To Explain the World: The Discovery of Modern Science (16 page)

It is sometimes said that the greatest contribution to science of the Abbasid caliphs was the foundation of an institute for translation and original research, the Bayt al-Hikmah, or House of Wisdom. This institute is supposed to have served for the Arabs somewhat the same function that the Museum and Library of Alexandria served for the Greeks. This view has been challenged by a scholar of Arabic language and literature, Dimitri Gutas.
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He points out that Bayt al-Hikmah is a translation of a Persian term that had long been used in pre-Islamic Persia for storehouses of books, mostly of Persian history and poetry rather than of Greek science. There are only a few known examples of works that were translated at the Bayt al-Hikmah in the time of al-Mamun, and those are from Persian rather than Greek. Some astronomical research, as we shall see, was going on at the Bayt al-Hikmah, but little is known of its scope. What is not in dispute is that, whether or not at the Bayt al-Hikmah, the city of Baghdad itself in the time of al-Mamun and al-Rashid was a great center of translation and research.

Arab science was not limited to Baghdad, but spread west to Egypt, Spain, and Morocco, and east to Persia and central Asia. Participating in this work were not only Arabs but also Persians, Jews, and Turks. They were very much a part of Arab civilization and wrote in Arabic (or at least in Arabic script). Arabic then had something like the status in science that English has today. In some cases it is difficult to decide on the ethnic background of
these figures. I will consider them all together, under the heading “Arabs.”

As a rough approximation, we can identify two different scientific traditions that divided the Arab savants. On one hand, there were real mathematicians and astronomers who were not much concerned with what today we would call philosophy. Then there were philosophers and physicians, not very active in mathematics, and strongly influenced by Aristotle. Their interest in astronomy was chiefly astrological. Where they were concerned at all with the theory of the planets, the philosopher/physicians favored the Aristotelian theory of spheres centered on the Earth, while the astronomer/mathematicians generally followed the Ptolemaic theory of epicycles and deferents discussed in
Chapter 8
. This was an intellectual schism that, as we shall see, would persist in Europe until the time of Copernicus.

The achievements of Arab science were the work of many individuals, none of them clearly standing out from the rest as, say, Galileo and Newton stand out in the scientific revolution. What follows is a brief gallery of medieval Arab scientists that I hope may give some idea of their accomplishments and variety.

The first of the important astronomer/mathematicians at Baghdad was al-Khwarizmi,
*
a Persian born around 780 in what is now Uzbekistan. Al-Khwarizmi worked at the Bayt al-Hikmah and prepared widely used astronomical tables based in part on Hindu observations. His famous book on mathematics was
Hisah-al-Jabr w-al-Muqabalah
, dedicated to the caliph al-Mamun (who was half Persian himself). From its title we derive the word “algebra.” But this was not really a book on what is today called algebra. Formulas like the one for the solution of quadratic equations were given in words, not in the symbols that are an essential element of algebra. (In this respect, al-Khwarizmi’s mathematics
was less advanced than that of Diophantus.) From al-Khwarizmi we also get our name for a rule for solving problems, “algorithm.” The text of
Hisah al-Jabr w-al-Muqabalah
contains a confusing mixture of Roman numerals; Babylonian numbers based on powers of 60; and a new system of numbers learned from India, based on powers of 10. Perhaps the most important mathematical contribution of al-Khwarizmi was his explanation to the Arabs of these Hindu numbers, which in turn became known in Europe as Arabic numbers.

In addition to the senior figure of al-Khwarizmi, there were collected in Baghdad a productive group of other ninth-century astronomers, including al-Farghani (Alfraganus),
*
who wrote a popular summary of Ptolemy’s
Almagest
and developed his own version of the planetary scheme described in Ptolemy’s
Planetary Hypotheses.

It was a major occupation of this Baghdad group to improve on Eratosthenes’ measurement of the size of the Earth. Al-Farghani in particular reported a smaller circumference, which centuries later encouraged Columbus (as mentioned in an earlier
footnote
) to think that he could survive an ocean voyage westward from Spain to Japan, perhaps the luckiest miscalculation in history.

