And so we finally come to the most important legacy of Andalusia. For it is through Spain that so much of Arabic science reached Europe. While there were other avenues of transmission and translation, such as through Sicily and along the trade routes with city-states like Venice, as well as through the efforts of Christian travellers in the East such as the Englishman Adelard of Bath (1080–1152), it was nevertheless first and foremost the recapture of Islamic Spain by the Christians that would give Europe access to the wealth of knowledge produced in the Islamic world. Just as Baghdad had been the centre of the thriving translation movement from Greek into Arabic, so cities like Toledo became the centres of translation of the great Arabic texts into Latin.
The very first translations took place as early as the tenth century, and consisted of a collection of treatises on the astrolabe discovered at the monastery of Ripoll in Catalonia.10 This Benedictine monastery in northern Spain was founded by the, presumably aptly named, Count Wilfred the Hairy (Guifré el Pilós, in Catalan) in 879, who used it as a centre to repopulate a region that sat as the buffer zone between the Islamic Empire to the south and the Christian Franks to the north. The monastery eventually became a great centre of learning and boasted an impressive library.11
One of the first scholars to study these early translations was Gerbert d’Aurillac (c. 945–1003), a French monk who developed a deep love for Arab culture and science and who had heard about their many wonderful achievements in mathematics and astronomy, including the work on the Hindu-Arabic numerals. He was introduced to the work of the great Islamic scholars by a bishop from Barcelona by the name of Atto, who had travelled to Córdoba to meet with al-Hakam and had returned smitten by Andalusian culture. Gerbert would later be the first Christian scholar to carry Arabic science across the Pyrenees into Europe.12 What makes this story so fascinating is that he eventually rose through the ranks of the Catholic Church to become Pope Sylvester II. It seems fitting that Christian Europe was first introduced to the science of the Islamic Empire by a pope!
Probably the most prolific translator of Arabic science into Latin was the Italian scholar Gerard of Cremona (1114–87), every bit as important in the story of the transmission of science as his Baghdadi Christian counterpart Hunayn ibn Ishāq three centuries earlier. He had travelled to Toledo in order to study Arabic and was famously the first to translate Ptolemy’s Almagest into Latin. He also edited for Latin readers the Toledan Tables, which despite their shortcomings were still the most accurate compilation of astronomical data in Europe at the time. Gerard translated many books, covering most areas of science, including the work of some of the biggest names, such as al-Khwārizmi, al-Kindi, al-Rāzi, Ibn al-Haytham, Thābit ibn Qurra, Hunayn ibn Ishāq and the Banū Mūsa brothers. He also translated al-Zahrāwi’s medical encyclopedia, meticulously copying his surgical illustrations. It is thanks to Gerard that al-Zahrāwi was for a long time held in higher regard in Europe, as the great Abulcasis,13 than he was in his native lands.
I still feel as though I have not done Andalusian scholarship justice in this brief chapter, for I have said nothing about the travel writer Ibn Jubayr (1145–1217) or the historian and philosopher Ibn al-Khatīb (1313–74). I will, however, say a little about the geographer al-Idrīsi (c. 1100–66). Educated in Córdoba, he travelled widely and would finally settle in Sicily, where he was employed by the Norman King Roger II to produce a new world atlas. It was completed in 1154 as The Book of Roger (al-Kitab al-Rujāri) and better known as the Tabula Rogeriana. It is regarded as the most elaborate and complete description of the world made in medieval times and was used extensively by travellers for several centuries, for it contained detailed descriptions of the Christian north as well as the Islamic world, Africa and the Far East (see Plate 21). The atlas describes the earth as a sphere with a circumference of 23,000 miles, and maps it in seventy rectangular sections. The historian S. P. Scott wrote a century ago that
the compilation of al-Idrīsi marks an era in the history of science. Not only is its historical information most interesting and valuable, but its descriptions of many parts of the Earth are still authoritative. For three centuries geographers copied his maps without alteration. The relative position of the lakes which form the Nile, as delineated in his work, does not differ greatly from that established by Baker and Stanley more than seven hundred years afterwards, and their number is the same.14
An interesting feature of al-Idrīsi’s map, as with all medieval Arabic maps, is that it is drawn upside down, with the north at the bottom.
