Pathfinders
Page 12
6
Big Science
Sleepless, I watch the heavens turn
Propelled by the motions of the spheres;
Those stars spell out (I don’t know how)
The weal and woe of future years.
If I flew up to the starry vault
And joined the heavens’ westward flow
I would learn, as I travelled the sky
The fate of all things here below.
The Caliph al-Ma’mūn
To counter the naive Eurocentric arguments of those who claim that the Abbāsids did nothing more than translate and assimilate existing Greek knowledge, historians tend to point out that once the translation movement was in full swing, the scholars of Baghdad began to question, extend and improve upon the knowledge they had acquired. While this is certainly true, it hardly smacks of a scientific revolution on the scale of what would later take place in Europe at the hands of Copernicus, Kepler and Galileo. So, while it is certainly vital to highlight the achievements and originality of a genius like al-Khwārizmi, it is equally important to understand what was so special about this time and place; what were the various ingredients that came together – whether due to complex socio-geopolitico-religious reasons or just serendipity – to produce something exceptional? We have already explored these in connection with the translation movement and, to some extent, the knock-on effects this had on the attitude of al-Ma’mūn and his contemporaries in Baghdad towards science in general. But there is another consequence of this that was revolutionary: they fundamentally changed the way scientific research was carried out.
What took place during the reign of al-Ma’mūn was something quite new in scholarship. A consequence of bringing together for the first time a wide range of different scientific traditions from around the world was that the scholars of Baghdad had at their disposal a far broader world-view than anyone before them. Differences in the translations of Persian, Indian and Greek astronomical texts, for instance, each with its own cosmological model, set of measurements and tables of astronomical values, not always in agreement with each other, meant they could not all be right. Of course these different scientific cultures had not developed in isolation. Cross-fertilization of ideas had taken place on many occasions and for different reasons in ancient times, with the Greeks acquiring the medical knowledge of the Egyptians and the mathematics and astronomy of the Babylonians, and the conquests of Alexander the Great in Central Asia leading to a two-way exchange of ideas and scientific knowledge with India. There was even a Babylonian ‘brain drain’ to Greece in the third and second centuries BCE.
But the scholars of Baghdad were starting their education from scratch. For the astronomers in particular, to be able to compare and comment objectively on the many texts they had translated with fresh eyes and open minds must have been gloriously exciting. Questions started to be asked and doubts raised. Clearly, discrepancies needed to be resolved, and it soon became apparent that there was an urgent need for a new set of comprehensive astronomical measurements to be made. But this was too big a task to be undertaken by just one scholar, and al-Ma’mūn was an impatient man.
Some time during the second decade of the ninth century, and associated with his new House of Wisdom, al-Ma’mūn ordered the building of the first astronomical observatory in Baghdad. This was the only way to check the accuracy of one of the greatest texts at the disposal of his scholars, which was, by then, translated and studied in Arabic: Ptolemy’s Almagest. Indeed, Ptolemy is rightly regarded as one of the greatest scientists in history because of his lasting influence on science for a millennium and a half.
Not much is known about the life of Ptolemy, other than that he flourished in Alexandria from 121 to 151 CE. However, his fame is due almost entirely to the publication of his masterpiece of ancient astronomy, the Mathematical Treatise. The title it is known by today derives from the Arabic al-Kitab al-Majisti (‘The Great Book’). This huge text brought together all Greek astronomical knowledge, such as the extensive work of that other great Greek astronomer Hipparchus three hundred years earlier, who had in turn been influenced by Babylonian astronomy. Books I and II of the Almagest deal with the different kinds of celestial motion, Books III–VI with solar and lunar theories, Books VII and VIII are catalogues of the stars, and finally Books IX–XII with the theory of the planets. Together with Euclid’s Elements, the Almagest was regarded as the most important scientific book to be translated into the Arabic language. But despite this incredible legacy, Ptolemy made surprisingly few astronomical observations himself – unlike Hipparchus – and what he did make he often failed to report correctly.1
The observatory that al-Ma’mūn commissioned in Baghdad to check many of the Greek observations reported in the Almagest was probably the world’s first state-funded large-scale science project. Modern science often involves the participation of thousands of scientists in multinational, multi-billion-dollar projects such as the Large Hadron Collider at CERN in Geneva. What al-Ma’mūn achieved, albeit on a far more modest scale, would produce no less spectacular results. He put together an impressive team of mathematicians, astronomers and geographers to work on three major projects that would have been impossible for one man working alone.
