Equatorie of the Planetis

28 May saw the launch at the Whipple Museum of the online Peterhouse Manuscripts Collection, housed in the Cambridge Digital Library. The collection aims to present highlights from the College’s collection of 276 medieval manuscripts, and will be developed as time and funding allow. High-quality images are presented alongside searchable transcription, commentaries and critical apparatus, making the Peterhouse manuscripts accessible to scholars around the world. Initial work on the collection has been made possible by generous funding from donors to the College, particularly Dr Joe Pesce.

The launch focused on the first manuscript to be digitised, the fourteenth-century Equatorie of the Planetis (MS 75.I). This manuscript has been at Peterhouse since at least 1538, but it was first brought to the world’s attention in the 1950s by the historian of science Derek (de Solla) Price. Price was a PhD student, conducting research into “the history of scientific instrument making”, and came to the Perne Library expecting to examine an unexceptional astrolabe treatise. He found something quite different, as he later recalled:

As I opened it, the shock was considerable. The instrument pictured there was quite unlike an astrolabe – or anything else immediately recognizable. The manuscript itself was beautifully clear and legible, although full of erasures and corrections exactly like an author’s draft after polishing (which indeed it almost certainly is) and, above all, nearly every page was dated 1392 and written in Middle English instead of Latin. [Science Since Babylon, enlarged edition, 1975, 26-27]

Peterhouse MS 75.I, f.74r

What Price saw, one cold December day in 1951…

Price realised straight away that the manuscript might be by the poet and astronomer Geoffrey Chaucer (c.1343-1400), whose Treatise on the Astrolabe, probably written in 1391, is a very early example of scientific writing in English. He quickly changed his PhD to focus exclusively on this manuscript, and the resulting thesis (published in 1955) included an edition and translation of the instrument treatise that takes up nine folios of the manuscript (alongside seventy folios of astronomical tables).

The instrument Price could not at first identify turned out to be an equatorium, a device designed to compute the positions of the planets. Few equatoria survive today, but they were popular tools of astronomy and astrology in the later middle ages. They were based on the models of planetary motion explained by the Greek astronomer Claudius Ptolemy (c.90-c.168) – essentially three-dimensional diagrams with moving parts. Medieval astronomers took pride in adapting and refining their predecessors’ designs, and the Peterhouse equatorium, whose construction is explained in detail in the manuscript, represents an improvement on the equatoria of notable astronomers such as Campanus of Novara (c.1220-1296) and Richard of Wallingford (1292-1336). Because the manuscript is a draft, we can see the author-translator working out and refining his ideas, learning new techniques and devising improvements as he goes.

Price decided to build the equatorium, following the manuscript’s instructions. In an era when historians favoured intensive textual scholarship and did not particularly value reconstruction, this was unusual. So why did Price do it? The answer perhaps lies in his biography. He was from a working-class, Jewish background in the East End of London, and had taken his first PhD in metal physics at the South-West Essex Technical College in 1946. He came to Cambridge from the University of Malaya, where he had been teaching applied mathematics. He arrived in Cambridge in 1951, the year that the University set its first exams in History of Science, and the Whipple Museum of the History of Science opened. The discipline of history of science was in its infancy, and scientists and historians were competing for authority as its boundaries were laid out. In this context, Price clearly felt he needed to establish himself; the publicity surrounding the discovery of a manuscript that might be written in the hand of Chaucer allowed him to do that. Price had worked in the Cavendish Laboratory, helping organise its archives and historic apparatus, and had a good relationship with the Cavendish Professor Sir Lawrence Bragg. Bragg helped him organise a full-scale model of the equatorium to display at an event at the Royal Society in 1952. The model, pictured below, is now at the Whipple Museum. An account of its construction, later loss and rediscovery, has just been published in the Royal Society journal Notes and Records, written by current Petrean Seb Falk.

Price's equatorium, Wh.3271, Whipple Museum

Equatorium made at Cavendish Laboratory for Derek de Solla Price, 1952. Now at Whipple Museum of the History of Science, Cambridge (Wh.3271).

Now, though, another model of the equatorium has been made – but this one is virtual. Produced by programmer and designer Ben Blundell, in collaboration with Scott Mandelbrote and Falk, the model is embedded in the Digital Library website alongside the manuscript. It allows users to gain the full experience of using the equatorium, giving results for the longitudes of the planets very close to those achieved by modern astronomical computation. In order to produce the model, Blundell needed to create his own calendar that transitioned seamlessly between the Julian and Gregorian systems, and to write new programming language to simulate movement of the equatorium’s silken threads!

Virtual equatorie

Virtual equatorie created by Ben Blundell for the Peterhouse collection at the Cambridge Digital Library

It is hoped that visitors to the website will gain a new understanding of how the equatorium works and might have been used. It is based on a simplified version of Ptolemy’s planetary models, ignoring the planets’ motions in latitude, and by scaling the parameters of the different planetary models to give them all equally sized deferent circles, their motions in longitude can all be modelled on a single disc. A single epicycle is used, its radius corresponding in size to the common deferent radius; a rotating rule is fixed at its centre and marked with the radii of the planets’ epicycles, which are thereby traced out as it rotates. The longitudes of the planets are found by taking easily calculated linear components of their motion from pre-prepared tables, and transferring those values to the equatorium by laying threads on the scales on the circumference of the disc and epicycle. (For more information, see the explanation on the Digital Library website, and try the model there!)

Study of the manuscript has not been confined to its technical content. At the launch, Professor Kari Anne Rand explained how linguistic and palaeographic evidence has been used to locate the manuscript’s production to the periphery of London, and to cast doubt on its attribution to Chaucer. She showed how certain characteristic features of the scribe’s practised, informal hand appeared in another manuscript that she has found, raising the possibility that an alternative candidate for the authorship of MS 75.I may soon be identified.

BL Burney 275 f390v

Detail from British Library MS Burney 275, f.390v (early 14th century). The illustrator of this copy of Ptolemy’s Almagest clearly had some understanding of the use of astronomical instruments.

Whoever wrote the manuscript was part of a thriving astronomical culture, based in but not restricted to the growing universities of Oxford and Cambridge. Instruments like this equatorium were used not just for astrology, or to model the movements of celestial bodies with greater ease, but as a route to greater comprehension of the cosmos. As the picture above indicates (and as Dr Catherine Eagleton reminded us at the launch), devices like astrolabes were familiar features of literate culture. The equatorium was undoubtedly a more complex device but, as the references to the Treatise on the Astrolabe in the Equatorie suggest, it might be a suitable next challenge for someone who had already mastered that more commonplace tool. If the fox could learn from nature with the help of his astrolabe, so too could the medieval English readers who, for the first time in the Peterhouse manuscript, had the opportunity to learn about equatoria in their mother tongue.


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