Georg Saliba-Greek Astronomy and the Medieval Arabic Tradition The medieval Islamic astronomers werenot merely translators. They may also have played a key role in the Copernican revolution.pdf

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Greek Astronomy and the Medieval Arabic Tradition: The medieval Islamic astronomers werenot merely translators. They may also have played a key role in the Copernican revolutionAuthor(s): George SalibaSource: American Scientist, Vol. 90, No. 4 (JULY-AUGUST 2002), pp. 360-367Published by: Sigma Xi, The Scientific Research SocietyStable URL: http://www.jstor.org/stable/27857695 .Accessed: 30/10/2014 11:41

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Greek Astronomy and the Medieval

Arabic Tradition

The medieval Islamic astronomers were not merely translators. They may also

have played a key role in the Copernican revolution

George Saliba

In 1957, two brilliant historians met to

consider an archaic manuscript writ

ten by a 14th-century Arab astronomer. The document they held in their hands was almost completely unknown to most historians of science, who would have considered it to be unbelievable had they known of its existence. It had been written by a timekeeper of the central Umayyad Mosque in Damas cus, a religious man by the name of Ibn al-Sh?tir. What was remarkable about the timekeeper's text was that it antici

pated some of the ideas of Copernicus more than 100 years before the Polish astronomer was born.

When the two historians first pre sented the writings of Ibn al-Sh?tir to their colleagues, they were greeted

with responses that ranged from total disbelief to total denial?some histori ans even walked out of public lectures when the manuscript was mentioned. The troubling thing about the discov ery was that it suggested that Coperni cus might not have come up with his ideas by himself. Some historians took

refuge in the notion that Copernicus and Ibn al-Sh?tir had arrived at the same place independently, and so this

was merely another instance of "inde

pendent discovery." In time, however, it became clear that there was a hidden connection between the works of

George Saliba is professor of Arabic and Islamic sci ence at Columbia University. He wor/cs on the his

tory of Arabic science with a special emphasis on the

development of planetary theories and their trans mission to Renaissance Europe. Address: 604 Kent

Hall Columbia University, 1140 Amsterdam Av

enue, MC 3942, New York, NY 10027. Internet:

[email protected]

Copernicus and those of medieval Is lamic astronomers. The acknowledg

ment of Ibn al-Sh?tir's work also meant that at some level the Renaissance?

which was at least partly inspired by the Copernican revolution?was not a

purely European creation. All of this posed a challenge to what

is at best a caricature of history, one that portrays the "torch" of science and knowledge as something that was handed down from the ancient Greeks to medieval Europe by way of Islamic scholars. In this view, the go-betweens

were seen as mere scribes, faithfully translating and preserving the ancient texts of Greek astronomy, philosophy and medicine until such time as Eu

rope would reawaken from its dark ness, pick up the books, and once

again carry the light. It is now evident that such a simple notion of history

miscasts the role of the Islamic civi lization in the scientific revolution and underestimates the often deep relation between different cultures and intel lectual movements.

So what else did the Islamic as tronomers do? And how much of what

they knew made its way to Europe but was not generally acknowledged as such? Assembling the scattered pieces of history is no easy thing. There are lit

erally thousands of Arabic scientific texts in major libraries around the

world, from Dublin in Ireland to Madras in southern India?many of them, like Ibn al-Sh?tir's manuscript, unknown to scholars. What historians have now pieced together suggests that unlike the traditional view, in which medieval Islamic civilization accepted the body of Greek astronomical thought

as the unalloyed truth, the Islamic as tronomers found much that was objec tionable and themselves attempted to

forge a new astronomy. Some of these new ideas were later adopted by Coper nicus, who launched his own revolution

against Greek astronomy.

The Trouble with Ptolemy When one thinks of ancient Greek as

tronomy, the name of Ptolemy usually comes to mind. In the second century a.D. Ptolemy codified the astronomical

knowledge of his time in three influen tial works, the Almagest, the Planetary

Hypothesis and the Handy Tables. Seven centuries passed before Ptolemy's writ

ings reached the Islamic world, and there appears to have been relatively little development of the science dur

ing the intervening period. The newly found interest among the

ninth-century Islamic scholars was at least partly attributable to the rapid spread of Islam. The recently acquired territories created administrative needs of an unprecedented scale, which in turn required sophisticated levels of science and technology. Ptolemy's

writings could offer the Islamic people geometry they could use to predict the motions of the sun, the moon and the

planets against the "fixed" stars. These were useful tools needed to maintain

among other things an accurate lunar calendar, determine the timings of their

daily prayers and, of course, ascertain the sacred orientation (the qibla) to

ward Mecca from a distant land (see Science Observer, May-June 2001).