Sanad ibn Ali al-Yahūdi was an ambitious young man who moved in the right circles of high society in Baghdad. The son of a Jewish astrologer who very probably worked in the caliph’s court, Sanad realized that he would have to convert to Islam if he really wanted to make a name for himself in the caliph’s circle. Like many bright young minds of his generation he studied the Almagest, but he felt that he needed to be part of the ‘in’ crowd of illustrious scholars associated with al-Ma’mūn to pursue his science further. One man in particular was to play an influential role in Sanad’s life. Already highly regarded and a few years older than Sanad, al-Abbās al-Jawhari held regular meetings in his home with a group of scholars. According to one account, the young Sanad convinced al-Jawhari of his impressive understanding of the Almagest and was welcomed into the circle.2 Al-Jawhari also then put in a good word on his behalf with the caliph, and Sanad was soon given work in the new House of Wisdom.
The senior astronomer in al-Ma’mūn’s court was a Persian by the name of Yahya ibn abi Mansūr, who was associated with the early days of the House of Wisdom and who is said to have been one of the tutors of the Banū Mūsa brothers.3 When al-Ma’mūn was convinced of the need to repeat the astronomical observation and measurements quoted in the Almagest, the two men he turned to were the wise old Yahya and the enterprising young Sanad. They were charged with building and heading up the new observatory in the year 828. While the notion of an observatory was certainly not new – although this would have been the first in the Islamic world – never before had one been created as a genuine scientific institution. The site chosen was in the north-east of the city, in a district known as al-Shammāsiyya. Some historians have referred to it by its name of the Mumtahan observatory,4 but it is often just called the Shammāsiyya. Historians cannot be sure about the types of instruments used at the observatory, but there would have been a sundial with brass gnomon to determine the height of the sun from the length of shadow it cast, as well as astrolabes and, most importantly, a mural quadrant (an instrument like a giant protractor from a school geometry set, cut in half to make a quarter of a circle and placed on its edge in order to measure the precise position in the sky of an object along a sighting rod or tube, known as a dioptra arm). In order to carry out this ambitious astronomical project, al-Ma’mūn also called upon al-Jawhari and, not surprisingly, also enlisted the help of the great al-Khwārizmi.
One other astronomer was also drafted in to help. He was not quite as highly regarded at the time as al-Khwārizmi or Yahya ibn abi Mansūr, but was destined to be among the many Islamic astronomers to influence the European Renaissance. His name was al-Farghāni and his main claim to fame, apart from a widely circulated compendium of Ptolemy’s Almagest (a deep knowledge of the Al
magest was one’s minimal entry ticket into this exciting collaboration), was his association with a device built to measure the water level of the Nile some years later called the Nilometer, which still exists to this day in central Cairo. His legacy also endures through the great Italian writer and poet Dante (1265–1321), who derived most of the astronomical knowledge he included in his Divine Comedy from the writings of al-Farghāni (whom he referred to by his Latin name, Alfraganus). Another famous Italian, Christopher Columbus, also used al-Farghāni’s value for the circumference of the earth in order to persuade his backers to fund his famous voyage. But al-Farghāni’s contribution to the Shammāsiyya project seems to have been his expertise with astronomical instruments. His treatise on the astrolabe still survives and provides the mathematical principles of astrolabe construction.5
In addition to writing the Almagest, Ptolemy had also produced a set of astronomical tables that proved to be a very useful tool for many astronomical calculations. They were known as the Handy Tables and contained all the data needed to compute the positions of the sun, moon and planets, the rising and setting of the stars as well as eclipses of the sun and moon, all far more rapidly and conveniently than similar tables he included in the more comprehensive Almagest. His Handy Tables became, with various modifications, the model for the later Arabic astronomical table, or star chart, known as the zīj.6
And so it was that over a period of a year or so in 828–9, the first critical appraisal of Ptolemy and his astronomy began in earnest when the earliest systematic astronomical observations in the Islamic world were made at Shammāsiyya. During this time, many observations of the sun and moon were made, mainly by Yahya ibn abi Mansūr and overseen by al-Khwārizmi, and a table with longitudes and latitudes of twenty-four fixed stars is recorded as having been drawn up at this time. Al-Ma’mūn then ordered the creation of a second observatory to carry out further measurements, this time at the Dayr Murrān monastery on the slopes of Mount Qasyūn overlooking Damascus.
