India, the Islamic world, medieval Europe, and China

India

Ptolemy was the last major figure in the Greek astronomical tradition. Commentaries were written on his works by Pappus of Alexandria in the 3rd century ce and by Theon of Alexandria and his daughter, Hypatia, in the 4th, but creative work was no longer being done. Babylonian astronomy traveled eastward into Persia and India, where it was adapted in original ways and combined with native Indian methods. Greek geometrical planetary theories, from the time between Hipparchus and Ptolemy, also made their way into India. This material is of great complexity and variety and is difficult to sort out. For example, Babylonian arithmetical procedures used for computing lunar and solar phenomena turn up in conjunction with a length for the solar year due to Hipparchus. Nevertheless, the Indian material, besides its own intrinsic interest, provides information about Greek astronomy during a vital period about which the Classical texts say little.

The Islamic world

In the 8th century, Arabic Muslim astronomers came into contact with this complicated astronomical material. Theories and methods that had passed from the Babylonians and Greeks through Persia to India now came back to the West. A good example is provided by the zīj of Muḥammad ibn Mūsā al-Khwārizmī (9th century). Al-Khwārizmī’s work is a confusing mixture of Indian, Persian, and Greek tables and techniques, but it helped establish an important genre of the zīj. A zīj is a handbook of astronomical tables, including tables for working out positions of the Sun, Moon, and planets, accompanied by directions for using them. The ancient prototype was Ptolemy’s Handy Tables.

Ptolemy’s Almagest was translated on at least four occasions into Arabic. Much of the translation activity centred on the Baghdad caliphate of the ʿAbbāsids (750–1258). With the pure geometrical form of Greek planetary theory now available, Arabic astronomers worked to master it and then to improve upon it. The zīj of al-Battānī (early 10th century ce) showed mastery of Ptolemaic planetary theory and improved values for some of Ptolemy’s parameters, such as the magnitude and direction of the Sun’s eccentricity. Hundreds of Arabic zījes from the 9th to the 15th century have been preserved. Some were based on Indian methods, but the great majority were in the tradition of the Almagest and the Handy Tables. A zīj that was very influential in the development of European astronomy was the Toledan Tables, compiled in Spain by a group of Muslim and Jewish astronomers, put into final form by Ibn al-Zarqallu around 1080, and translated into Latin soon after. (The Toledan Tables are mentioned by Chaucer in The Canterbury Tales.)

With the passage of time, it became possible for astronomers to make new discoveries, including those that depended on detecting slow changes in the heavens. In the 9th century the Baghdad astronomers observed that the obliquity of the ecliptic had decreased from the value given in Ptolemy’s Almagest. The obliquity of the ecliptic is the angle between the celestial equator and the tropic of Cancer. It corresponds to the northward displacement of the Sun between the equinox and the summer solstice and can be measured by means of noon altitudes of the Sun taken at key times of the year. Between Ptolemy’s time and the present day, the obliquity of the ecliptic has decreased by about a quarter of a degree. Arabic astronomers also noted that the seasons had changed slightly in length from the values recorded by Ptolemy. This implied that the solar apogee has a slow motion to the east. Thus, the centre of the Sun’s circle can be regarded as revolving very slowly about Earth. This motion was represented in al-Battānī’s zīj.

Ptolemy’s planetary theory was criticized, but minor disagreements between Ptolemy’s tables and actual observations of the planets did not play a significant role in this criticism. Most of the criticism centred on Ptolemy’s violation of the Aristotelian principle of the uniformity of the celestial motions. About 1000 ce Ibn al-Haytham criticized the equant point in Shukūk ʿalā Baṭlamyūs (“Doubts About Ptolemy”). Ibn al-Haytham also objected to Ptolemy’s habit of defining motions with respect to immaterial points and lines as if they were real material bodies. (Complaints about the artificiality of Ptolemy’s constructions had been made even in late antiquity—for example, by the Greek philosopher Proclus in his Diadochi hypotyposis astronomicarum positionum [“Sketch of Astronomical Hypotheses”].)