All of this resulted in a massive translation effort that took place in var ious courts under various patrons in

360 American Scientist Volume 90


Figure 1. Model for the moon's motion across the sky proposed by the 14th-century Islamic astronomer Ibn al-Sh?tir is identical to one proposed by Copernicus nearly two centuries later. Copernicus did not credit Ibn al-Sh?tir with the origin of this model, nor did he acknowledge other Islamic as tronomers who anticipated some of his ideas for the motions of the planets in his heliocentric cosmology. The author discusses the contributions of me dieval Islamic astronomers and whether Copernicus might have known about their work. Here the moon's sphere resides on the rotating circumfer ence of the smallest circle. It is shown at eight positions along its orbital path around the Earth. Ibn al-Sh?tir's model improved considerably on

descriptions of the moon's motion by the second-century Greek astronomer Ptolemy.

2002 July-August 361


11

?%t3w^*^S!3*?i^?i^ii .^^^i&i?'SWftisjMJii? f?ll

Figure 2. Arabie translations of Ptolemy's Almagest were heavily annotated, as evidenced by the commentaries in the margins of these pages. All of Ptolemy's writings were critically con sidered beginning almost immediately upon their introduction to the Islamic world in the ninth century. (Courtesy of The British Museum.)

Figure 3. Earth's orbital motion around the sun produces illusions that must be explained in a

geocentric universe. In the course of a year, the sun appears to move through the sky along a

path?the ecliptic?that passes through the 12 constellations of the zodiac (white figures). The two solstices mark the times of the year when the Earth's axis is tilted most directly toward or

away from the sun. The equinoxes mark the two points where the celestial equator (the pro jection of Earth's equatorial plane into space) intersects the ecliptic. Ancient Greek and me dieval Islamic astronomers attempted to explain these phenomena with models that assumed a geocentric universe. Ptolemy's model provided reasonable predictions for these celestial

events, but the Islamic astronomers improved on his work significantly.

eluding the most famous House of Wisdom, in Baghdad, which was ap parently established for such a pur pose. In due course some of these Arab translations made their way back to

Europe through Spain and were then

eventually translated to Latin for use

by European scholars. Although some of the original Greek manuscripts (the Almagest, for example) survived to the later Middle Ages, others would have been lost were it not for the Arabic translations. It was an important act of conservation, but from the very begin ning the Arabic translators did more than provide verbatim translations?

they took it upon themselves to com ment on the text and to correct errors

(Figure 2). Some of these were trivial, such as the faulty statements of the Greek texts, but other mistakes raised some eyebrows.

Among these were some errors that

required correction if the work was to be used for practical matters. This in cluded such things as the rate of preces sion. The Earth wobbles on its axis over time, and this can be detected by the

movement of the ecliptic?the apparent path of the sun relative to the back

ground stars (Figure 3). Over the course of 700 years, Ptolemy's work predicted a change of some seven degrees, where as the Baghdad astronomers measured a change on the order of 10 or 11 de grees. Ptolemy also didn't get an accu rate read on the Earth's inclination, the tilt of its equatorial plane relative to the

ecliptic. The ninth-century astronomers measured a value that is much closer to what we now accept. Finally, Ptolemy determined a fixed position for the "so lar apogee"?the sun's greatest distance from the Earth in its "geocentric orbit." (Recall, of course, that Ptolemy and the

medieval Arabs assumed a geocentric universe.) But the Islamic astronomers observed that the solar apogee had in fact moved by about 10 degrees over the course of seven centuries and that its motion was more or less similar to the motion of precession.

