50 January 2003 Sky & Telescope

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 as-

tronomer, 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 im-

proving upon classical astronomy and its allied sciences of

mathematics and geometry for the eventual benefit of all

the world’s peoples.

Sky & Telescope January 2003 51

Alexandria

MediterraneanSea

Ptolemy’s Legacy. This print from Andreas Cellarius’s Atlas Coelestis seu Harmonia Macrocos-

mica (1660) shows the Ptolemaic view of the Moon’s motion: our natural satellite revolves clockwise

in its epicycle (blue-filled circle) as it moves counterclockwise along its deferent orbit. To better ac-

count for the Moon’s location in the sky, the deferent’s center revolves clockwise around the Earth

(at the very center) along the innermost circle. Unfortunately, this embellishment “pulls” the Moon

Earthward at first and last quarter, erroneously increasing its apparent diameter.

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Ptolemy’s EraIn his Almagest, written around A.D. 150, Ptolemy wove hisown ideas with strands from Plato, Aristotle, Hipparchus, andother Greek philosophers and astronomers. The resulting cos-mological compendium not only was descriptive; it also hadpredictive powers. For example, at that time the Earth wasviewed as being in the center of the cosmos, surrounded bythe Moon, Sun, and planets, which revolved around it in largespherical orbits called deferents. In turn, Ptolemy picturedthese heavenly bodies revolving in smaller orbits, called epi-cycles, each centered on a point that traveled along the defer-ent. While not necessarily intended to represent reality, thesemathematical devices could be used quite effectively to predicta heavenly body’s location in the sky at a given time.

The ability to foretell celestial events had practical applica-tions in planting crops, observing religious festivals, and keep-ing time. But Ptolemy’s model clearly also had philosophicalsignificance, and it addressed the Greek urge to understand thecosmos and humanity’s place in it.

In addition to his cosmographic theories, Ptolemy devel-oped a catalog of 1,022 stars in 48 constellations named formythological 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 astheir ecliptic longitudes and latitudes. Ptolemy’s constellationsare still with us; in fact, they dominate today’s star maps andplanispheres.

As an Empire DeclinedMuch of Ptolemy’s work disappeared in Europe over the cen-turies to follow. How did this happen? For one thing, politicalinfighting, social decay, and invasions by Germanic tribes ledto a gradual decline in the western part of the Roman Empire,centered on Rome itself. In A.D. 330, such pressures led Em-peror Constantine the Great to move his capital to the easterncity of Byzantium on the Bosporus Strait, in what is nowTurkey. (He immodestly renamed the new seat of his empireConstantinople.)

As the collapsing western Roman empire headed toward itsfinal fall in the 5th century A.D., several factors contributed tothe loss of classical astronomical knowledge. In his 1998book, Astronomies and Cultures in Early Medieval Europe, Uni-versity of West Virginia historian Stephen McCluskey explainsthat the Roman educational system increasingly was geared

52 January 2003 Sky & Telescope

The Mythological Sky.Within this first edition of Johann

Schaubach’s Eratosthenis’ Catasterismi

(1795) lies this foldout chart of Ptole-

my’s Northern Hemisphere constella-

tions. Schaubach’s was the first Latin

translation, from the Greek, summa-

rizing the mythology of the constella-

tions as described by Eratosthenes

(c. 284–192 B.C.).

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The Church and the CalendarBut other aspects of astronomy were preserved dur-ing the Middle Ages in the Latin-speaking West, pri-marily because of the Catholic Church’s growing in-fluence and its dependence on calendrical events.For example, it was important to establish whenthe solstices and equinoxes occurred because theseevents became associated with the conceptionand birth of Jesus Christ and John the Baptist.The Julian calendar became the basis for Christ-ian rituals, since with it many religious holidayscould be given fixed dates that were independ-ent of celestial events. (Easter Sunday was animportant exception, as the relevant biblicalevents were based not on specific calendardates but on the Jewish Passover and its as-sociated full Moon.)

Another concern of medieval astronomerswas determining the time for monastic prayers. In

addition, monks were involved in a number of feasts and cere-monies throughout the month, and it was important for them tokeep track of their dates. Until about the 10th century A.D., whenwater clocks began to be used more commonly, the stars were theprincipal means of nocturnal timekeeping at most monasteries.

