by Ehsan Masood
We know that observatories were popular with many more of Islam’s caliphs and rulers. We also know that most observatories did not qualify for financing under Islam’s special system of religious endowments, which helped to pay for mosques, schools, colleges and hospitals. As a result, the vast majority of observatories did not survive beyond the lifetime of their founding patron. Whereas mosques, hospitals, universities and schools built during the Islamic era survived many centuries, the longest-lasting observatory had a working life of just 30 years. In almost every case, once a patron died, it wouldn’t be long before his observatory would follow him to the grave.
Such a relatively short life also suggests that, while observatories were undoubtedly important in the conduct of religion, they were not seen as essential or critical for the conduct of faith – at least not in the same way that a mosque or a hospital might have been. Another reason for this is their use in astrology.
Historians of Islamic astronomy, such as the late Aydin Sayili from Turkey and David King, agree that the needs of faith did help to drive astronomy. At the same time, there is no doubt that, as far as rulers, governors and caliphs were concerned, astrology was an important motivation for their funding of, and interest in, the work of astronomers. In some ways, this interest in astrology was similar to their desire to fund the translation of Greek works of astrology into Arabic.
In contrast to its relationship with modern science today, astrology during the Islamic middle ages (as in much of the Latin West) was regarded by the ruling classes as an important application of astronomy, or as applied astronomy. The argument went that if the moon could influence tides, then surely it should be possible for planets to influence other physical things, as well as events and people.
A few observatories were based inside or close to royal palaces, and most of the leading astronomers were often asked for astrology-based advice by their rulers. This included advice on political appointments, impending wars and invasions, as well as advice on who (and when) to marry. This meant that astronomers and astrologers were very powerful people, and had the ability to influence royal decisions.
The astrolabe
One of the early astronomers was Ibrahim al-Fazari. He was either a Persian or, judging from his name, an Arab who learned his craft in Persia. He clearly knew his astronomy, for under the direction of the Caliph al-Mansur (754–75 CE) he worked with his son to translate the ancient Indian astronomical text by Brahmagupta which came to be known as the Sindhind. The Muslim willingness to learn from other astronomical traditions is part of the reason for their extraordinary success. But the translation of the Sindhind was valuable not only for its astronomical insights – it is thought that it also brought Indian numerals into the Arabic world for the first time; a task later completed by al-Kharwizmi, who also produced an improved version of the Sindhind.
Under the instructions of Caliph Harun al-Rashid, al-Fazari also made the first known astrolabe in the Islamic world. In the hands of the Arabic craftsmen and astronomers, the astrolabe became one of the most beautiful scientific instruments ever made. It was not only the wonderful craftsmanship that made these brass mechanical computers such attractive objects; it was the increasingly intricate and precise design that meant that they were the medieval equivalent of a GPS unit. The astrolabe was a model of the universe that you could hold in your hand. By using it to measure the angle of stars and the sun above the horizon, it could tell you anything from your current latitude to where stars will appear in the sky. It became the main navigational aid for many centuries, celebrated in Chaucer’s Treatise on the Astrolabe, until it was superseded by the simpler quadrant.
The coming of Ptolemy
It was just a few years after al-Fazari’s astrolabe, in the reign of Caliph al-Mamun, that astronomy really began to take off. The catalyst was the translation of a number of the works of the Greco-Roman astronomer Ptolemy. A Syriac version of Ptolemy’s al-Majisti or Almagest was followed by three Arabic versions, Arabic versions of his Planetary Hypotheses (which explained his theory of how the planets moved) and his Handy Tables for predicting the movement of the planets and stars. The impact of Ptolemy’s books was dramatic, and would shape the course of Islamic astronomy throughout the medieval period.
First of all, though, the demand was for up-to-date and accurate zij, the tables of celestial movements. New tables were needed for religious purposes, and as navigational aids. And so began a massive and never-ending project to produce zij based on both observations and recalculations. The astronomers who produced these tables could be found at all levels of society. They were employed by patrons, they worked in mosques, and many were enthusiastic amateurs.
The great skywatch
To get the new observations, rulers and wealthy patrons began to establish observatories. The first were set up in the 820s by al-Mamun in Baghdad, and on Mount Qasiyun near Damascus. Their task was to reconcile the data from the three different traditions – Persian, Indian and Greek. Thereafter, all new zij were based essentially on the Ptolemaic model of the Handy Tables. Other famous observatories were at Rayy (near modern Tehran), Isfahan and Shiraz. Over the centuries, observatories became ever bigger and more spectacular; partly no doubt for status reasons, but also to achieve greater and greater precision, with giant sextants and quadrants as big as artificial ski-slopes. The largest and most spectacular were at Maragha in Persia and at Samarkand – these obervatories were set up in the 13th and 15th centuries by invading Mongol and Turkic descendants of Genghis Khan and Tamerlane whose hordes invaded the eastern Islamic empire and took over its institutions. The Samarkand observatory, run personally by Tamerlane’s grandson Ulugh Beg, was the largest of all, and its great 130-foot radius arc can still be seen today plunging into the ground.
