

Worth had three copies of sixteenth-century commentaries on Sacrobosco’s De Sphaera: one by Theodor Graminaeus and two separate editions by the renowned Jesuit mathematician and most famous commentator of Sacrobosco in the early modern period: Christopher Clavius.

Theodor Graminaeus,
Uberior enarratio eorum quae a Ioanne de Sacro Bosco proponuntur
(Cologne, 1567), title page.

Theodor Graminaeus was a publisher based in Cologne who produced works concerned with map-making and astronomy. In 1567 he produced this octavo edition of Uberior enarratio eorum quae a Ioanne de Sacro Bosco proponuntur, ita vt adiecta difficilioribus locis commentarij vicem supplere possit… (Cologne, 1567), and continued his astronomical publishing two years later by publishing a Cologne edition of Guillaume Morel’s Parisian 1569 edition of Aratus Solensis’s Phaenomena. We can see that Sacrobosco’s 16 page text had expanded considerably since its genesis sometime in the 1230s. Graminaeus’s commentary also added a host of astronomical illustrations, including depictions of the shadow of the Sun of the Earth; and the moon Moon fully eclipsing the Sun as seen from the Earth. Graminaeus’s octavo edition comes thirty-six years after the initial octavo edition, produced by the University of Wittenberg to cater for their student market. The format itself shows that this was a work that would be sold to a range of different book-buyers.

Graminaeus-p273 Graminaeus
Theodor Graminaeus,
Uberior enarratio eorum quae a
Ioanne de Sacro Bosco proponuntur
(Cologne, 1567), p. 273
(Depicting the shadow of the sun on the earth).
Theodor Graminaeus,
Uberior enarratio eorum quae a
Ioanne de Sacro Bosco proponuntur

(Cologne, 1567), p. 277 (
Depicting how the Moon can fully
eclipse the Sun when viewed from earth)

While little is known about Theodor Graminaeus, Christopher Clavius, 1538-1612, was one of the jewels in the Jesuit mathematical crown of the sixteenth century. One of the founders of the Collegio Romano, Clavius’s influence spread throughout the sphere of Jesuit influence. As Feingold (2003) and others have emphasised, Jesuit mathematicians were at the cutting edge of mathematical research. Clavius may have been best known for his seminal works in mathematics but his commentary on Sacrobosco’s sphere was the most comprehensive ever written, running to over 800 pages. First published at Rome in 1570 it ran to more than sixteen printings between 1570 and 1618. Worth’s copy on display here was a 1608 Geneva printing of the 1581 Roman edition which had been substantially augmented by Clavius to take into account the new astronomical challenges facing the Ptolemaic astronomical view.


Christoph Clavius, Christophori Clauii Bamb. ex Societate
Iesu, in sphæram Ioannis de Sacro Bosco, commentarius ([Geneva], 1608), title page.

When medieval astronomers explored the heavens they did so predominantly in an aristotelian framework – there was no rival cosmological system on offer. This situation changed with the Renaissance editorial drive to produce editions of newly available ancient texts, not only aristotelian commentaries but also Platonic and Stoic works. However, sixteenth-century aristotelians faced far greater challenges than their medieval counterparts: the advent of the Copernican challenge and the disquieting fact that the heavens themselves seemed to be undermining key aristotelian tenets: the new star of 1572 and the comet of 1577 provided ample testimony that Aristotle’s neat division between the corruptible terrestrial sphere and the incorruptible heavens where change could not take place was demonstratively wrong. Clavius’s 1581 edition of his commentary on Sacrobosco’s Sphaera portrays an aristotelianism at bay: Clavius’s first edition of 1570 of Sacrobosco might have said little of Copernicus and focused instead on another perceived threat, the homocentric theory of Fracastoro, but the many later editions, such as Worth’s copy, could not afford to ignore the celestial events of the 1570s.

manuscripts  Clavius-pp-208-9
Christoph Clavius, Christophori Clauii Bamb. ex Societate
Iesu, in sphæram Ioannis de Sacro Bosco, commentarius ([Geneva], 1608),
Manuscript notes at end of work and p. 209 (notes on nova in Cassiopeia).