As a practical matter, those later as tronomers handled these mistakes by creating their own astronomical tables

(ephemerides) that could satisfy their

daily concerns. However, the recogni tion that Ptolemy had made such fun damental errors set the tone for even

greater questions about the astronomi cal legacy of the Greeks. Were Ptole

my's instruments the reason for his mistakes? Or were his methods of ob

362 American Scientist, Volume 90


Ptolemy's eccentric solar model Ptolemy's epicyclic solar model

Figure 4. Ptolemy proposed two geocentric models to explain the sun's motion, which appeared to slow down during part of the year, suggest ing that it was farther from the Earth. His "eccentric" model (left) assumed that the center of the Earth did not coincide with the center of the

sphere that carried the sun. Ptolemy's "epicyclic" model (right) envisioned a carrying sphere?the deferent?which was concentric with the cen

ter of the Earth, but it placed the sun on another sphere?the epicycle?which was carried by the deferent. Although either model could account

equally well for the motion of the sun, both violated Aristotle's cosmology, since both assumed a "center of heaviness" other than the Earth.

Ptolemy said nothing of these violations, but Islamic astronomers were unsettled by these inconsistencies because Ptolemy had supposedly adopted Aristotle's cosmology. Here the action of the epicyclic model (lower right) can be understood by considering the motions of the defer ent and the epicycle in two steps. First, imagine that the deferent carries the epicycle 45 degrees counterclockwise. The epicycle then rotates clockwise by an equal amount. Carried full circle, these combined motions will describe the sun's path around the Earth (brown dash).

servation in error? Or perhaps it was

something else? These questions creat ed an environment in which every as

pect of the Greek astronomical tradi tion was subjected to close scmtiny.

The Big Questions Eventually the theoretical astronomers were able to sort out some of the issues

surrounding the source of Ptolemy's errors. They discovered, for example, that his observational techniques left

something to be desired. Ptolemy's cal culations of the solar eccentricity (a de

scription of its apparent "orbit") and the position of the solar apogee were in error because he observed the sun at four points, the two equinoxes and the two solstices. The trouble is that during solstice in late June and December, the

sun appears to rise and set over the same points above the horizon for sev eral days, and so it's very difficult to determine exactly when it arrives. The Islamic astronomers realized that ob servations of greater accuracy could be

made during mid-season (when the sun passes through the midpoints of Taurus, Leo, Scorpio and Aquarius). With one stroke they had bettered

Ptolemy and solved the problems of the solar apogee and solar eccentricity.

The theoreticians also developed an intense interest in the cosmological foundations of Ptolemaic astronomy. Ptolemy received his cosmology from Aristotle, who held that the planets and the stars were embedded in concentric celestial spheres that surrounded the Earth. All of the spheres and the celes

tial bodies were supposed to be made of a single, simple "element": ether. Unlike the other elements?earth, fire, water and air?ether was perfect and divine, and did not possess worldly properties such as friction. On the surface this was fine, and Ptolemy accepted Aristotle's elements unquestioningly.

The trouble began when Ptolemy at

tempted to explain celestial mechanics within the framework of Aristotle's cos

mology. Ptolemy had suggested that an outermost ninth sphere was responsi ble for the daily motion of the neigh boring eighth sphere, which carried the fixed stars, and so could account for the

phenomenon of precession. The ninth

century Arab astronomer Muhammad b. M?s? considered Ptolemy's ninth

sphere, and realized that it simply



Figure 5. Apparent motion (yellow line) of an "upper planet/' such as Mars, relative to the

"fixed stars" in the sky involves a retrograde motion (from east to west) as the Earth passes the

upper planet on the "inside track" of their respective orbits around the sun. Ptolemy con

trived exotic geocentric models to explain this behavior (see Figure 6).

could not be. How could a frictionless

sphere move another frictionless sphere if they were both revolving about the same center?

This kicked off a philosophical de bate among the Islamic scholars. What

exactly was the ether? What was the true nature of the spheres? Could the celestial bodies have properties that contradicted their originally defined natures? In some ways, the quest for

consistency between the original defin itions of the celestial bodies and their

apparent properties became the main concern of Islamic astronomy. This may be the fundamental difference be tween Islamic astronomers and their

Greek predecessors. Once that line of questioning began,

the medieval Islamic scholars saw that Greek astronomy was fraught with

cosmological absurdities. They imag ined their role as reformers of that as

tronomy and creators of an alternative

astronomy without the contradictions. The cosmological issues that most dis turbed the Islamic astronomers were

simply stated but not so easily solved. It took generations of them to articu late the questions properly first, and then generations after that to solve the

problems, ultimately producing an al ternative astronomy?one that would benefit the astronomy of Copernicus.

At this point I must add that we should not be quick to judge Ptolemy and his followers as naive for holding on to the concepts of the Aristotelian

spheres and the geocentric universe.