The poetic and philosophical aspects of astronomy also con-tinued to be pursued throughout the Middle Ages. Religiousovertones influenced cosmology, with the Earth remaining in

Sky & Telescope January 2003 53

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Alexandria

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ME D I T E R R A N E A N S E A

Counting Out the Days. This calendar leaf for October is from a

Book of Hours handwritten by an un-

known scribe on vellum (calfskin)

around 1460, probably in a French or

Low Countries monastery. It depicts

saints’ days, feasts, dominical letters,

and other calendrical information.“KL”

denotes “Kalendarium,” and the top

two lines inform the user that October

has 31 calendar and 30 lunar days.

(The remaining days for October are

listed on the other side of the page.)

Countdown to Easter. Calendrical — and thus ultimately as-

tronomical — knowledge was essential to observing Christian holi-

days. The outer circles, with letters from A to G, represent the days of

the week on this printed calculating device, or volvelle, here seen in

the Spanish edition of Giovanni Galluci’s Theatro del Mundo y del Tiem-

po, first published in Venice in 1588. On any given year the inner

pointer could be set to an important date (January 1, say), and if one

knew the year’s dominical letter one could determine the number of

Sundays until Easter.

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NICK AND CAROLYNN KANAS COLLECTION

toward making civic leaders out of the sons of aristocrats. Inastronomy, Ptolemy’s mathematics was not emphasized asmuch as Aratus’s poetry or Plato’s philosophy. Mathematicalastronomy did not meet the needs of people who were con-cerned with war and political survival, and scholars began tolose the technical skills needed to comprehend Greek theory.In addition, Latin translations of earlier Greek works wereimperfect.

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principles of spherical geometry (unlike Ptolemy’s works);their tables of the heavenly bodies described only mean (aver-age) motions and not variations therefrom; and they placedstars inaccurately within the constellations.

Islamic InfluencesIn contrast to the Latin West, classical Greek astronomicalconcepts were well known in Islamic lands. The Islamic reli-gion was founded by the prophetMuhammad (A.D. 570–632), whowas persecuted and driven out of hisnative Mecca but fled to Medinawith his followers in A.D. 622. Histeachings took hold, and throughfaith and warfare they rapidly spreadthroughout the Middle East, NorthAfrica, and into Spain. In A.D. 762,Muhammad’s successors founded anew capital, Baghdad, which soon

54 January 2003 Sky & Telescope

the center of the cosmos according to God’s divine laws. For thisreason, the clergy admired Plato’s notions of a divine creatorand his geocentric emphasis. Another writer who influenced as-tronomical thought during the Middle Ages was MartianusCapella (c. A.D. 365–440) His popular textbook, written inLatin, used allegory and poetry to describe the seven liberal arts.In his astronomy section, Capella presented a model of the solarsystem, stemming from earlier Greek sources, that had Mercuryand Venus orbiting the Sun while the Moon, the Sun, and theother planets orbited the Earth. Nicholas Copernicus later citedCapella when he developed his famous heliocentric model.

In the 8th and 9th centuries, the court of Charlemagne inAachen attempted to systematize astronomical learning alongreligious lines. Schools were established for the clergy and forthe children at court. Ancient texts were collected, copied, anddisseminated, and newly written anthologies included solarphenomena, weather, computational tables, the structure ofthe heavens, and constellation descriptions. However, as Mc-Cluskey points out, these anthologies lacked the mathematical

Evolving World Views. This print, from an atlas by Jean-Baptiste Nolin (c. A.D. 1750), shows an armillary sphere (center) surrounded

by four models for the solar system: the Sun-centered Copernican model (upper left), the Earth-centered Ptolemaic model (upper right), and

two hybrids (Tycho’s, lower left; and Capella’s, lower right).

Baghdad

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became a center of astronomical learning as the Islamic em-pire expanded into Christian and northern Indian lands.Scholarly Muslims were exposed to classical manuscripts andtranslated many, including those of Ptolemy and other ancientastronomers, from the original Greek into Arabic.

Besides a desire for learning encouraged by enlightenedcaliphs, Muslims had astrological and religious reasons forpursuing astronomical knowledge. Such knowledge was help-ful in locating the direction of Mecca for daily prayers and inprecisely determining when to pray and to fast.

