A medieval computer

In a little more than a month, Manchester (UK) will host the largest conference in the history of science, technology and medicine in recent years. The 24th International Congress of History of Science, Technology and Medicine will take place over a whole week (21-28 July) and already promises to be a memorable event. The Congress meets every four years around the globe (last time it was in Budapest, and before then in Beijing), so it is particularly exciting to see it happening in the UK (although, sadly, I won’t be able to attend it!). The organisers have set up a website with a wealth of information: along with the programme (which is searchable, of course), there is a terrific blog where participants at the Congress can showcase their own research. The piece below is one such example, and a particularly exciting one at that: it is written by Seb Falk, a PhD student at Cambridge University currently working on a unique medieval manuscript, The Equatorie of the Planetis. The text describes a ‘medieval computer’ (i.e. calculating device) that could be used to predict the position of the luminaries and the five known planets. Among other things such a device may have assisted the medieval astrologer when casting horoscopes.

Seb has kindly agreed to have his post re-blogged here, so it is appended below for your enjoyment. His paper at the iCHSTM is part of a series of exciting panels on pre-modern astronomy and cosmology. There is, of course, a wealth of other relevant papers in the history of astronomy/astrology that one could attend at the iCHSTM: on the theory and practice of Ottoman sundials, on astronomical data in early modern conjunctionist astrology, and on the role of astronomy in ancient Egyptian funerary culture, to name just a few! No shortage of fascinating talks for those interested in the science of the stars! You just have to put ‘astronomy’ or ‘astrology’ into the programme search engine and … Puff! They all magically appear! So, if you are in the Manchester area (or further afield but a little adventurous) consider checking them out!

Seb’s paper promises to be particularly exciting as you will be able to see the real thing, a reconstructed equatorium! And if you want to know more about his project, you can check out his own blog, Astrolabes and Stuff. You can even follow Seb on Twitter, both during the conference and after. His Twitter name is @Seb_Falk (the Congress itself has its own Twitter account: @ichstm2013, and so have many other participants).

If you attend the iCHSTM have fun, and let us know what were the highlights!


How to cast a medieval horoscope, by Seb Falk

In preparation for iCHSTM 2013, I’ve spent the last few weekends indulging my creative side.  Sawing and filing wood and brass into a disc, ring and pointer may have disturbed the peace of my neighbours’ Saturday afternoons, but it has meant I will be able to demonstrate a particularly ingenious, user-friendly medieval device: a planetary equatorium.I have recently begun PhD research into a unique fourteenth-century manuscript.  Known as The Equatorie of the Planetis, it describes how to construct an equatorium.  This makes it one of the earliest pieces of writing about a scientific instrument in the English language.  The first person to study it, Derek de Solla Price, was convinced not only that it was written by Geoffrey Chaucer, but that it was a draft in Chaucer’s own handwriting.  The authorship debate still rages; meanwhile, I am looking at some of the other fascinating aspects of this manuscript.

Much like their better known cousins, astrolabes, equatoria were medieval calculating devices.  These devices made use of astronomical theories and models that were long-established, having first been refined around 150 CE by the Greek astronomer Ptolemy.  In both cases, they existed in something close to their complete form in the late Classical period, before being further developed in the Islamic world from around the tenth century, and refined still further in western Europe between the thirteenth and sixteenth centuries. While astrolabes could be used for a range of functions, from telling the time to measuring the height of a building, equatoria just did one thing: modelled the motions of the planets.

They did this by recreating the essentials of Ptolemy’s planetary theories as a kind of diagram with moving parts.  These became progressively simplified, so that a single device could model the motion of the Sun, the Moon and the five known planets.  After an initial investment of time making his equatorium, an astronomer could then predict the location of the planets to a high degree of accuracy, far faster than by the alternative method – trigonometric calculation.  Using this basic computer, planetary astronomy could be as simple as looking up a couple of values in a table, and using them to place some pieces of brass, wood and string.  The question is: why?

For early modern astronomers such as Johannes Schöner, who included cut-out-and-build equatoria in his 1521 Aequatorium Astronomicum, they had a largely educational purpose: they could be used to demonstrate the fundamentals of the Ptolemaic theories, just as many classrooms today use globes (another favourite device of Schöner’s) to teach children about latitude and longitude. [I’m no longer so confident about this claim: Schöner’s equatoria could be used for practical astrology, though it’s hard to be sure that they actually were.]

But equatoria also had practical importance.  Although nowadays we are dismissive of astrology, and think of horoscopes as a simple matter of making (up) predictions about people’s future fortunes based on the month of their birth, it wasn’t always that way.  In the medieval period there was no hard distinction between astronomy and astrology, and the calculations that could be made using personal and planetary information were complex and varied.  They had a range of possible uses, too, guiding anything from political decision-making to the timing of medical procedures.

In the case of The Equatorie of the Planetis, the simplifications made by its designer make it less suitable as a demonstration device, but much easier to make, transport and use to calculate planetary positions.  The designer has shown great imagination in paring the instrument down to its bare essentials.  It could be argued that by simplifying the Ptolemaic model, he demonstrated a lack of understanding and precision, but I think it is the reverse: he showed great sophistication in understanding where approximations could be made for the sake of greater usability, without sacrificing too much accuracy.

It’s sometimes suggested that these medieval “instruction” texts were not really designed to be followed except in the reader’s imagination. Certainly it’s true that it would be expensive and rather unwieldy to make it at its full six-foot scale! (Though that is precisely what Derek de Solla Price did in 1952.)  But with my newly built equatorium I’m looking forward to showing people at iCHSTM that these six-hundred-year-old instructions can be followed to produce a user-friendly, and useful, little computer.

PS. I have modified my views slightly since writing this, mainly about how sophisticated an astronomer the equatorium’s creator was, and how sure we can be about Schöner’s purposes.  I’m looking forward to discussing these issues with people at the conference.
This blog post is based on the paper , “Putting classical astronomy to work: the design and use of a medieval equatorium,” which [I am] due to give as part of symposium T157, “Pre-modern astronomy and cosmology,” on Saturday 27th July at iCHSTM.



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