The Arab who was most influential among European astronomers was al-Battani (Albatenius), born around 858 BC in northern Mesopotamia. He used and corrected Ptolemy’s
Almagest
, making more accurate measurements of the ~23½° angle between the Sun’s path through the zodiac and the celestial equator, of the lengths of the year and the seasons, of the precession of the equinoxes, and of the positions of stars. He introduced a trigonometric quantity, the sine, from India, in place of the closely related chord used and calculated by Hipparchus. (See
Technical Note 15
.) His work was frequently quoted by Copernicus and Tycho Brahe.

The Persian astronomer al-Sufi (Azophi) made a discovery whose cosmological significance was not recognized until the twentieth century. In 964, in his
Book of the Fixed Stars
, he described a “little cloud” always present in the constellation Andromeda. This was the earliest known observation of what are now called galaxies, in this case the large spiral galaxy M31. Working at Isfahan, al-Sufi also participated in translating works of Greek astronomy into Arabic.

Perhaps the most impressive astronomer of the Abbasid era was al-Biruni. His work was unknown in medieval Europe, so there is no latinized version of his name. Al-Biruni lived in central Asia, and in 1017 visited India, where he lectured on Greek philosophy. He considered the possibility that the Earth rotates, gave accurate values for the latitude and longitude of various cities, prepared a table of the trigonometric quantity known as the tangent, and measured specific gravities of various solids and liquids. He scoffed at the pretensions of astrology. In India, al-Biruni invented a new method for measuring the circumference of the Earth. As he described it:
⁴

When I happened to be living in the fort of Nandana in the land of India, I observed from a high mountain standing to the west of the fort, a large plain lying south of the mountain. It occurred to me that I should examine this method [a method described previously] there. So, from the top of the mountain, I made an empirical measurement of the contact between the Earth and the blue sky. I found that the line of sight [to the horizon] had dipped below the reference line [that is, the horizontal direction] by the amount 34 minutes of arc. Then I measured the perpendicular of the mountain [that is, its height] and found it to be 652.055 cubits, where the cubit is a standard of length used in that region for measuring cloth.
*

From these data, al-Biruni concluded that the radius of the Earth is 12,803,337.0358 cubits. Something went wrong with his calculation; from the data he quoted, he should have calculated the radius as about 13.3 million cubits. (See
Technical Note 16
.) Of course, he could not possibly have known the height of the mountain to the accuracy he stated, so there was no practical difference between 12.8 million cubits and 13.3 million cubits. In giving the radius of the Earth to 12 significant figures, al-Biruni was guilty of misplaced precision, the same error that we saw in Aristarchus: carrying out calculations and quoting results to a much greater degree of precision than is warranted by the accuracy of the measurements on which the calculation is based.

I once got into trouble in this way. I had a summer job long ago, calculating the path of atoms through a series of magnets in an atomic beam apparatus. This was before desktop computers or pocket electronic calculators, but I had an electromechanical calculating machine that could add, subtract, multiply, and divide to eight significant figures. Out of laziness, in my report I gave the results of the calculations to eight significant figures just as they came from the calculating machine, without bothering to round them off to a realistic precision. My boss complained to me that the magnetic field measurements on which my calculation was based were accurate to only a few percent, and that any precision beyond this was meaningless.

In any case, we can’t now judge the accuracy of al-Biruni’s result that the Earth’s radius is about 13 million cubits, because no one now knows the length of his cubit. Al-Biruni said there are 4,000 cubits in a mile, but what did he mean by a mile?

Omar Khayyam, the poet and astronomer, was born in 1048 in Nishapur, in Persia, and died there around 1131. He headed the observatory at Isfahan, where he compiled astronomical tables and planned calendar reform. In Samarkand in central Asia he wrote about topics in algebra, such as the solution of cubic equations. He is best known to English-speaking readers as a poet, through the magnificent nineteenth-century translation by Edward Fitzgerald of 75 out of a larger number of quatrains
written by Omar Khayyam in Persian, and known as
The Rubaiyat.
Unsurprisingly for the hardheaded realist who wrote these verses, he strongly opposed astrology.