Faulty or not, the cosmology provided excellent observational results, allow

ing one to predict the positions of the

planets for any time and place. There was simply no other cosmology that

systematically explained so much of the observable universe before Isaac

Newton's universal law of gravitation in the 17th century.

It's All Spheres... At some level, Ptolemy must have been aware of the difficulties inherent in a strict adherence to Aristotelian cos

mology. He could not account for even the simplest planetary motions with out relaxing Aristotle's restrictions. Consider the motion of the sun. If the Earth were at the very center of the ce lestial sphere, then the sun would have a uniform rate of motion through the

sky all year round because its distance from us would not change. The facts are otherwise: During the spring and summer months in the Northern

Hemisphere, the sun appears to be

moving more slowly than it does in the autumn and winter. (Of course, we now know that there are simply more

days in spring and summer than there are in autumn and winter because the Earth is farthest away from the sun

during these months and so takes

longer to travel between equinoxes.) Ptolemy treats the problem in Al

magest III (the third of thirteen books), where he offers his readers a choice of two models that might explain the variation in the length of the seasons

(Figure 4). His eccentric model pro posed that the center of the Earth did not coincide with the center of the

sphere that carried the sun in its yearly motion. So as the sun orbited the Earth it would appear to be farther away (and so move slowly) during part of the year. In his epicyclic model, Ptole

my assumed a "carrying sphere," later called the deferent, which was concen tric with the center of the Earth, and then placed the sun on another sphere, called the epicycle, which was carried

by the deferent and had a radius equal to the eccentricity of the first model. As the epicycle and the deferent moved at the same speed, but in opposite direc tions, the body of the sun on the sur face of the epicycle would describe a circle whose diameter equaled the dif ference between the sun's nearest and farthest points from the Earth.

Either model could account equally well for the apparent motion of the sun. In making his choice, Ptolemy opted for the classical axiom of sim

plicity and adopted the eccentric mod



Ptolemy's model for the upper planets

Figure 6. Ptolemy's model for the motion of the upper planets involved a nonsensical construction that proved deeply disturbing to the medieval

Islamic astronomers. Ptolemy proposed that a carrying sphere, or deferent (light blue), spun in place around an axis that passes through "the cen

ter of equation of motion" (later called the "equant"), and not through the center of the sphere. Such a motion is physically impossible, and the

problem was not solved until the 13th century by the Islamic astronomer Mu'ayyad al-D?n al-'Urd?. Copernicus used 'Urdi's theorem in his mod

els of planetary motion. Here the planet is embedded within an epicycle sphere that travels within the shell-like surface of the deferent.

el simply because it involved a single sphere. What was left unsaid, however, was that both models violated Aristo tle's cosmology. In the eccentric model, the Earth was not the center of "heavi ness," a strict violation of the Aris totelian assumption that the Earth was at the very center of the universe. On the other hand, the epicycle model as sumed an epicyclic sphere that had its own center of heaviness, which was not moved by its own motion, but rather by the motion of the carrying deferent. This conflicted with the sim

plicity of the ether and created a center of heaviness outside the Earth.

Ptolemy said nothing about the man ner in which he would account for these violations, leaving the reader to think that such transgressions did not really

matter. At the end of the 12th century, Arabic philosophers of Andalusia, Spain, attacked this very problem of Ptolemaic astronomy and focused on these specific transgressions, but most

practicing astronomers in the Islamic world went along with Ptolemy's si lence. After all, Ptolemy's models made

reasonably accurate predictions. It wasn't till the 14th century that Ibn

al-Sh?tir broke this silence, claiming that the eccentric model was indeed a violation of Aristotle's cosmology and should be abandoned. Ibn al-Sh?tir banished all such models from his own

geocentric astronomy, and then he took an interesting tack: He questioned the nature of Aristotle's ether. If everything in the heavens was made of ether?the celestial spheres, the planets and the stars?how was it that the stars emitted

light and the carrying spheres did not? Ibn al-Sh?tir concluded that the ether

must have some composition (tark?bun ma) and could not be as simple as had been assumed. If this kind of composi tion could be allowed in the celestial realm, he argued, then epicycles should be tolerated as well, since even the

largest planetary epicycle could not

compare in size to the smallest fixed star. By including epicycles, Ibn al Sh?tir managed to construct purely geo centric models that were consistent with this new vision of Aristotelian cosmolo

gy as well as Ptolemy's observations and the more refined observations of later astronomers.

