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From Ptolemy to the Renaissance: How Classical Astronomy Suvived the Dark Ages.

Alexandria, Egypt, Second Century A.D. An unprecedented community of poets, philosophers, scientists, and other scholars has contributed to one of the greatest libraries ever built by humankind. Amid this fervent intellectual backdrop, classical astronomy has reached its zenith in the writings of the Hellenistic astronomer, mathematician, and geographer Claudius Ptolemy. His recently completed astronomical magnum opus, the Syntaxis, will be revered as "The Greatest" (Almagest) by generations of learned people.

Little does Ptolemy know, however, that the empire so richly supporting Alexandria's intellectual endeavors soon will decline and fall, depriving western Europe of any knowledge of his work or that of his contemporaries for centuries to come.

Fortunately, while Europe slid into its long slumber through the Dark Ages, Islamic and Byzantine scholars would enjoy their own Renaissance, preserving and improving upon classical astronomy and its allied sciences of mathematics and geometry for the eventual benefit of all the world's peoples.

Ptolemy's Era

In his Almagest, written around A.D. 150, Ptolemy wove his own ideas with strands from Plato, Aristotle, Hipparchus, and other Greek philosophers and astronomers. The resulting cosmological compendium not only was descriptive; it also had predictive powers. For example, at that time the Earth was viewed as being in the center of the cosmos, surrounded by the Moon, Sun, and planets, which revolved around it in large spherical orbits called deferents. In turn, Ptolemy pictured these heavenly bodies revolving in smaller orbits, called epicycles, each centered on a point that traveled along the deferent. While not necessarily intended to represent reality, these mathematical devices could be used quite effectively to predict a heavenly body's location in the sky at a given time.

The ability to foretell celestial events had practical applications in planting crops, observing religious festivals, and keeping time. But Ptolemy's model clearly also had philosophical significance, and it addressed the Greek urge to understand the cosmos and humanity's place in it.

In addition to his cosmographic theories, Ptolemy developed a catalog of 1,022 stars in 48 constellations named for mythological figures. This catalog identified individual stars by their locations within constellations ("on the end of the tail," for example) and listed their relative brightnesses as well as their ecliptic longitudes and latitudes. Ptolemy's constellations are still with us; in fact, they dominate today's star maps and planispheres.

As an Empire Declined

Much of Ptolemy's work disappeared in Europe over the centuries to follow. How did this happen? For one thing, political infighting, social decay, and invasions by Germanic tribes led to a gradual decline in the western part of the Roman Empire, centered on Rome itself. In A.D. 330, such pressures led Emperor Constantine the Great to move his capital to the eastern city of Byzantium on the Bosporus Strait, in what is now Turkey. (He immodestly renamed the new seat of his empire Constantinople.)

As the collapsing western Roman empire headed toward its final fall in the 5th century A.D., several factors contributed to the loss of classical astronomical knowledge. In his 1998 book, Astronomies and Cultures in Early Medieval Europe, University of West Virginia historian Stephen McCluskey explains that the Roman educational system increasingly was geared toward making civic leaders out of the sons of aristocrats. In astronomy, Ptolemy's mathematics was not emphasized as much as Aratus's poetry or Plato's philosophy. Mathematical astronomy did not meet the needs of people who were concerned with war and political survival, and scholars began to lose the technical skills needed to comprehend Greek theory. In addition, Latin translations of earlier Greek works were imperfect.

The Church and the Calendar

But other aspects of astronomy were preserved during the Middle Ages in the Latin-speaking West, primarily because of the Catholic Church's growing influence and its dependence on calendrical events. For example, it was important to establish when the solstices and equinoxes occurred because these events became associated with the conception and birth of Jesus Christ and John the Baptist. The Julian calendar became the basis for Christian rituals, since with it many religious holidays could be given fixed dates that were independent of celestial events. (Easter Sunday was an important exception, as the relevant biblical events were based not on specific calendar dates but on the Jewish Passover and its associated full Moon.)

Another concern of medieval astronomers was determining the time for monastic prayers. In addition, monks were involved in a number of feasts and ceremonies throughout the month, and it was important for them to keep track of their dates. Until about the 10th century A.D., when water clocks began to be used more commonly, the stars were the principal means of nocturnal timekeeping at most monasteries.

