The start of summer?

Facebook greeted me today with this little graphic:

Given that this is the radar picture over the UK this morning, it's not surprising that several Facebook users felt a certain irony.

But in what sense is it the first day of summer? As the Met Office explain in a lovely clear blog post, it depends whether you're talking about the meteorological summer or the astronomical summer.  For our forecasting friends, summer began three weeks ago on 1 June.  (Though as my wife often reminds me, summer in Ireland is often thought of as beginning on 1 May.)

But of course I'm interested in the astronomical calendar.  There, summer begins at the summer solstice.  That's today.  In astronomical terms, the solstice occurs when the Earth's axis is most inclined towards the Sun.  Or to put it another way, it's when the Sun stops in its passage north, pausing directly over the Tropic of Cancer before beginning its return journey back towards the south.

Since our calendar years are of unequal length (remember 29 February?), the solstice doesn't occur at exactly the same time every year.  Normally it's on 21 June, but this year, because it's a leap year, it's on the 20th.  To be precise, the moment of solstice will be at 23:34 BST tonight.  After that the Sun will head back south and the days will get gradually shorter.

Medieval astronomers understood this very well.  In his Treatise on the Astrolabe, Chaucer explains that

the Head of Capricorn [start of the zodiac sign of Capricorn] is the lowest point that the Sun goes in winter, and the Head of Cancer is the highest point that the Sun goes in summer. And therefore understand well that any two degrees [of the ecliptic] that are alike far from either of these two Heads, are of alike declination, be it southward or northward; and the days of them are alike in length and the nights also, and the shadows alike, and the altitudes alike at midday forever. (II.16)

This makes the connection between the Tropic of Cancer and the zodiac sign of Cancer quite clear.  (Not the constellation Cancer though; the Sun won't get there for another month, because of the precession of the equinoxes.)  Chaucer points out that any two places that the Sun passes in the zodiac, which are equally far from one of the Heads/solstices, will have the same length days (and nights).

Making use of shadows - if there are any.

They'll also have the same declination - that is, the Sun will be equally far from the equator (which it crosses at the equinox).  The Sun's declination, combined with its noon altitude, allows Chaucer to find his latitude.  I'm sure you've noticed that the Sun gets higher in the sky in summer.  If you measure its altitude at noon, subtract the Sun's declination at that date, and then subtract the whole lot from 90°, you have your latitude.

Because the Sun is higher in the summer than in the winter, a shadow cast by any object at noon will be shorter.  Astronomers (and even architects) had, since the ancient Greeks if not the Babylonians, been well aware of the usefulness of shadows for marking the seasons; for finding local time, the direction of north, and the direction of sunrise and sunset.

Chaucer wasn't shy of dropping his astronomical knowledge into his poetry, of course.  In the Canterbury Tales, the Merchant describes the weather a few days before the solstice.  It's rather better than today's:

Bright was the day, and blue the firmament;
Phebus [the Sun] hath of gold his streams down sent
To gladden every flower with his warmnesse.
He was that time in Gemini, as I guess,
But little from his declination
Of Cancer, Jove's exaltation [an astrological reference to Jupiter].

Today the Sun will set further north than any other day this year.  But I can't guarantee the weather will be summery enough for you to see it.

Geoffrey Chaucer's Inspirational Astronomy

OK, so I missed Andy Murray's Wimbledon semi-final.  And I missed seeing England hammer the Aussies in Cardiff.  But I didn't care.  Because I was incredibly excited to be at my first Chaucer conference.

The Biennial London Chaucer Conference took place at the Institute of English Studies last Friday and Saturday.  Given that the manuscript I work on has been (wrongly) attributed to Chaucer, and that the theme of this year's conference was "Science, Magic, and Technology", this was one conference I couldn't miss.

And I'm so glad I was there.  An inspiring series of presentations took hugely varied approaches to the theme.  Literary, historical or scientific, they were all fascinating and I learned so much.  If you want a flavour, check out the #Chaucer2015 hashtag on Twitter.  My cardboard astrolabe featured in the conference's most popular tweet!

You know it's an awesome #Chaucer & science conference when SOMEBODY busts out an #astrolabe. #medieval #Chaucer2015 pic.twitter.com/IUEn5iIvd9

— Jonathan Hsy (@JonathanHsy) July 10, 2015

If you want to know what I had to say about Chaucer and his influence on the Equatorie of the Planetis, why not watch the video of my presentation (above)?  Hope you enjoy it, and don't forget to comment!

The vernacular in medieval astronomy - at Kalamazoo

I recently returned from the International Congress on Medieval Studies.  It's held every year at the University of Western Michigan in Kalamazoo (USA), and this year was the 50th congress.

3000 medievalists met to discuss everything from Viking archaeology to Middle English poetry - and of course there was a mead and ale tasting.  I'll put together some tweets from the congress in a future post.  For now, here's a recording (with PowerPoint slides) of my presentation at the conference.  Hope you enjoy it, and don't forget to comment!

Launch of the Digital Equatorium

I wrote this post for the Peterhouse Perne and Ward Libraries blog.  It is cross-posted from there with their kind permission.

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]

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.

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

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 above, 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.

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 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.

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.

The Scholar as Craftsman: Notes and Records article

Regular readers of this blog (if there are any, I should apologise to them for my long silence) will be familiar with the story of King Arthur's Table.  You won't need me to remind you how I was lucky enough to discover this forgotten object in the stores of the Whipple Museum of the History of Science, and then spent much of 2013 investigating its biography.

You'll already know, I'm sure, that the story takes in Sir Lawrence Bragg, the Cavendish Laboratory and its workshops.  You'll be aware that the hero of this story, Derek de Solla Price, was a complex character who came to Cambridge more or less on a whim, discovered a manuscript (possibly) by Chaucer in the library at Peterhouse, and thereby launched his career as a superstar historian of science.  My research into Price and his work took me from Cambridge to Chicago, visiting many archives and museums in between (and getting lots of help from Price's lovely family).

Why am I reminding you of all this?  Because my article telling this story has just been published by Notes and Records, the Royal Society's journal for the history of science.  It will be printed in the journal in June, but before then it is completely free to read and download online.  Just click here and read all about it!

And of course don't forget to come back here and leave a comment to let me know what you think!

How to cast a medieval horoscope

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: 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.