In the next chapter, I want to focus on astronomy and trace its development during medieval times. For, more than any other discipline, this story highlights the way scientific progress is a continuum. While progress may speed up and slow down, see highs and lows as civilizations rise and fall, and get passed on like a baton in a relay race of discovery, one story exemplifies more than any other the debt owed by European scholars to the giants of the Islamic world. Throughout this book I have highlighted the achievements of the astronomers of the Islamic Empire in the period between Ptolemy and Copernicus. But how important were they really? After all, they believed almost to a man that the sun revolves around the earth, and they had no telescopes that would persuade them otherwise. So, prepare to meet some new characters, without whom the father of modern astronomy, the man who finally delivered the heliocentric model of our solar system, Copernicus, might as well have followed his own father into the copper trade from which he derived his name.
14
The Marāgha Revolution
Accordingly, since nothing prevents the earth from moving, I suggest that we should now consider also whether several motions suit it, so that it can be regarded as one of the planets. For, it is not the centre of all the revolutions.
Nicolaus Copernicus
By this stage in our journey the reader should no longer be in any doubt that the scientific revolution in sixteenth- and seventeenth-century Europe could not have taken place had it not been for the many advances made in the medieval Islamic world in philosophy, medicine, mathematics, chemistry and physics. But one discipline in particular deserves rather more careful consideration and analysis. As I write these words in 2009, the scientific community is celebrating an important anniversary that may have passed the wider world by. Amid all the publicity over Charles Darwin – the year 2009 being the two-hundredth anniversary of that great man’s birth and 150 years since the publication of his Origin of Species – it has gone somewhat unnoticed that it is also designated as International Year of Astronomy. For 2009 marks the four-hundredth anniversary of an event that, more than any other, signalled the birth of modern astronomy. It was during the summer of 1609 that Galileo first pointed his new telescope into the sky to reveal the wonders of the cosmos (just as Robert Hooke would half a century later use the microscope to reveal the wonders of the world of the very small).
But Galileo’s telescope did so much more than simply bring the distant heavenly bodies closer to us; it banished millennia of confusion and guesswork about our place in the universe. The traditional historical narrative, however, is that it was not Galileo who deserves the mantle of founder of modern astronomy but the earlier Polish astronomer Nicolaus Copernicus (1473–1543).
It is a fact of life that oversimplified accounts of the development of science are often necessary in its teaching. Most scientific progress is a messy, complex and slow process; only with the hindsight of an overall understanding of a phenomenon can a story be told pedagogically rather than chronologically. This necessitates the distilling of certain events and personalities from the mêlée: those who are deemed to have made the most important contributions. It is inevitable therefore that the many smaller or less important advances scattered randomly across hundreds of years of scientific history tend to be swept up like autumn leaves into neat piles, on top of which sit larger-than-life personalities credited with taking a discipline forward in a single jump. Sometimes this is perfectly valid, and
one cannot deny the genius of an Aristotle, a Newton, a Darwin or an Einstein. But it often leaves behind forgotten geniuses and unsung heroes.
In astronomy, this ‘coarse-graining’ appears, superficially, to be particularly tempting. For Ptolemy’s Almagest was so influential as the culmination of Greek astronomical thought that it is not regarded as having been truly replaced until Copernicus wrote his De revolutionibus thirteen hundred years later. That work was to signal a true paradigm shift, one of the most dramatic in the history of human thought. For that is the moment when mankind ceased to occupy the centre of the universe as described by Ptolemy’s geocentric cosmology. Copernicus would show that it is in fact the earth that revolves around the sun rather than the other way round.