Ptolemy's models of the sun's motion

may seem contrived, but his configura tions for the planets are even more so. The apparent motions of Saturn, Jupiter, Mars and Venus are strange?seeming to move slower at times, occasionally appearing to be stationary or even mov

ing retrograde relative to the stars (Fig ure 5). To explain these motions Ptolemy needed to abandon simplicity and in

corporate both eccentric and epicyclic spheres. (His descriptions of the mo

tions of the moon and Mercury are even more complicated!)

For each planet in these combined models, Ptolemy assumed the exis tence of an eccentric sphere (the defer ent), which was sufficiently thick to

carry a solid epicycle in its shell-like structure. In turn, the planet was im

mersed in the epicycle's surface. Ptole

my again remained silent on these vio lations, but the situation was even

worse from a cosmological point of view. Unlike his model of the sun's motion, the motion of the epicycle was now no longer equal to the motion of the deferent. Instead the epicyclic mo tion itself had to account for the mo tion of the individual planet. Because its motion was independent of the def erent, it could no longer be used to hide the violation of the eccentricity as had been done with the sun.

Perhaps the most disturbing aspect to this model was that the planetary deferents were no longer moving about their own centers. Instead, Ptole

my proposed that they moved uni

formly in place around an axis that

passed through "the center of equation of motion," which in medieval times was called the equant. The notion of the equant was the "last straw" for some Islamic astronomers, simply be cause it was physically nonsensical.

Try to imagine a sphere moving in



Figure 7. Tusi Couple can generate linear motion (white dashed line) from uniform circular motion (pink circle and blue circle), and so solved a

number of problems that troubled the ancient astronomers. Here the blue circle carries the pink circle in a clockwise direction, while the pink circle rotates counterclockwise on its central axis. The two circular motions result in the linear motion (white arrow) of an object (red ball) rid

ing on the pink circle. The mechanism, devised by the 13th-century astronomer Nas?r al-D?n al-T?s?, was used to explain the latitudinal motion

of planets in geocentric cosmologies, and it still has a wide range of modern applications.

place around an axis that does not pass through its center (Figure 6). Many tried to solve the problem of

the equant, including a student of the

11th-century Islamic philosopher Avi cenna, but none was able to do so. It wasn't until the 13th century that a Damascene astronomer, Mu'ayyad al

D?n al-'Urd?, managed to find a solu tion for the planetary equant. The theo rem, now called the 'Urd? lemma, could reproduce the apparent motions of the planets with a deferent that moved uniformly in place around an axis that passed through its center. Centuries later, Copernicus was to use this theorem to account for planetary

motions in his heliocentric cosmology.

In a fashion, Ptolemy's model could

explain the "longitudinal" motion of the planets through the sky, but their

motion in "latitude" required another mechanism. In the case of Venus, for

example, Ptolemy assumed that the

"equatorial plane" of the deferent

sphere would oscillate up and down, performing a seesaw motion. He pro posed a mechanism that consisted of two small circles, perpendicular to the

equatorial plane and lying at its cir cumference. The size of the small cir cles would correspond to the size of the planet's latitudinal movement, so that when the equatorial plane rotated

along the circles it would describe a seesaw motion.

This mechanism doesn't work, how ever, because attaching the tip of the

equatorial diameter of the deferent to the small circles would generate a

wobble that destroys the longitudinal motions (which were otherwise accu rate). And needless to say, it could nev er be accommodated within Aristotle's

cosmology, which held that all celestial motions were uniformly circular. Noth

ing should seesaw in the heavens. In the words of the 13th-century as tronomer Nas?r al-D?n al-T?s?, "this kind of talk was outside the craft of as

tronomy." This was his polite way of

saying that Ptolemy's description was utter nonsense. And Ptolemy seems to have known this. In his own defense, he resorted to the frailty of mortals who dare to understand the mind of God: "Now let no one, considering the

complicated nature of our devices, judge such hypotheses to be over-elab orated. For it is not appropriate to com

pare human [constructions] with di vine, nor to form one's beliefs about such great things on the basis of very dissimilar analogies...."

Ultimately, it was T?s? who came up with a solution to the problem of latitu dinal motion. His brilliant theorem con sisted of two spheres, one half the diam eter of the other and placed tangentially inside it (Figure 7). T?s?'s mechanism could generate linear motion from uni form circular motion and so destroyed the long-held Aristotelian division be tween the linear motions of the world and the circular motions of the heavens. The mechanism, now called the T?s?