Using observatories like these, along with increasingly sophisticated calculations in spherical geometry and trigonometry, the Arabic astronomers gradually made more and more accurate measurements of the earth and the heavens. They calculated the tilt of the earth on its axis, reaching a figure that was remarkably close to the modern one, and refined the measurement of precession – the slow rotation of the earth’s tilt over nearly 26,000 years. They also calculated the circumference of the earth to be 24,835 miles (compared with current measurements of 24,906 miles), and measured how the earth’s furthest point from the sun shifted by a few seconds each year.
Islamic superstars
One observation in particular stands out. In 1006, a brilliant new star suddenly appeared in the night sky. A young astronomer in Cairo called ibn-Ridwan described this startling event with the precision that became the hallmark of the Arab astronomers:
The sun on that day was 15 degrees in Taurus and the spectacle in the 15th degree of Scorpio. The spectacle was a large circular body, two and a half to three times as large as Venus. The sky was shining because of its light. The intensity of its light was a little more than a quarter that of moonlight. It remained where it was and it moved daily with its zodiacal sign until the sun was in sextile with it in Virgo, when it disappeared at once.
So exact and full was this young boy’s description that astronomers can be certain today that what he was seeing was a supernova 7,000 light years from earth which they have named Supernova 1006, after the year when it was first seen.
Nearly all the great Islamic scholars contributed their ideas and observations to astronomy, from al-Khwarizmi and ibn-Sina (Avicenna), to ibn-Rushd (Averroes) and Musa bin Maymun (Maimonides). The zij tables of al-Khwarizmi and al-Battani were picked up in Spain by astronomers like Maslama al-Majriti in the 10th century, who not only updated them with his own remarkable observations but translated them into Latin, and so began the gradual process of the transmission of Islamic astronomical data and ideas into Europe.
Over the centuries, hundreds of zij were produced by Islamic scientists. On the whole, the new observations and more precise calculations meant that they became more accurate as time went on. And yet it was no
t simply a matter of getting better and better observations and calculations. Each time a new table was issued it was accurate for a little while, but sooner or later a discrepancy began to appear between the predicted position of the planets and their actual position. There was clearly a flaw in Ptolemy’s basic model, and as the centuries went on, this problem began to occupy Islamic astronomers more and more.
The Ptolemaic system
Very little is know about Claudius Ptolemy, beyond the fact that he was Greek and lived in Alexandria between 90 and 168 CE. Yet he wrote two profoundly influential works. One was his Geography, which became the standard atlas of the world for the next 1,300 years. The other was the Almagest. This work provided a complete model for the movement of the sun, moon, planets and stars that was developed over more than five centuries, but came to be called the Ptolemaic system. It was an entirely mechanical model, based on the scientific world view of ancient Greek thinkers such as Aristotle. It is all based on the rotation of perfect spheres, since nobody could contemplate any other shape for heavenly bodies until Kepler introduced the idea of less-than-perfect spheres in the early 17th century.
At the centre of Ptolemy’s system is the fixed earth. Around it rotates a vast sphere carrying with it, in a series of seven perfectly spherical layers, the sun and the moon, the five planets that were known at the time and, furthest out, the stars. As these transparent ‘crystal’ spheres rotate, they carry all the celestial bodies with them, so we see them moving through the skies. This was not simply meant to be an attractive theoretical picture of how things were, but a model for predicting the movements of these bodies precisely – and this is where it became complicated.
Matching theory with reality
With the earth fixed in one place, it’s difficult to get the observed movements of the heavenly bodies to match the model – particularly the movements of the planets. Unfortunately, only the stars move in perfect circles. It is bad enough that the sun’s path through the sky changes through the year. How could it possibly do that if it were simply swept around on the surface of a ball? The planets make even less sense, since their path seems even more variable than that of the sun. That’s how they got their name, which is the Greek word for ‘wanderers.’ In particular, the planets not only seem to shift a little further eastwards against the background of the stars each night; they also seem to loop back and travel westwards for a few months each year, a phenomenon called ‘retrograde motion’. Nowadays this looping back is easily explained by the fact that the earth is constantly overtaking slower-moving planets further out from the sun, and constantly being overtaken by faster-moving planets closer in. Yet if the earth is fixed, this motion is very hard to explain, and it caused astronomers problems for millennia.
Because of the model that was accepted at the time, the ancient Greeks had to think in terms of perfect circles and perfectly uniform motion. And yet they had to get their model of the spheres to match the observed movement of the planets, sun and moon precisely, or it would not work for prediction. Over the centuries, they gradually found answers to all these problems, or at least they seemed to. In the 3rd century BCE, Apollonius suggested that there are wheels within wheels. All the while the planets are going round in a big circle (the ‘deferent’), he suggested, they are also whirring round in a small circle or ‘epicycle’ too, like some celestial waltzer. A century later, Hipparchus then ‘explained’ the sun’s motion by suggesting that its rotation is eccentric – that is, its centre of rotation is slightly offset from the centre of the earth.