The poetic and philosophical aspects of astronomy also continued to be pursued throughout the Middle Ages. Religious overtones influenced cosmology, with the Earth remaining in the center of the cosmos according to God's divine laws. For this reason, the clergy admired Plato's notions of a divine creator and his geocentric emphasis. Another writer who influenced astronomical thought during the Middle Ages was Martianus Capella (c. A.D. 365-440) His popular textbook, written in Latin, used allegory and poetry to describe the seven liberal arts. In his astronomy section, Capella presented a model of the solar system, stemming from earlier Greek sources, that had Mercury and Venus orbiting the Sun while the Moon, the Sun, and the other planets orbited the Earth. Nicholas Copernicus later cited Capella when he developed his famous heliocentric model.

In the 8th and 9th centuries, the court of Charlemagne in Aachen attempted to systematize astronomical learning along religious lines. Schools were established for the clergy and for the children at court. Ancient texts were collected, copied, and disseminated, and newly written anthologies included solar phenomena, weather, computational tables, the structure of the heavens, and constellation descriptions. However, as McCluskey points out, these anthologies lacked the mathematical principles of spherical geometry (unlike Ptolemy's works); their tables of the heavenly bodies described only mean (average) motions and not variations therefrom; and they placed stars inaccurately within the constellations.

Islamic Influences

In contrast to the Latin West, classical Greek astronomical concepts were well known in Islamic lands. The Islamic religion was founded by the prophet Muhammad (A.D. 570-632), who was persecuted and driven out of his native Mecca but fled to Medina with his followers in A.D. 622. His teachings took hold, and through faith and warfare they rapidly spread throughout the Middle East, North Africa, and into Spain. In A.D. 762, Muhammad's successors founded a new capital, Baghdad, which soon became a center of astronomical learning as the Islamic empire expanded into Christian and northern Indian lands. Scholarly Muslims were exposed to classical manuscripts and translated many, including those of Ptolemy and other ancient astronomers, from the original Greek into Arabic.

Besides a desire for learning encouraged by enlightened caliphs, Muslims had astrological and religious reasons for pursuing astronomical knowledge. Such knowledge was helpful in locating the direction of Mecca for daily prayers and in precisely determining when to pray and to fast.

Advances were made in Ptolemaic theory and empirical astronomy alike at Islamic observatories. For example, Islamic astronomers developed a type of astronomical table called the zij listing quantities like the mean motions and true positions of the heavenly bodies as well as calendrical information related to the risings and settings of the Sun and Moon. These tables were based on Greek, Indian, and Islamic observations. Especially influential was the zij developed by the Baghdad astronomer and mathematician Muhammad Ibn Musa al-Khwarizmi around A.D. 840. As one of the earliest Arabic astronomical documents to be translated, it was to circulate widely in western Europe.

Islamic astronomers also refined the astrolabe, a calculating device originated by the Greeks that projected the heavens onto a metal plate, enabling them to predict positions of the heavenly bodies and to tell time by the stars. In the 10th century A.D., the Baghdad astronomer Abd Al-Rahman al-Sufi (A.D. 903-986) integrated Ptolemy's star catalog with Arab traditions, and in his Book of the Fixed Stars he presented detailed constellation boundaries as well as Arabic star names that were later incorporated into the Greek system used today (Fomalhaut, Algol, and Aldebaran are but three familiar examples).

Three centuries later, the influential Persian astronomer and mathematician Nasir Al-Din al-Tusi (A.D. 1201-1274) critiqued Ptolemy's system and developed new geometric planetary models of his own. He also founded the great Maragha Observatory, whose foundations still survive some 80 kilometers south of Tabriz in what is now northwestern Iran.

One of al-Tusi's most influential accomplishments lay in the area of planetary orbital theory. He noted that if a circle rolls inside the circumference of another circle twice as large, then any point on the inner circle would move back and forth along a straight line. This "Tusi couple" theorem could be proven geometrically, in the spirit of Ptolemy, and could be illustrated visually to create a model of planetary motion. Models incorporating versions of the Tusi couple appeared in later Byzantine manuscripts, and Copernicus made use of its principles when discussing variations in precession (the motion of the Earth's axis around the ecliptic pole), determining ecliptic latitudes for the planets, and describing Mercury's orbit.