Couple, has a wide range of applications, mcluding the translation of a piston's lin ear motion into the circular motion of a wheel. Its astronomical origin was recog nized in its metaphorical name?"the

sun-and-planet mechanism"?when it

was applied to the steam engine. And

Figure 8. Copernicus's proof of the T?s? Couple (left) was identical to that used by T?s? him

self (right)?including the alphabetical names of the corresponding points in the diagrams.

Copernicus also applied the T?s? Couple to explain Mercury's motion in a manner that was

identical to that used by Ibn al-Sh?t?r. How did Copernicus become familiar with the works

of the Islamic astronomers? Historians have proposed a number of possibilities, but whether or not the mystery is ever solved, it is clear that there are deep connections between the as

tronomical traditions of the ancient Greeks, the medieval Arabs and the Renaissance Euro

peans. (Image from Copernicus's De Revolutionibus courtesy of the American Philosophical

Society. Image of the Arabic text courtesy of the Vatican Library.)



Copernicus himself applied the mecha nism to explain planetary motions with in his heliocentric cosmology (Figure 8).

Coincidence?

Understanding the distinction between Arabic astronomy and Greek astrono

my is key to appreciating the founda tions of modern astronomy. The transi tion from classical Greek astronomy to the astronomy of the European Renais sance would have been very different had it not been for the intellectual con tributions of the medieval Islamic as tronomers. The problems inherent to

Ptolemy's work were simply too deep, and it took many generations of Arabic scholars to articulate them and then to resolve them.

The biggest problem was that the mathematical language used by Ptole my to describe planetary motions un dercut the physical basis of Aristotle's

geocentric cosmology. If it were simply a

problem of mistaken observations or even flawed methods, it would not have been critical. But by representing Aris totle's cosmology with a mathematical

description that deprived it of its funda mental properties, Ptolemy created a fic titious world that contradicted common sense. The equant, for example, de

scribed a sphere that did not have the

properties of a sphere. This was the cen tral problem of Greek astronomy, and it

required a major overhaul. In his early works, Copernicus too

was troubled by the mathematical in consistencies of Ptolemy, but it was the

problem of the equant that disturbed him more than the geocentric cosmolo gy. A heliocentric universe would not solve the problem of the equant in any case, since Copernicus still viewed ce lestial motions as circular rather than el

liptical and so still required the equant to describe elliptical motions. (Incorpo rating the Earth's orbit obviated the need for epicycles.) A close inspection of Copernicus's work shows that the

only two theorems he used that weren't

already in the classical Greek sources were the 'Urdi Lemma and the T?s?

Couple. And he was using them in the 16th century to solve precisely the same

problems that faced the Islamic as tronomers in the 13th century. (For the

most part, the shift to a heliocentric cos

mology only reorients the vector con

necting the Earth and the sun, but it also

plays havoc with the other aspects of Aristotle's cosmology that Copernicus was trying to preserve.)

There were other similarities be tween the works of Copernicus and

medieval Arab astronomy. The Coper nican reconfiguration for describing the motion of the moon, a strictly geo centric body, proved to be identical? vector for vector?to the configuration proposed by Ibn al-Sh?tir at least two centuries before. And Copernicus's

model of Mercury's motion employed the T?si Couple in a way that was identical in placement and function to Ibn al-Sh?tir's model of Mercury.

So we must ask, how could some one like Copernicus become familiar with these ideas? Apparently he could not read Arabic, and as far as we know the works had not been trans lated into Latin.

As it happens, there are some inter

esting clues lying about. One possibili ty was raised by the Austrian-Ameri can historian Otto Neugebauer, who drew attention to a Byzantine Greek

manuscript, translated from Arabic, which contained some of the results obtained by the Islamic astronomers.