The problem is that these theoretical motions still didn’t match real-life observations. So with mind-boggling ingenuity, Ptolemy combined epicycles with doubly offset eccentric rotations for the planets around points called ‘equants’ to create a clockwork machine of immense complexity. Remarkably, though, it seemed to work, and its predictions were always pretty nearly right, which is the main reason why the Islamic astronomers took it on board so completely. But it was the ‘pretty nearly’ that eventually caused to them to begin asking questions. There was always that gradual slippage between the tables and the observations that meant that the tables constantly had to be updated.
Doubts about Ptolemy
Gradually, Islamic astronomers began to think that there might be problems with the Ptolemaic model. The model was meant to be a picture of the real world, describing how the celestial bodies actually moved. But the continual adjustments that it needed drew their attention to its basic conceptual flaws. Arab astronomers began to ask how some of Ptolemy’s epicycles and equants could work in the real world. Just as al-Razi had written his Doubts about Galen, so the great poly-math ibn al-Haitham (Alhazen) wrote his Doubts about Ptolemy (al-Shukuk ala Batlamyus). And just as al-Razi had simply raised questions, so did ibn al-Haitham. He focused on Ptolemy’s concept of the eccentric motions and equants, because he could not see how they could possibly be real. Real objects, he knew, simply did not move like that. A real sphere simply can’t rotate offcentre and yet stay in the same place. Yet ‘no motion exists in this world in any perceptible fashion,’ ibn al-Haitham argued, ‘except the motion of [real] bodies.’ There just had to be a central point about which everything rotated.
A few centuries later, in the 12th century, ibn-Rushd (Averroes) went further, declaring that:
To assert the existence of an eccentric sphere or an epicyclic sphere is contrary to nature … The astronomy of our time offers no truth, but only agrees with the calculations and not with what exists.
And if the calculations were beginning to look uncertain, too, then it was clear that the smooth clockwork of the Ptolemaic system was begin to rattle alarmingly.
Tuning the model
Over the next few centuries, Islamic astronomers began to make adjustments to Ptolemy’s model to try to make it conform with motion that was believable in the real world. Interestingly, no major astronomer ever really gave much time to the idea that the earth moved, even though it had been suggested – because it did not correspond to any real motion that they could imagine. The idea of the fixed earth at the centre of concentric spheres, on the other hand, certainly did.
So with as much ingenuity as the Greeks had shown, the Arab astronomers began to tinker and fiddle to get rid of equants and make all the celestial motions, as far as they could see, possible in reality. A key breakthrough was made by a brilliant astronomer born in the Persian Khorasan city of Tus in 1201. Nasir al-Din al-Tusi was born into a frightening time, when the forces of Genghis Khan were just beginning to spread across Asia.
By the time al-Tusi was thirteen years old the Mongols had dealt with China, and were soon storming west into Central Asia, generating horror story after horror story as they moved on towards the heartland of Islam. As they neared Tus, the young al-Tusi was sent away to Nishapur. Nishapur was not attacked at first, but he must have heard the devastating news that his home town had been ravaged by the Mongols. Nowhere, it must have seemed, was going to be safe, especially on the plains where the Mongol horsemen could ride easily. This may be why al-Tusi decided to take a job with the governor of Alamut, up in a secure mountain fortress. Alamut was the centre of power of the Ismaili branch of Islam, and al-Tusi made himself at home in the city, taking on the Ismaili faith.
The Khan’s astronomer
For 30 years, Alamut was a place of safety, and there al-Tusi devoted himself to the study of astronomy and mathematics, writing a number of important books which only reached Europe much later, including his radical rethink of Ptolemy. Even Alamut, however, would not be safe forever. In 1256, the Mongols arrived on the plains below the fortress under the leadership of Genghis Khan’s grandson Helagu, and soon managed to find a way into the impregnable fortress, perhaps by treachery.
Extraordinarily, al-Tusi not only survived the general slaughter but was taken on by Helagu as his personal astrologer. Not only that, but Helagu built for him the biggest, best-equipped observatory that had yet been constructed, at Maragha i
n Persia. It had the largest quadrant ever made, four metres across and made of solid copper, and a library that soon possessed some 400,000 books. Interestingly, the line of communication that opened up to China across the vast Mongol empire gave al-Tusi access to a whole new set of astronomical data and ideas, while Muslim astronomers who had been trained at Maragha trekked eastwards to create a new generation of observatories in China.
The Tusi Couple
Recalculations made at Maragha enabled al-Tusi to compile the most complete and accurate set of tables so far, known as the Zij al-Ilkhani after his patron. He also clearly established trigonometry as a separate branch of maths independent of spherical geometry, dramatically streamlining calculations about distances and directions in the heavens. But his greatest breakthrough was in finding a way to get rid of most of the equants from the Ptolemaic model and to replace them with believable uniform motion. He did this with an idea that came to be called the Tusi Couple.
The Tusi Couple was a way of showing how realistically uniform motion in circles can actually end up making something appear to move in a straight line. This sounds impossible, but it works like this: imagine a wheel rolling around the inside wheels of a drum. If the wheel is exactly half the diameter of the drum, then any point on the rim of the wheel will appear to move in a straight line across the drum.