Here we see Clavius trying to cope with the new star of 1572 – which, as the illustration makes clear, was visible in the constellation Cassiopeia. The problem of the nova in Cassiopeia for aristotelians was that it indicated change in a region where, according to Aristotle, no change should take place: the celestial sphere. The nova of 1572 and the comet of 1577 produced not only a range of anti-Aristotelian treatises but also a diversity of aristotelian responses to the issue of celestial incorruptibility. Clavius agreed with Tycho Brahe that the nova in Cassiopeia was indisputably located in the celestial regions but argued that it was only apparently a new star – suggesting that it had pre-existed in the heavens but hadn’t been visible up until then. This was at least nominally aristotelian but subsequent Jesuit astronomers such as Giambattista went much further, trying other tactics: Riccioli agreed that the heavens were subject to change but based his acceptance not only on the sighting of new stars and comets but also the Bible and Church Fathers. For Clavius too, the ultimate judge was Scripture. The religious underpinning of his commentary on Sacrobosco is reflected in the title page of the collected works by Clavius in the Worth Library – his Mainz 1612 edition.


Christoph Clavius, Opera Mathematica (Mainz, 1612), 5 v., title page of v1.

Other Works by these authors in the Worth Library:
Aratus Solensis, Phaenomena… Ioachimi Perionij opera… (Paris, 1540). 4o. This work is also included in a number of compilations of Greek poetry and works on astronomy.

Selected Reading:

Feingold, M. (ed.) (2003) The New Science and Jesuit Science: Seventeenth Century Perspectives. Dordrecht: Kluwer.
Feingold, M. (ed.) (2003) Jesuit Science and the Republic of Letters. Massachusetts Institute of Technology.
Gingerich, O. (1988) ‘Sacrobosco as a Textbook’, Journal for the History of Astronomy 19, no. 4, 269-273.
Goldstein, B. R. (1972) ‘Theory and Observation in Medieval Astronomy’, Isis 63 no. 1 (Mar.), 39-47.
Grant, E. (1985) ‘A New Look at Medieval Cosmology, 1200-1687’, Proceedings of the American Philosophical Society, 129 no. 4 (Dec.), 417-432.
Lattis, J. M. (1994) Between Copernicus and Galileo. Christoph Clavius and the Collapse of Ptolemaic Cosmology. Chicago: University of Chicago Press.
Leff, G. (1992) ‘The Trivium and the Three Philosophies’ in Hilde De Ridder-Symoens (ed.) A History of the University in Europe vol I. Universities in the Middle Ages. Cambridge: Cambridge University Press, pp. 307-336.
Maxwell, H. C. ‘Hipparchus and the Precession of the Equinoxes’, Proceedings of the Royal Irish Academy 91889-1901), 6 (1900-1902), 450-456.
North, J. (1992) ‘The Quadrivium’ in Hilde De Ridder-Symoens (ed.) A History of the University in Europe vol I. Universities in the Middle Ages. Cambridge: Cambridge University Press, pp. 307-336.
Okruhlik, K. (1978) ‘The Interplay between Theory and Observation in the Solar Model of Hipparchus and Ptolemy’, Proceedings of the Biennial Meeting of the Philosophy of Science Association 1, 73-82.
Pantin, I. (1998) ‘Is Clavius worth reappraising? The impact of a Jesuit mathematical teacher on the eve of the astronomical revolution’, Studies in History and Philosophy of Science 27 no. 4, 593-598.
Pedersen, O. (2004) ‘Sacrobosco, John de (d. c.1236)’, Oxford Dictionary of National Biography, Oxford University Press.
Pedersen, O. (1985) ‘In Quest of Sacrobosco’, Journal for the History of Astronomy XVI, 175-221.
Thorndike, L. (1949) The Sphere of Sacrobosco and its commentators. Chicago: University of Chicago Press.
Toomer, G. J. (ed.) (1998) Ptolemy’s Almagest. Princeton University Press.
Zhitomirsky, S. (1999) ‘Aratus’ ‘Phaenomena’: Dating and Analysing its Primary Source’, Astronomical and Astrophysical Transactions 17, 483-500.
Toomer, G. J. (1978): ‘Hipparchus’, Dictionary of Scientific Biography 15, 207-224

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