His senior astronomer, Yahya ibn abi Mansūr, died in the early summer of 830 and so the work at the new observatory was overseen by Khālid al-Marwarrūdhi, who designed several new instruments to be used there. He also built a 16-foot mural quadrant to measure solar angles. It was made of brass and mounted on a marble base built into the side of the mountain and aligned with the meridian. With his instruments in place, al-Marwarrūdhi led another series of solar and lunar observations during 832–3 to compliment those made at Shammāsiyya. However, it seems he encountered some difficulties with the warping and expansion of the metal instruments in the summer heat.7
On completion of all the observations, a new zīj was produced for al-Ma’mūn, with a compilation of all the results from the two observatories. It is known as al-Zīj al-Mumtahan,8 which translates as ‘The Verified Tables’. The name of Yahya ibn abi Mansūr is often associated with this zīj but, since he was not involved with the Syrian observatory measurements, it clearly could not have been down to him alone. In fact, this endeavour was equivalent to a modern-day scientific paper with multiple authors, and the group of astronomers involved in producing it were referred to as Asshāb al-Mumtahan (‘Companions of the Verified Tables’) – hence the alternative name of Shammāsiyya as the Mumtahan observatory.
A widespread misconception is that until Christopher Columbus discovered America everyone believed the earth to be flat. However, even the ancient Greeks had figured out that our world is a sphere. For Pythagoras in the sixth century BCE, this was obvious from a purely aesthetic point of view: surely the gods would have created the earth as a perfect sphere, the most pleasing of mathematical forms. This model was later endorsed by Plato, Aristotle and Archimedes, based on more practical evidence. A flat earth, for example, could not explain how the Pole Star is seen lower in the sky as one travels further south, but a curved surface could. They had even gone so far as to make crude guesses about the size of the earth. For instance, Aristotle had cited a value for its circumference of 400,000 stadia, while Archimedes estimated 300,000 stadia. Since one stadion equates to roughly a tenth of a mile, these figures correspond to 40,000 and 30,000 miles, respectively – not far off the correct value of just under 25,000 miles. Even Plato, whom I do not regard as having been as good a scientist as either Aristotle or Archimedes, provides a remarkable description of our planet as a large sphere floating in space: ‘First of all the true earth, if one views it from above, is said to look like those twelve-piece leather balls, variegated, a patchwork of colours, of which our colours here are, as it were, samples that painters use.’9 Not only did Plato know that the earth was spherical but his description of its surface as having a ‘patchwork of colours’ evokes the images we are so familiar with today of our planet viewed from space with its weather patterns swirling above seas, deserts and snow-capped mountains.
As for its size, another Greek scholar decided he could go one better than educated guesswork. He believed he could actually measure it. His name was Eratosthenes (c. 275–195 BCE) and he was the chief librarian of Alexandria, as well as being a brilliant astronomer and mathematician. His method for working out the size of the world was, like so many great ideas in science, beautifully simple: if he could measure the distance along the surface of the earth corresponding to just one of the 360 degrees around its circumference, then all he would have to do is multiply this distance by 360.
He knew that on the longest day of the summer solstice the midday sun shone vertically down to the bottom of a well in Syene (modern Aswan) in southern Egypt. But on that same day each year, he had observed that the sun was not directly overhead in Alexandria in the north; instead, its rays shone down at an angle equal to one-fiftieth of a full circle (that is, one-fiftieth of 360 degrees, or 7.2 degrees). He assumed that Syene was directly south of Alexandria, so if he knew the distance between the two cities, then multiplying this distance by fifty would give him the complete circumference of the earth. There are no details about exactly how this was done but apparently he had someone walk from Alexandria to Syene, counting paces! The distance reported back to him was precisely 5,000 stadia (about 500 miles). This gave a value of 250,000 stadia, or 25,000 miles, for the circumference of the world – a value so close to the modern measurement of 24,900 miles that it would seem churlish and pedantic to find any fault with it.