Advances were made in Ptolemaic theory and empirical as-tronomy alike at Islamic observatories. For exam-ple, Islamic astronomers developed a type of astro-nomical table called the zij listing quantities likethe mean motions and true positions of the heav-enly bodies as well as calendrical information relat-ed to the risings and settings of the Sun and Moon.These tables were based on Greek, Indian, and Is-lamic observations. Especially influential was the zijdeveloped by the Baghdad astronomer and math-ematician Muhammad Ibn Musa al-Khwarizmiaround A.D. 840. As one of the earliest Arabic as-tronomical documents to be translated, it was tocirculate widely in western Europe.

Islamic astronomers also refined the astrolabe, acalculating device originated by the Greeks thatprojected the heavens onto a metal plate, enablingthem to predict positions of the heavenly bodiesand to tell time by the stars. In the 10th centuryA.D., the Baghdad astronomer Abd Al-Rahman al-Sufi (A.D. 903–986) integrated Ptolemy’s star cata-log with Arab traditions, and in his Book of theFixed Stars he presented detailed constellationboundaries as well as Arabic star names that werelater incorporated into the Greek system usedtoday (Fomalhaut, Algol, and Aldebaran are butthree familiar examples).

Three centuries later, the influential Persian as-tronomer and mathematician Nasir Al-Din al-Tusi(A.D. 1201–1274) critiqued Ptolemy’s system anddeveloped new geometric planetary models of his

own. He also founded the great Maragha Observatory, whosefoundations still survive some 80 kilometers south of Tabriz inwhat is now northwestern Iran.

One of al-Tusi’s most influential accomplishments lay in thearea of planetary orbital theory. He noted that if a circle rollsinside the circumference of another circle twice as large, thenany point on the inner circle would move back and forth alonga straight line. This “Tusi couple” theorem could be provengeometrically, in the spirit of Ptolemy, and could be illustratedvisually to create a model of planetary motion. Models incor-porating versions of the Tusi couple appeared in later Byzan-

Sky & Telescope January 2003 55

Islamic Astronomers at Work. This hand-

painted reproduction depicts a piece of art entitled “As-

tronomers in their Observatory in Galatasaray,” which ap-

pears in the Ottoman Turkish manuscript Sehinsehname,

or “Book of Kings” (1581). The astronomer standing at the

bottom is using a plumb bob to adjust the meridian circle

in his observatory’s huge armillary sphere.

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tine manuscripts, and Copernicus made use of its principleswhen discussing variations in precession (the motion of theEarth’s axis around the ecliptic pole), determining ecliptic lati-tudes for the planets, and describing Mercury’s orbit.

From the 11th to the 13th century A.D., much of Spain wastaken back from the Moors, Islamic invaders originally fromAfrica. The victors were Christians from independent king-doms, such as Castile, to the north. In the process, Greek andIslamic astronomical knowledge was brought into western Eu-rope. By the 11th century A.D. European scholars possessedastrolabes and were teaching others how to use them. In thefollowing century a number of classical works were translatedfrom Arabic into Latin, among them Euclid’s Elements ofGeometry and al-Khwarizmi’s zij. The 12th century also wouldsee a complete Latin translation of Ptolemy’s Almagest fromthe Arabic, though it was very literal and hard to follow.

The stage was set for Latin astronomy to move from the po-etic and philosophical to the precise and mathematical. How-ever, this process was slow and incomplete. The main centersof learning were the new universities, which gained promi-nence as towns grew and cathedral schools became increasing-ly secular. But astronomy remained a liberal art more than amathematical science, and for several more centuries it was toremain dominated by the Aristotelian concept of heavenlybodies moving around the central Earth in perfect, unchang-ing, concentric, crystalline spheres made from the ether.

The Byzantine ConnectionEven less widely known than theIslamic impact on European as-tronomy is that of the other greatrepository of classical Greek learn-ing: the Greek-speaking ByzantineEmpire, especially its capital cityof Constantinople. Founded byGreeks in the 7th century B.C.under its original name of Byzan-tium, Constantinople became thecapital of the entire Roman Em-

pire under Constantine the Great, and it remained the capital ofthe empire’s eastern realm when Rome fell.

