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  • Writer's pictureSeb Falk

How to cast a medieval horoscope

Updated: May 13, 2020

I wrote this post for the blog of the 24th International Congress of History of Science, Technology and Medicine (iCHSTM), which takes place in Manchester on 21-28 July 2013.  Loyal readers of this blog won’t find much new here, but it’s a fair summary of my research so far.

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.

Posted on 20 June 2013 by admin

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.

The equatorium nears completion

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: they 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.

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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