The greatest Arab contributions to physics were made in optics, first at the end of the tenth century by Ibn Sahl, who may have worked out the rule giving the direction of refracted rays of light (about which more in
Chapter 13
), and then by the great al-Haitam (Alhazen). Al-Haitam was born in Basra, in southern Mesopotamia, around 965, but worked in Cairo. His extant books include
Optics, The Light of the Moon, The Halo and the Rainbow, On Paraboloidal Burning Mirrors, The Formation of Shadows, The Light of the Stars, Discourse on Light, The Burning Sphere
, and
The Shape of the Eclipse.
He correctly attributed the bending of light in refraction to the change in the speed of light when it passes from one medium to another, and found experimentally that the angle of refraction is proportional to the angle of incidence only for small angles. But he did not give the correct general formula. In astronomy, he followed Adrastus and Theon in attempting to give a physical explanation to the epicycles and deferents of Ptolemy.

An early chemist, Jabir ibn Hayyan, is now believed to have flourished in the late eighth or early ninth century. His life is obscure, and it is not clear whether the many Arabic works attributed to him are really the work of a single person. There is also a large body of Latin works that appeared in Europe in the thirteenth and fourteenth centuries attributed to a “Geber,” but it is now thought that the author of these works is not the same as the author of the Arabic works attributed to Jabir ibn Hayyan. Jabir developed techniques of evaporation, sublimation, melting, and crystallization. He was concerned with transmuting base metals into gold, and hence is often called an alchemist, but the distinction between chemistry and alchemy as practiced in his time is artificial, for there was then no fundamental scientific theory to tell anyone that such transmutations are impossible. To my mind, a distinction more important for the future of science is between those chemists or alchemists who followed Democritus
in viewing the workings of matter in a purely naturalistic way, whether their theories were right or wrong, and those like Plato (and, unless they were speaking metaphorically, Anaximander and Empedocles), who brought human or religious values into the study of matter. Jabir probably belongs to the latter class. For instance, he made much of the chemical significance of 28, the number of letters in the alphabet of Arabic, the language of the Koran. Somehow it was important that 28 is the product of 7, supposed to be the number of metals, and 4, the number of qualities: cold, warm, wet, and dry.

The earliest major figure in the Arab medical/philosophical tradition was al-Kindi (Alkindus), who was born in Basra of a noble family but worked in Baghdad in the ninth century. He was a follower of Aristotle, and tried to reconcile Aristotle’s doctrines with those of Plato and of Islam. Al-Kindi was a polymath, very interested in mathematics, but like Jabir he followed the Pythagoreans in using it as a sort of number magic. He wrote about optics and medicine, and attacked alchemy, though he defended astrology. Al-Kindi also supervised some of the work of translation from Greek to Arabic.

More impressive was al-Razi (Rhazes), an Arabic-speaking Persian of the generation following al-Kindi. His works include
A Treatise on the Small Pox and Measles.
In
Doubts Concerning Galen
, he challenged the authority of the influential Roman physician and disputed the theory, going back to Hippocrates, that health is a matter of balance of the four humors (described in
Chapter 4
). He explained, “Medicine is a philosophy, and this is not compatible with renouncement of criticism with regard to the leading authors.” In an exception to the typical views of Arab physicians, al-Razi also challenged Aristotle’s teaching, such as the doctrine that space must be finite.

The most famous of the Islamic physicians was Ibn Sina (Avicenna), another Arabic-speaking Persian. He was born in 980 near Bokhara in central Asia, became court physician to the sultan of Bokhara, and was appointed governor of a province. Ibn Sina was an Aristotelian, who like al-Kindi tried to
reconcile with Islam. His
Al Qanum
was the most influential medical text of the Middle Ages.

At the same time, medicine began to flourish in Islamic Spain. Al-Zahrawi (Abulcasis) was born in 936 near Córdoba, the metropolis of Andalusia, and worked there until his death in 1013. He was the greatest surgeon of the Middle Ages, and highly influential in Christian Europe. Perhaps because surgery was less burdened than other branches of medicine by ill-founded theory, al-Zahrawi sought to keep medicine separate from philosophy and theology.