As the principal city of what later would be called theByzantine Empire, Constantinople became an importantstrategic, trade, and cultural center (a position it holds to thisday as Istanbul, Turkey). There a number of classical works

were preserved and discussed in their native Greek. Islamicleaders sent envoys to purchase many of these, and they weretranslated into Arabic in the 8th and 9th centuries.

In addition, evidence suggests that Byzantine scholars notonly were well versed in the mathematical astronomy of Ptol-emy and Islamic writers and taught it in their universities; theyalso conceptually advanced the classical theories with new ele-ments of their own. For example, Emmanuel Paschos (Univer-sity of Dortmund, Germany) and Panagiotis Sotiroudis (Uni-versity of Thessaloniki, Greece) recently have translated andanalyzed a 13th-century Byzantine manuscript, The Schemata ofthe Stars, which had been uncovered from the Vatican Libraryin Rome more than 30 years earlier by the late astronomy his-torian Otto Neugebauer. Paschos and Sotiroudis attribute theSchemata to Gregory Chioniades (c. A.D. 1240–1320), a profes-sor of medicine and astronomy in Constantinople who studiedin Persia and later became Bishop of Tabriz.

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

The Schemata also contains a number of variations and im-

56 January 2003 Sky & Telescope

Epicycles Updated. This photograph shows a page from The

Schemata of the Stars, a recently translated 13th-century Byzantine

manuscript. The lower figure shows how the two circles constituting a

Tusi couple can generate back-and-forth motion along a straight line.

The upper figure places a Tusi couple upon a deferent to describe the

Sun’s motion around the Earth; the resulting solar motion is equiva-

lent to the eccentric (off-center) trajectory that Ptolemy had postulat-

ed centuries earlier.

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provements upon these earlier works, including an epicyclicmodel for the Sun’s orbit around the Earth (Ptolemy had fa-vored a simpler eccentric approach); a new model, with eccen-tric orbits, for the revolutions of the superior planets; and im-provements in the trajectory of Mercury’s epicycle.

Paschos and Sotiroudis explain that the Schemata made itsway to Italy, possibly in the 15th century A.D. There it may haveinfluenced Copernicus, who had learned Greek and studiedchurch law, medicine, and astronomy in several Italian cities.

Other evidence suggests that Byzantine documents madetheir way into Europe through Italy. In her 1998 book, WorldlyGoods: A New History of the Renaissance, University of LondonEnglish professor Lisa Jardine recounts that on February 8,1438, Byzantine Emperor John VIII, Eastern Orthodox Patri-arch Joseph II, and an entourage of some 700 bishops, monks,and learned laymen arrived in Florence, where the court ofPope Eugenius IV then was located. The meeting had beencalled to reconcile the Roman Catholic and Eastern Orthodoxchurches. The Byzantines brought a number of books andtexts in the original Greek, including the works of Plato, Aris-totle, Euclid, and Ptolemy. While the leaders continued to hag-gle over church doctrine and negotiate (unsuccessfully) themerger of their two churches, the intellectual experts on boththe Byzantine and Latin sides exchanged philosophical andmathematical ideas. Jardine emphasizes the importance of thiscontact:

It was books written in Greek [that] most impressed the schol-ars in Florence. The inability of monastic copyists to transcribethe unfamiliar alphabet of Greek script, and the difficulty inlearning classical Greek anywhere in the West had, for instance,cut the intellectual tradition off from the work of the greatGreek mathematicians and geometers — Euclid, Apollonius,Pappus, Ptolemy.

She further notes that such books — along with lecturesgiven by Greek scholars during this meeting — contributed tothe vogue for Greek learning in Italy, and that they led thewealthy Florentine patron of the arts, Cosimo de Medici, tofound his Platonic Academy.

Around A.D. 1453, when Constantinople fell to the Otto-man Turks, a number of Byzantine scholars moved to Italy,bringing with them their personal libraries of rare Greekbooks. Venice contained so many such émigrés that the Greekscholar and immigrant Cardinal Bessarion likened the city toanother Byzantium, and in A.D. 1468 he donated his magnifi-cent collection of more than 600 books and manuscripts(which included mathematical works by Archimedes, Apol-lonius, and Ptolemy) to St. Mark’s Cathedral.