We know that Copernicus did read Greek, and he may have had the op portunity to see it early in the 16th cen

tury in the course of his studies in Italy (where the manuscript now resides). More recently, I have been consider

ing another possibility. In my travels to various European libraries I have un covered several Arabic manuscripts on

planetary astronomy?including a

copy of T?s?'s critique of Ptolemy. The

manuscripts appear to have been owned by Copernicus's contempo raries, who could read Arabic very well as evidenced by many Latin notes

they left on the margins. Did those con

temporaries, or their colleagues, ever communicate this knowledge to

Copernicus? Noel Swerdlow of the

University of Chicago and Neugebauer have even suggested that the contents of many Arabic works were common

knowledge in Italy by the 1500s. Other questions remain. Why would

the Arabic astronomers, who worked so desperately to overhaul Ptolemaic

astronomy, still cling to Aristotelian

cosmology? Why would Copernicus go through all the steps to mathematically account for an Aristotelian cosmology, and then throw it all away by shifting the center to the sun? He had nary a shred of evidence that anything like

Newton's gravity was coming?which did explain how the universe was held

together. Based on what he knew at the

time, we could even accuse Copernicus of behaving just like Ptolemy: accept ing a mathematical expediency without

having the cosmology to back it up. The Arabic astronomers at least re mained consistent.

In any case, all of this begs for a re finement of our analytical concepts if

we are to distinguish what was Arabic in the science of the European Renais sance or what was Greek in Arabic sci ence. When there are such intimate con nections between scientific traditions it becomes almost meaningless to speak of a Greek, Arabic or European science as if each had a character of its own.

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sophical Society 117:423-512.

Swerdlow, N., and O. Neugebauer. 1984. Math ematical Astronomy in Copernicus's De Revo lutionibus. New York: Springer Verlag.

Toomer, G. 1984. Ptolemy's Almagest. New York: Springer Verlag.

Links to Internet resources for further

exploration of "Greek Astronomy and the

Medieval Arabic Tradition" are available

on the American Scientist Web site:

http: / / www.americanscientist.org/ articles / 02articles / saliba.html



Article Contentsp. 360p. 361p. 362p. 363p. 364p. 365p. 366p. 367

Issue Table of ContentsAmerican Scientist, Vol. 90, No. 4 (JULY-AUGUST 2002), pp. 298-392Front MatterLetters to the Editors [pp. 300-302]Errata: Engineering: The Civil Engineer: On the occasion of a sesquicentennial [pp. 302-302]Macroscope: Sharing in Science [pp. 304-307]Computing Science: The World According to Wolfram [pp. 308-312]Engineering: Art and Iron and Steel [pp. 313-317]Marginalia: Carbides [pp. 318-320]SCIENCE OBSERVERELECTRIC STARS [pp. 321-322]HUYGENS'S CLOCKS REVISITED [pp. 322-323]

Serendipitous Radiation Monitors: Past radiation doses can be measured by studying the tracks that speeding particles have left in ordinary solidsdetectors that just happened to be there [pp. 324-331]The Evolutionary Ecology of Escherichia coli: Abundantly studied and much feared, E. coli has more genomic plasticity than once believed and may have followed various routes to become a pathogen [pp. 332-341]Cluster Dynamics: Fast Reactions and Coulomb Explosion: The femtochemistry of these unusual aggregates reveals much about the fleeting instant between reactant and product [pp. 342-349]Protein Structures: From Famine to Feast: Thousands of protein structures are known and accessible. Structural genomics is building on new technology to fill in the missing pieces [pp. 350-359]Greek Astronomy and the Medieval Arabic Tradition: The medieval Islamic astronomers were not merely translators. They may also have played a key role in the Copernican revolution [pp. 360-367]Scientists' BookshelfA More Modern Synthesis [pp. 368-371]A Spy or Not a Spy, That Was the Question [pp. 371-373]Explaining Linguistic Diversity [pp. 374-375]Andrew Carstairs-McCarthy [pp. 376-377]BIOLOGYA Bird's-Eye View [pp. 378-379]In Biophilia He Trusts [pp. 379-380]Genes on the Move [pp. 380-381]ARTIFICIAL INTELLIGENCE: Robot Futures [pp. 381-382]MATHEMATICS: The Search for Rigor [pp. 382-384]PALEONTOLOGY: Adventures in the Fourth Dimension [pp. 384-385]GEOLOGY: The Plate Tectonic Revolution [pp. 385-386]The Wild, Wild East [pp. 386-387]ANTHROPOLOGY: Should Ideology Color Science? [pp. 387-388]

Sigma Xi Today: JULY/AUGUST, 2002, VOLUME: 11, NUMBER: 4 [pp. 389-392]Back Matter

Documents

Georg Saliba-Greek Astronomy and the Medieval Arabic Tradition The medieval Islamic astronomers werenot merely translators. They may also have played a key role in the Copernican revolution.pdf