But the truth is that Eratosthenes was very lucky to have got so close. There were a number of serious errors, inaccuracies and crude guesses involved in his method that conspired by chance to give an answer close to the correct one. While the midday sun at the summer solstice is indeed directly overhead at the Tropic of Cancer, the city of Syene was not on the tropic but about 22 miles north; nor was it exactly due south of Alexandria. Most importantly, it would not have been possible to measure the distance between the cities with any degree of accuracy at all. Counting paces would have been unreliable and the path taken would in all likelihood have followed the meandering course of the Nile, including the complex Delta region around Alexandria. Lastly, we do not know the exact length of his unit of distance (the stadion); I said ‘a tenth of a mile’, but this is rather approximate. In any case, the fact that the number of paces came to exactly 5,000 stadia is suspicious and most modern historians do not believe Eratosthenes ever did have the distance measured in this way but had unwittingly used instead a value for the distance that itself had been calculated from an even earlier estimate of the earth’s circumference;10 a sort of circular logic whereby an estimate of the earth’s circumference is used to deduce a distance that is then itself used to recalculate the circumference.
And so we move forward in time a thousand years to Abbāsid Baghdad and the band of astronomers working for al-Ma’mūn. They knew about Eratosthenes’ method from the writings of Ptolemy. In fact, Ptolemy quoted a later, revised but incorrect value for the circumference of the earth of just 180,000 stadia by another Greek astronomer, by the name of Posidonius.11 Ten years after his arrival in Baghdad, al-Ma’mūn wished to
know what all this meant: exactly how long was one Greek stadion? No one could agree. This called for another project for the Companions of the Verified Tables.
Al-Ma’mūn dispatched a group that included his top astronomers, Sanad, Yahya, al-Jawhari and Khālid al-Marwarrūdhi, along with carpenters and metal-workers, to the north-west corner of Iraq in the flat plains of Sinjar, about 70 miles west of Mosul. There, the group split into two teams who headed out in opposite directions, due north and due south, counting paces as they went and placing arrows in the ground as markers along the way. They stopped when they had measured a 1-degree angle of the earth’s curvature based on the positions of the stars. Both groups then turned around and re-measured the distance back to base. The average of the measurements was taken and found to be 56.6 Arabic miles. We know that an Arabic mile, or mīl, is about 20 per cent bigger than our modern-day ‘statute’ mile, so this distance is actually about 68 miles. Multiplying this number by 360 gives a figure of 24,500 miles, which is a more reliable figure than the one arrived at a thousand years earlier by Eratosthenes.
Good scientist that he was, al-Ma’mūn then commissioned another expedition to carry out a second measurement, this time in the Syrian desert. Starting from the city of Palmyra in central Syria, his astronomers measured the distance to the city of Raqqah to its north. They found the two cities separated by 1 degree of latitude and 66.6 mīl, giving a larger circumference of 24,000 mīl, or 28,700 statute miles.
Of course, while the whole project is admirable, all these numbers just added to the confusion. Everyone seems to have been in the right ballpark and it is probably pointless trying to credit those who arrived at the closest value. Al-Ma’mūn’s astronomers will have had to contend with the same issues as Eratosthenes. For instance, al-Raqqah is in fact about 1.5 degrees of latitude north of Palmyra as well as being about a degree of longitude to the east. In any case, the true distance between the two cities is more than 100 miles (around 90 mīl). Many historians and geographers throughout history have quoted these numbers, none of them having an accurate idea of the exact length of a stadion or a mīl. Even Marco Polo and Christopher Columbus used them, but apart from confusion over units (these explorers were unaware of the difference between a Roman and Arabic mile), they were often unwittingly quoting al-Farghāni quoting Ptolemy quoting Posidonius quoting Eratosthenes.