Astronomy’s Western RebirthA number of factors influenced the rebirth and advancement ofclassical astronomy, especially its more mathematical and scien-tific aspects, in Europe’s Renaissance. First, adequate translationsof classical Greek texts became freely available in Latin; in manycases these were translated directly by native Greek speakers fromConstantinople. Second, these translations included valuablecommentaries and additions by Islamic and Byzantine scholars.Third, the spirit of the Renaissance encouraged the advancementof knowledge for its own sake and not solely to address religiousneeds. Fourth, secular universities had become well established,and they were increasingly inclined to transmit new scientific in-formation to their students.

But how could the reemerging as-tronomical ideas reach people whowere not involved with universities?This revolutionary fifth factor in therebirth of classical astronomical learn-ing was the mid-1400s developmentof printing using movable type. By thelate 1470s, Erhard Ratdolt was pub-lishing scientific books in Venice,complete with woodcut illustrations.Between 1495 and 1498, the Venetianprinter Aldo Mannucci issued Aristo-tle’s complete works, and his AldinePress continued to publish high-

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Astronomical Publication Ascending. This page from

Scriptores Astronomici Veteres, an A.D. 1499 book from Venice’s Aldine

Press, describes (in Latin) the now-defunct constellation Argo Navis by

quoting the Greek poem Phaenomena, which Aratus (c. 315–240 B.C.)

had written nearly two millennia previously. The figure comes from one

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quality books by classical writers.Another center of printing wasNuremberg, where in 1493 Hart-mann Schedel wrote the NurembergChronicle, an influential geographicaltext and world history with woodcutillustrations of important people,places, and events.

Although printed books were rel-atively expensive initially, they found readers among peoplewith commercial interests such as shipbuilding and navigationand in aristocratic families. In fact, Jardine states, having agreat library became an important status symbol among pow-erful Renaissance men. These new collectors competed for rarebooks, stimulating the book trade and motivating more trans-lations and printings of old masterpieces. Gradually, new mar-kets opened up for more affordable textbooks in schools anduniversities, where the less privileged were exposed to theworks. Some of these books included volvelles, movable attach-ments to book pages that could be used to perform astronom-ical calculations. Some volvelles provided simpler but afford-able alternatives to astrolabes made of metal.

Thus by the time of the Renaissance, classical Greek astron-omy had returned to western Europe from Islamic and Byzan-

tine sources, at times substantially improved. The secular uni-versities, the availability of printed books, and the humanismof the times all set the stage for new mathematical and obser-vational developments in astronomy, beginning with NicholasCopernicus and progressing through Tycho Brahe to JohannesKepler and Galileo Galilei.

In a parallel manner, classical Greek descriptions of the con-stellations could be illustrated on paper in early celestial worksby Albrecht Dürer, Alessandro Piccolomini, and Giovanni Gal-lucci, and later in the great star atlases of Johann Bayer, Jo-hannes Hevelius, John Flamsteed, and Johann Bode. Over thequiet lake of the Middle Ages, Islamic and Byzantine scholarshad built a great bridge connecting Ptolemy’s mathematical as-tronomy to the great Renaissance thinkers, and the resultingdynamic flow of knowledge continues to this day.

Nick Kanas (nick21@itsa.ucsf.edu) is a professor of psychiatry at theUniversity of California, San Francisco, and the Veterans Administra-tion Hospital, where he studies psychosocial issues affecting astronauts.A member of the San Francisco Amateur Astronomers, he has collectedantiquarian celestial books, atlases, and prints for more than twodecades.

An Affordable Astrolabe. This volvelle (complete with its

calculating string) is from Peter Apianus’s Cosmographia (1533), and it

contains a number of features found on an astrolabe, including zodiac

and month circles, unequal hour lines, and a shadow square that uses

trigonometric methods to determine the height of a distant object.

Nuremberg

Awakening Astronomical Awareness. In this 1493

Latin edition of Hartmann Schedel’s Nuremberg Chronicle, 9th-century

clerical leaders share the page with a supposedly calamitous comet.

As the comet is described in the paragraph covering the years A.D.

804–813, it is probably not Halley’s Comet, which appeared in 837.

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