Ibn al-Haytham’s doubts about Ptolemaic planetary theory inspired some creative mathematical modeling by 13th-century astronomers associated with the observatory of Marāgheh (now in Azerbaijan). Naṣīr al-Dīn al-Ṭūsī described a construction through which two circular motions can give rise to the oscillation of a point back and forth along a straight line. Ptolemy’s theories of Mercury and of the Moon involved oscillatory movements for which the standard mechanisms seemed philosophically questionable. Al-Ṭūsī applied his two-circle mechanism (called an “al-Ṭūsī couple” by modern scholars) to produce the same phenomena in what seemed to him a physically more plausible way. Al-Ṭūsī’s student al-Shīrāzī went farther, using a minor epicycle to eliminate the need for an equant point. In the 14th century Ibn al-Shāṭir of Damascus built on the works of the Marāgheh school in his Nihāyat al-suʾl fi taṣḥīḥ al-uṣūl (“Final Inquiry Concerning the Rectification of Planetary Theory”), which was also characterized by the elimination of nonuniform motions in favour of minor epicycles. However, these efforts did not transform common practice, since the overwhelming majority of late medieval planetary tables are Ptolemaic in their underlying theory. In the 16th century Nicolaus Copernicus used models identical to those of Ibn al-Shāṭir and the Marāgheh school. How he came by them is unknown, but there are too many of them to make independent discovery credible. These technical “improvements” on Ptolemy had nothing to do with the heliocentric hypothesis, but they show that Copernicus was heir to a tradition of critical engagement with Ptolemy.

Medieval Europe

In the Latin West the level of scientific learning had sunk to a low level. None of the Greek works most important for ancient astronomy and cosmology—Aristotle’s On the Heavens and Ptolemy’s Almagest, Handy Tables, and Planetary Hypotheses—were available. The teaching of astronomy was based on a number of low-level Latin accounts. Book II of Pliny the Elder’s Naturalis historia (Natural History, 1st century ce) contained a summary of astronomical matters. In the 4th century Martianus Capella wrote an allegorical poem, De nuptiis Philologiae et Mercurii (The Marriage of Philology and Mercury). In the two introductory books, Philology, personified as a maiden, is wed to Mercury, patron god of learning. In the following seven books, each of the Liberal Arts, including Astronomy, personified as a handmaid to Philology, steps forward to give an account of her art. Martianus’s Marriage was widely admired in the early Middle Ages as a compendium of all useful learning.

In the 12th and 13th centuries, two developments were key to the revival of astronomy in the Latin West. The translation movement rapidly made available key works of Greek astronomy that had long been out of reach. One of the most important translators was Gerard of Cremona. As his students later wrote of him, he had learned everything that was known to the Latins, but for love of the Almagest, which he had heard of but which was unavailable in Latin, he went to Spain and learned Arabic well enough to translate it. Thus, one could maintain that a major reason for the revival of learning in the West was one man’s desire to be able to read Ptolemy. Gerard translated from Arabic versions not only Ptolemy’s Almagest but also Aristotle’s On the Heavens, Euclid’s Elements, and about two dozen other works of astronomy and geometry. In a single generation most of the key works of ancient astronomy became available.

The second important development was the foundation of the European universities, starting with those of Bologna, Paris, and Oxford. Because astronomy figured among the liberal arts, it had a place in the university core curriculum. Of course, the astronomy of the liberal arts curriculum was at a rudimentary level. The students might be taken though an introduction to the celestial sphere—for example, the De sphaera mundi (“On the Sphere of the World,” c. 1230) by Johannes de Sacrobosco, which might be followed by the anonymous Theorica planetarum (“Theories of the Planets”), a superficial introduction to eccentrics and epicycles. Nevertheless, in every university town there had to be someone charged with teaching astronomy.

In the 1270s a new set of astronomical tables was compiled in Spain under the patronage of the Christian king Alfonso X of Léon and Castile. These were based on standard Ptolemaic astronomy, with some differences in the treatment of precession (now considered to occur at a variable speed). By 1320 the Alfonsine Tables had reached Paris, where they were reworked by several Parisian astronomers. From there they spread all over Latin Europe, and for more than two centuries they were the standard.

China

Though “oracle bones” exist from the late 2nd millennium bce that mention observations of lunar and solar eclipses as well as the appearance of a new star (nova), astronomical reports begin to be fairly numerous only from about 200 bce. In China astronomy had an imperial function. The emperor was considered the Son of Heaven. Thus, the regulation of the calendar, as well as the success or failure of his astronomers to predict an eclipse, reflected either well or badly on him. Many different astronomical summaries were written in conjunction with the ascent of a new emperor. Usually these emphasized the lunisolar calendar, but later they also included tables for predicting the motions of the planets, as well as eclipses. Chinese predictive astronomy used repeating arithmetical cycles and was thus more like Babylonian astronomy than like Greek astronomy. Perhaps because the Chinese were less tied up with cosmological theories and “laws” of nature than the Greeks and their medieval European successors were, the Chinese astronomers were much more interested in singular events, such as comets, novae, meteor showers, solar eclipses, and sunspots (which the Chinese discovered before the Europeans), and they kept detailed records of them.