Post 48: Science & Islam: Astronomy

Book CoverA few weeks ago I posted a review of the book Science & Islam by Ethan Masood, a tie in with a BBC series from about five years ago. I felt that the book was a bit of a let down, but loved the topic and wanted to read more on it. As a follow up I then wrote a post giving a bit more detail on the Islamic contributions to Mathematics. I wouldn’t pretend that I’m a better writer than Masood, but I wanted to focus a bit more on some of the techniques and details of the work than he did. I do have a scientific background, but I don’t want to make this into a science blog so I’m attempted to strike a bit of an awkward line between the history and the science. With that introduction/disclaimer out of the way – here’s a short summary of the Islamic world’s contribution to Astronomy.

Looking at it now, Mathematics may be the headline grabbing topic for the Islamic golden age but Astronomy (and its unfortunate and misguided relative Astrology) were at least as important. Not only did they provide the motivation for a lot of the work in mathematics and physics, but they also did a lot of very underrated work in moving the topic forward from its ancient roots towards the early heliocentric model of Copernicus. Islamic scholars invented technologies like the astrolabe, published tables of data that later scientists would draw on, and worked out a lot of the mathematical difficulties for the later models. Unfortunately, the political and educational system in the Islamic world meant that they weren’t fully able to capitalize on this; the wonderful observatories were only ever short term institutions and the whole thing stagnated around the turn of the sixteenth century.


Why was the Islamic world so interested and successful in this topic? Well, there were several motivations that could be picked out: improved navigation by the stars in the deserts of Arabia, astrological predictions for wealthy patrons, better calculations of the direction to Mecca and the timing of prayers. These are all certainly reasons why patrons funded the work, but it’s all a bit superficial – the more complex timings for prayer never caught on, and astrology didn’t exactly required all this intellectual effort. Fundamentally it probably comes down to the usual reasons for pure scientific research – a quest for prestige and knowledge for the sake of knowledge.

Astronomy before Islam

The work on astronomy didn’t spring from nothing, as with mathematics the scholars of the Islamic world put great effort into tracking down and translating works from the Greeks, Romans and Indians. Key amongst these was Claudius Ptolemy, a Greek-Egyptian philosopher who lived in Roman Alexandria in the early second century A.D. He developed what became know as the Ptolemaic system – a geometric model to describe the movements of the planets. This is the classic medieval set of spheres with the earth at the centre, moving out through the sun, moon and planets and finishing with a fixed sphere of stars. Previous Greek philosophers had also worked with a similar model, but Ptolemy refined and recalculated it based on centuries of measurements. He also set up a format of data tables that would be used for centuries to come. Basically, despite his flaws, he formalized much of Astronomy.

AstrolabeThese flaws did not go unnoticed and much of the world in Islamic astronomy would be to find and correct these errors. In fact, the great scientist Al-Haitham (previously mentioned in my Maths post) would write a book called “Doubts Concerning Ptolemy” discussing the contradictions in Ptolemy’s work. For one thing his mathematical models and his suggested orbits for the planets did not quite match up, the Greek was aware of this and tried to explain it away as relatively minor but Al-Haitham was doubtful – no amount of hand waving would make him ignore these errors and these mathematical orbits didn’t seem to make sense physically. Despite these doubts, one shouldn’t get the impression that the old work was dismissed; the ancient astronomers were greatly respected and the progress was seen as building on what had gone before, not tearing it down.

The other great work of pre-Islamic astronomy was Zij al-Sindhind, a great set of astronomical tables from India that was brought to the court in Baghdad and translated in the eighth century. This translation was by Muhammad al-Fazari and his father Ibrahim al-Fazari (who we’ll come back to later when discussing the astrolabe). These tables were said to be compiled by the great Indian mathematician Brahmagupta, and he added insights to explain the illumination of the moon by the sun, the timing of eclipses and the fact that the earth is spherical (although, some of these had already popped up in Greek works). The development of trigonometry began to play a big part in the use of these tables to calculate and predict the movement of the planets, spurring on both the Golden Age of maths and of astronomy.

Measuring the stars

Before the likes of Al-Haitham would come up with a truly new development in the planetary model, the muslim scholars would first get to grips with taking data and keeping records on the stars. The work of their Greek and Indian predecessors would provide the format for this, but the great observatories that sprung up at Baghdad, Cairo, Damascus and Samarkand would rival these. The results and revised calculations from these would not just spread in the Islamic world, but also in Christendom – providing an impressive set of foundations for later scientists to work with. It’s hard to stress just how useful this data could be, even if it was taken with an inferior model in mind – one account by a young physician Ali Ibn Ridwan describes a strange event in so much detail that nowadays it can be clearly identified as a super nova.