But you are perhaps wondering whether you are really now expected to abandon even this most established of narratives. Is even the great Copernican revolution about to be relegated to the ignominy of a hidden debt to earlier Arabic astronomers who arrived there first? The history of astronomy that leads up to Copernicus is rich and subtle and deserves to be unpicked carefully. I shall tell it, as I have tried to do throughout this book, as objectively and clearly as I can, leaving you to make up your own mind at the end about the issue of – yes – that inevitable hidden debt.
*
Let us first review just why astronomy was so important in Islam. There are two distinct ways in which religion played a role in medieval Islamic astronomy. The first is the obvious one. Astronomy was from the start seen as a science ‘in the service of Islam’. Careful astronomical measurements could provide the faithful with tables, charts and techniques that were crucial in determining the times of prayer, the beginning and end of Ramadan (the month of fasting), as well as the all-important qibla (the direction towards Mecca for prayer). This relationship turned out to be mutually beneficial, for it is clear that Islam provided social legitimacy to astronomy and even gave astronomers the excuse and opportunity to tackle interesting scientific problems that were not necessarily part of this ‘service’.1
The second way that Islam played an important role was in its insistence on a clear distinction being made between astronomy as an exact science and the superstitions associated with astrology, which was seen as a direct challenge to Islamic doctrine by giving powers to the stars that should be reserved only for God. It therefore encouraged astronomy to become ‘metaphysically neutral’.2 Thus, we find that astrology, where it continued, was seen as part of natural philosophy rather than the exact science of astronomy. Despite this split, there were nevertheless the inevitable attacks on astronomy and astronomers from the religious orthodox who still associated it with the ‘ancient sciences’ of the Greeks and, as such, deemed it un-Islamic.
And yet astronomy flourished. It did much more than repeat and check the measurements found in Greek texts like the Almagest. Initially, this was certainly its whole extent, with al-Ma’mūn’s astronomers producing new and improved star charts and tables. These men were followed by a long line of wonderful astronomers who would carry out meticulous measurements far more accurate that anything the Greeks could have managed. Among the greatest of these were the Syrian astronomer al-Battāni (Albatenius) (c. 858–929) and the Egyptian Ibn Yūnus (c. 950–1009); both are widely regarded as the greatest observational astronomers of Islam.
Among the many improvements and corrections to Ptolemy’s astronomical measurements were those to his values for the length of the year, the obliquity of the ecliptic (the angle of tilt of the earth’s axis of rotation relative to the plane of its orbit), the precession of the fixed stars (now known to be due to the gradual shift in orientation of the earth’s axis of rotation) and the solar apogee (the furthest distance of the sun from the earth).3 So extensive and important were these new measurements that I am tempted to describe them more fully, but I will do no more than record that Copernicus himself was well aware of the work of al-Battāni in particular in this regard and indeed quoted him regularly in his De revolutionibus.
I wish, however, to return to Ptolemy and the cosmological model that the Muslim astronomers inherited from him. For without understanding its successes and faults we cannot hope to navigate our way through the centuries of Islamic astronomy that led to Copernicus. To begin with, as already mentioned, the most important distinction one can make between Ptolemy and Copernicus is the monumental switch from a geocentric to heliocentric model of the universe. Yet Copernicus was not the first to suggest that the earth went round the sun.
The very first known astronomer to propose a heliocentric model was the Greek Aristarchus of Samos (c. 310–230 BCE), who stated correctly that the earth rotated around its own axis, and in turn revolved around the sun. Like his contemporary, Eratosthenes, Aristarchus had calculated the size of the earth, and estimated the size and distance of the moon and sun. From these, he concluded that the sun was six to seven times wider than the earth and thus hundreds of times larger in volume. Some have suggested that his calculation of the relative size of the earth and sun led Aristarchus to conclude that it made more sense for the earth to be moving around the much larger sun than the other way round. His writings on the heliocentric system are lost, but some information is known from surviving descriptions and critical commentary by his contemporaries, such as Archimedes. In a famous passage in The Sand Reckoner, Archimedes writes:
You know that most astronomers designate by the word cosmos the sphere whose centre coincides with the centre of the Earth … But Aristarchus, the Samian, published in writing certain hypotheses in which it follows from the suppositions that the cosmos must be many times greater than the one mentioned before. He assumes namely that the fixed stars and the Sun remain stationary, while the Earth moves round the Sun through the circumference of a circle.4
Little did Archimedes know that Aristarchus’ description was spot on. Aristarchus also believed the stars to be very far away, and saw this as the reason why there was no visible parallax.
