String Theory – Medieval-style
Updated: May 12, 2020
Apologies for the long hiatus! I’ve been busy with lots of things, among them the birth of my first child. I’ll post his horoscope here soon! But first, here’s a few thoughts I’ve been mulling over for a while…
My research centres on scientific instruments. I study descriptions and images of them, attempt to follow the instructions to make and use them, examine the instruments themselves, and sometimes review or help curate exhibitions of them. With all this, I spend a lot of time thinking about materials.
When we think about astrolabes, we tend to think of the shiny brass objects that are most common in museums. Lots of attention has been paid to the kinds of metals that were used, and how they were shaped. (Recent increases in the affordability of analytical technology means that this area is ripe for new discoveries – and I hope to blog soon about some research I’ve been involved in.)
But there are other materials too. You don’t have to think about this subject for very long to realise that the ornate astronomical instruments now on display in museums were probably not the same ones used for practical navigation at sea: sailors would have taken advantage of simpler designs and cheaper materials, principally wood. Nor were they the same ones used for study in the new universities: teaching and learning took place with instruments made of parchment or paper.
But one material is rarely mentioned in the scholarly literature: string. This is despite the fact that it appears in many descriptions of instruments, and a good number of the surviving examples too. The image on the right shows the equatorium for Jupiter, from Peter Apian’s Astronomicum Caesareum (1540). This sumptuous work, dedicated to Emperor Charles V, is hardly an ordinary equatorium treatise, but in its use of string it is entirely typical.
Before being incorporated into astronomical compendia like this, threads had been used for centuries in practical surveying instruments such as quadrants. The easiest way to measure an angle, such as the height of a building, was via a plumb-bob (a lead weight hanging on a string) that could move over a circumference marked on a brass or wood quarter-circle (see the image below).
It’s a small step from that to the use of threads as pointers, to read angles on scales on the circumference of more theoretical instruments. It’s easy to see why this step was taken: they were flexible, easy to attach, use and replace; they were narrow and thus relatively precise. And crucially, of course, they were cheap.
How cheap? Of course that depended on what the threads were made of. Sadly most instrument treatises are silent on the subject of what kind of string to use, where to find it and how to cut it. So we have to assume that instrument makers used the cheapest thing they could find, or whatever was to hand. This probably meant threads made of hemp or flax.
Unusually, though, the Equatorie of the Planetis (the manuscript that’s the focus of my research) does talk about materials. (One of the reasons I find it so fascinating is that it goes into many of the practical details that are absent from most medieval scientific treatises.) It says:
Note that every centre [of each planet’s equant circle] must be also small as a needle, and in every equant must be a silk thread.
Why silk? Was silk finer than other threads, and thus more precise? Was silk in this context meant metaphorically, and the writer really just wanted some very soft and flexible thread? Was it included to give a sense of luxury or importance to the astronomical work, as if only the finest materials were suitable for the tasks undertaken? Or is the whole thing a flight of fancy, in which the writer was indulging his imagination in describing an instrument that he had no intention of making?
I’m not sure, but I’d like to think there is a practical reason. After all, the Equatorie of the Planetis design also includes a revolving metal pointer, which was the standard device used on astrolabes. A metal pointer was needed on the brass epicycle because the radius of each of the planets needed to be marked at the appropriate point along its length. But on the face of the equatorium, with an equant centre for each planet, something more manageable was required. So we can see the designer of the equatorium choosing appropriate techniques and materials at each stage, with the clear goal of producing a user-friendly, effective planetary computer.
And what difference would a material other than silk, say flax, hemp or polypropylene, make? I don’t know, but I’m starting to think another reconstruction experiment is required!