“I will now describe a spectacle [the Supernova 1006] which I saw at the beginning of my studies. This spectacle appeared in the zodiacal sign Scorpio, in opposition to the Sun. The sun on that day was 15 degrees in Taurus and the spectacle in the 15th degree of Scorpio. This spectacle was a large circular body, two and a half to three times as large as Venus as large as Venus. The sky was shining because of its light. The intensity of its light was a little more than a quarter of 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. …
Because the zodiacal sign Scorpio is a bad omen for the Islamic religion, they bitterly fought each other in great wars and many of their great countries were destroyed. Also many incidents happened to the king of the two holy cities [Mecca and Medina]. Drought, increase of prices and famine occurred, and countless thousands died by the sword as well as from famine and pestilence. At the time when the spectacle appeared, calamity and destruction occurred which lasted for many years afterwards .”

Ignore the stuff about the zodiac for now, the information here is presented concisely, clearly and neutrally despite the author having little idea how to explain it. It’s a perfect model for how one should record things.

How did they take these measurements? At this point we’ll bring back Mo al-Fazari, one of the translators of the ancient Indian texts. He also introduced a greek instrument, the Astrolabe, to Arabic science. This had many uses (Al-Sufi listed over 1000 – I haven’t seen the list, but you do have to suspect that paperweight and door stop are going to be in there somewhere) and it does have that Swiss army knife look to it. Basically it could be used for measuring angles – thus it could be used to find distances, positions, time and anything else. The basic device would be added to with more elaborate designs and even mechanisms. Other tools such as the Quadrant (a variation on the astrolabe also used for measuring angles), the Planisphere (a kind of star chart) and the Equitorium (a tool for geometrically calculated planetary positions) were also used and improved during this period. They’re fascinating things and well worth reading about in more detail(or seeing in places like the British Museum or the Oxford History of Science Museum). Somehow, however, they never quite reached the telescope stage despite Al-Haitham in particular performing a lot of work in optics that would lay the ground for its later invention.

From Ptolemy to Copernicus

Tusi CoupleAs I said in my introduction, doubts were growing about the old Ptolemaic model and, although the new sets of measurements could help to refine a few of the parameters in it, the whole thing was looking a bit shake-y. New ideas were required. No one went quite as far as to ditch the entire thing, and the new models remained geocentric, but the issues were discussed in detail and some scholars (like al-Biruni) even began to suggest that the earth could in principle be moving. One of the obvious problems with Ptolemy was his equant – a sort of offset added into his model so that he could use nice, uniform circles like everyone supposed the orbits to be. Many of the great astronomers tackled this in their own way, producing models with some way or other to remove this. Al-Haitham (of course), Nasir al-Din al-Tusi and al-Shatir were all notable names in this. As the former has had more than enough attention already, I’ll give a bit of a run through the other two.

Al Shatir's ModelThe first of these, al-Tusi came up with a nice geometric technique, now called a Tusi Couple. In this, a point on the edge of a rotating circle (that is itself rotating) can appear to be moving linearly. This allowed al-Tusi to take Ptolemy’s complex system of epicycles, more or less fix the equant problem and make the whole thing a lot neater. And it would later allow Copernicus to do away with the system altogether. In this, he also argued for the possibility of a rotating earth – he wasn’t necessarily in favour of the idea, just insistent that the thing was yet unproven one way or the other. His arguments here would also find their way into Copernicus.

The second, al-Shatir, improved al-Tusi’s model further; making it work for all of the planets and finally producing results that were dramatically better than Ptolemy’s. He also argued against Aristotle’s idea of an ether (it would be a while before the whole ether business got solved). His whole project was getting closer and closer to modern science – he had a physical theory with a mathematical model and observable evidence to support it! Another scholar named ali Qushji joined him in this, explicitly arguing for a study of astronomy that was based on empirical evidence rather than the grand philosophic theories of the ancient Greeks (and Aristotle in particular).

Things were going so well, but unfortunately that was more or less that for Islamic astronomy. From this high point in the late fifteenth century, the focus shifted to Europe as the renaissance scientists began to truly revolutionize and formulate the study of the world around us. However, this shouldn’t be a reason to ignore this work and the great flourishing of astronomy under Islam – I’ve been pushed for space (no pun intended), so there’s less biographical details here than previous posts, but it really does make for a fascinating subject. Chemistry (and the strange world of Alchemy) is up next, so look out for another post soon.


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