From the 11th to the 13th century A.D., much of Spain was taken back from the Moors, Islamic invaders originally from Africa. The victors were Christians from independent kingdoms, such as Castile, to the north. In the process, Greek and Islamic astronomical knowledge was brought into western Europe. By the 11th century A.D. European scholars possessed astrolabes and were teaching others how to use them. In the following century a number of classical works were translated from Arabic into Latin, among them Euclid's Elements of Geometry and al-Khwarizmi's zij. The 12th century also would see a complete Latin translation of Ptolemy's Almagest from the Arabic, though it was very literal and hard to follow.

The stage was set for Latin astronomy to move from the poetic and philosophical to the precise and mathematical. However, this process was slow and incomplete. The main centers of learning were the new universities, which gained prominence as towns grew and cathedral schools became increasingly secular. But astronomy remained a liberal art more than a mathematical science, and for several more centuries it was to remain dominated by the Aristotelian concept of heavenly bodies moving around the central Earth in perfect, unchanging, concentric, crystalline spheres made from the ether.

The Byzantine Connection

Even less widely known than the Islamic impact on European astronomy is that of the other great repository of classical Greek learning: the Greek-speaking Byzantine Empire, especially its capital city of Constantinople. Founded by Greeks in the 7th century B.C. under its original name of Byzantium, Constantinople became the capital of the entire Roman Empire under Constantine the Great, and it remained the capital of the empire's eastern realm when Rome fell.

As the principal city of what later would be called the Byzantine Empire, Constantinople became an important strategic, trade, and cultural center (a position it holds to this day as Istanbul, Turkey). There a number of classical works were preserved and discussed in their native Greek. Islamic leaders sent envoys to purchase many of these, and they were translated into Arabic in the 8th and 9th centuries.

In addition, evidence suggests that Byzantine scholars not only were well versed in the mathematical astronomy of Ptolemy and Islamic writers and taught it in their universities; they also conceptually advanced the classical theories with new elements of their own. For example, Emmanuel Paschos (University of Dortmund, Germany) and Panagiotis Sotiroudis (University of Thessaloniki, Greece) recently have translated and analyzed a 13th-century Byzantine manuscript, The Schemata of the Stars, which had been uncovered from the Vatican Library in Rome more than 30 years earlier by the late astronomy historian Otto Neugebauer. Paschos and Sotiroudis attribute the Schemata to Gregory Chioniades (c. A.D. 1240-1320), a professor of medicine and astronomy in Constantinople who studied in Persia and later became Bishop of Tabriz.

The Schemata listed and illustrated the constellations and their constituent stars; the mechanisms of lunar and solar eclipses; and the uses of epicycles, deferents, and eccentric (off-center) orbits to describe the motions of heavenly bodies around the Earth. The work shows that Chioniades and his contemporaries knew spherical geometry and trigonometry, and that they were influenced not only by Ptolemy but also by al-Tusi and other Arabic and Persian scholars.

The Schemata also contains a number of variations and improvements upon these earlier works, including an epicyclic model for the Sun's orbit around the Earth (Ptolemy had favored a simpler eccentric approach); a new model, with eccentric orbits, for the revolutions of the superior planets; and improvements in the trajectory of Mercury's epicycle.

Paschos and Sotiroudis explain that the Schemata made its way to Italy, possibly in the 15th century A.D. There it may have influenced Copernicus, who had learned Greek and studied church law, medicine, and astronomy in several Italian cities.

Other evidence suggests that Byzantine documents made their way into Europe through Italy. In her 1998 book, Worldly Goods: A New History of the Renaissance, University of London English professor Lisa Jardine recounts that on February 8, 1438, Byzantine Emperor John VIII, Eastern Orthodox Patriarch Joseph II, and an entourage of some 700 bishops, monks, and learned laymen arrived in Florence, where the court of Pope Eugenius IV then was located. The meeting had been called to reconcile the Roman Catholic and Eastern Orthodox churches. The Byzantines brought a number of books and texts in the original Greek, including the works of Plato, Aristotle, Euclid, and Ptolemy. While the leaders continued to haggle over church doctrine and negotiate (unsuccessfully) the merger of their two churches, the intellectual experts on both the Byzantine and Latin sides exchanged philosophical and mathematical ideas. Jardine emphasizes the importance of this contact:

It was books written in Greek [that] most impressed the scholars

in Florence. The inability of monastic copyists to transcribe

the unfamiliar alphabet of Greek script, and the difficulty in

learning classical Greek anywhere in the West had, for instance,

cut the intellectual tradition off from the work of the great

Greek mathematicians and geometers--Euclid, Apollonius,

Pappus, Ptolemy.