The only other astronomer known to have been a supporter and follower of Aristarchus’ heliocentric theory was a Babylonian by the name of Seleucus (c. 190 BCE). In fact, Aristarchus and Seleucus were probably the only astronomers in antiquity to embrace the notion that the earth revolves around the sun.5 According to the Greek biographer Plutarch, Seleucus was the first to prove the heliocentric system through logical reasoning, but it is not known what arguments he used, and even this interpretation of Plutarch’s writing is itself disputed6 since Seleucus’ ‘proof’ of the heliocentric theory may have amounted to no more than computing a table of numbers based on the theory.
The heliocentric model was quickly rejected, however, by some of the greatest minds in ancient Greece in favour of a geocentric one. Indeed, the best observational astronomer of antiquity, Hipparchus (fl. c. 162–127 BCE), dismissed the heliocentric model of Aristarchus completely. Hipparchus carried out his work on the island of Rhodes, but had close contact with other astronomers in Alexandria and Babylon. His emphasis on careful measurement, his willingness to revise his own beliefs in the light of new evidence and his renowned unforgiving criticisms of sloppy reasoning of other scholars such as Eratosthenes place him, almost uniquely among the Greeks, as an early adherent of the scientific method. Indeed, he would not have looked out of place among great Islamic astronomers such al-Battāni, Ibn al-Haytham and al-Bīrūni. Hipparchus’ most important astronomical work concerned the orbits of the sun and moon (within a geocentric model) and their distances from the earth. He famously worked out that the moon’s mean distance from the earth is sixty-three times the earth’s radius, just a few percentage points larger than the correct value. He also discovered the precession of the equinoxes (the path traced by the earth’s axis of rotation, like the wobble of a spinning top).
But the man who had the greatest influence on astronomy in ancient times was none other than Aristotle himself, almost a century before Aristarchus. And Aristotle has a lot to answer for. It is his model of the cosmos, as described in his
great work On the Heavens, which would colour and shape humanity’s notions about the nature of the universe for almost two thousand years. But he was barking up the wrong cosmic tree; it is Aristotle, rather than Ptolemy, to whom humanity owes its long and mistaken fixation with the geocentric universe. In fact, of all the fallacies, muddles, wrong turns and dead ends in the history of science, the Aristotelian universe was the most dramatically wrong.
Aristotle’s basic idea was that the earth occupies the privileged centre of what was known as the celestial spheres, or orbs, with all other heavenly bodies (moon, sun, Mercury, Venus, Mars, Jupiter, Saturn – only five planets were known – and the ‘fixed’ stars) moving in perfect circular orbits around it. His cosmological model comprised a very complex system of fifty-five spheres, which did rather well at predicting the observed motion of all these bodies across the sky. One can even go so far as to say that this was almost a proper scientific theory.7 It seems to have been accepted universally soon after its inception, even though new observational data from Hipparchus and others necessitated certain modifications to it.
One of the most serious anomalies was to do with the motion of the planets, particularly Mars. It was known that planets moved across the sky from east to west at a faster rate than the fixed stars. But this rate was not constant. In fact, relative to the stars, they would slow down, speed up, and sometimes even double back on themselves. This ‘retrograde’ motion could not be accommodated in Aristotle’s model and a fix was devised by Hipparchus and later perfected by Ptolemy.
Pathfinders Page 26
