Developed in the Islamic world from the eleventh century CE and refined in western Europe between the thirteenth and sixteenth centuries, equatoria were material instantiations of the principles of Ptolemaic planetary astronomy. Although they are often categorised as astronomical instruments akin to the more familiar astrolabes, they represented a more complex planetary science: they were not normally used for observation but functioned either as computers or demonstration devices: either simplifying astronomical theories for practical solutions or providing manipulable diagrams for educational purposes.

Few equatoria survive; most are known from treatises that describe their manufacture and use. Among the best known of these is an almost complete fourteenth-century work that survives in a single manuscript copy, bound with a set of astronomical tables modelled on the thirteenth-century Alphonsine Tables. The manuscript was first studied and named The Equatorie of the Planetis by Derek de Solla Price, who attributed it to Geoffrey Chaucer, but its provenance cannot be known with certainty.

What is clear is that its author was an accomplished practical astronomer: he was able to manipulate Ptolemaic principles to produce an instrument of impressive clarity and usability: a single large disc, open ring, moving pointer and set of threads allow the calculation of the locations of the sun, moon and planets with great accuracy. The theories have been pared down to ensure that this calculator is relatively simple to produce and still easier to use, saving its user a great deal of time that would otherwise be taken to compute planetary positions geometrically.

I will explain how the author improved upon earlier simplifications such as the “epicycle tail” model to produce an instrument that, while not without its limitations, provides an outstanding balance of simplicity and accuracy. I will argue that, though it would be unwieldy, the instrument described in the manuscript was certainly designed to be made, and was not simply a thought experiment; I will use a model I have constructed according to the instructions in the manuscript to show this. This assemblage would not have served particularly well to demonstrate Ptolemaic principles; what it was was an intensely user-friendly computing device. I will discuss the various purposes to which it might have been put.