She further notes that such books--along with lectures given by Greek scholars during this meeting--contributed to the vogue for Greek learning in Italy, and that they led the wealthy Florentine patron of the arts, Cosimo de Medici, to found his Platonic Academy.

Around A.D. 1453, when Constantinople fell to the Ottoman Turks, a number of Byzantine scholars moved to Italy, bringing with them their personal libraries of rare Greek books. Venice contained so many such emigres that the Greek scholar and immigrant Cardinal Bessarion likened the city to another Byzantium, and in A.D. 1468 he donated his magnificent collection of more than 600 books and manuscripts (which included mathematical works by Archimedes, Apollonius, and Ptolemy) to St. Mark's Cathedral.

Astronomy's Western Rebirth

A number of factors influenced the rebirth and advancement of classical astronomy, especially its more mathematical and scientific aspects, in Europe's Renaissance. First, adequate translations of classical Greek texts became freely available in Latin; in many cases these were translated directly by native Greek speakers from Constantinople. Second, these translations included valuable commentaries and additions by Islamic and Byzantine scholars. Third, the spirit of the Renaissance encouraged the advancement of knowledge for its own sake and not solely to address religious needs. Fourth, secular universities had become well established, and they were increasingly inclined to transmit new scientific information to their students.

But how could the reemerging astronomical ideas reach people who were not involved with universities? This revolutionary fifth factor in the rebirth of classical astronomical learning was the mid-1400s development of printing using movable type. By the late 1470s, Erhard Ratdolt was publishing scientific books in Venice, complete with woodcut illustrations. Between 1495 and 1498, the Venetian printer Aldo Mannucci issued Aristotle's complete works, and his Aldine Press continued to publish high-quality books by classical writers. Another center of printing was Nuremberg, where in 1493 Hartmann Schedel wrote the Nuremberg Chronicle, an influential geographical text and world history with woodcut illustrations of important people, places, and events.

Although printed books were relatively expensive initially, they found readers among people with commercial interests such as shipbuilding and navigation and in aristocratic families. In fact, Jardine states, having a great library became an important status symbol among powerful Renaissance men. These new collectors competed for rare books, stimulating the book trade and motivating more translations and printings of old masterpieces. Gradually, new markets opened up for more affordable textbooks in schools and universities, where the less privileged were exposed to the works. Some of these books included volvelles, movable attachments to book pages that could be used to perform astronomical calculations. Some volvelles provided simpler but affordable alternatives to astrolabes made of metal.

Thus by the time of the Renaissance, classical Greek astronomy had returned to western Europe from Islamic and Byzantine sources, at times substantially improved. The secular universities, the availability of printed books, and the humanism of the times all set the stage for new mathematical and observational developments in astronomy, beginning with Nicholas Copernicus and progressing through Tycho Brahe to Johannes Kepler and Galileo Galilei.

In a parallel manner, classical Greek descriptions of the constellations could be illustrated on paper in early celestial works by Albrecht Durer, Alessandro Piccolomini, and Giovanni Gallucci, and later in the great star atlases of Johann Bayer, Johannes Hevelius, John Flamsteed, and Johann Bode. Over the quiet lake of the Middle Ages, Islamic and Byzantine scholars had built a great bridge connecting Ptolemy's mathematical astronomy to the great Renaissance thinkers, and the resulting dynamic flow of knowledge continues to this day.

NICK KANAS ( is a professor of psychiatry at the University of California, San Francisco, and the Veterans Administration Hospital, where he studies psychosocial issues affecting astronauts. A member of the San Francisco Amateur Astronomers, he has collected antiquarian celestial books, atlases, and prints for more than two decades.
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Author:Kanas, Nick
Publication:Sky & Telescope
Geographic Code:00WOR
Date:Jan 1, 2003
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