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Understanding the medieval mind

Why were medieval Christians and Jews so fascinated by their calendars?  What do their calendars tell us about medieval society? And what part do astrolabes play in representing the medieval world-view?

Two AHRC research projects are examining medieval Jewish texts about astronomy and calendars, touching on a wide range of related disciplines including, mathematics, literature, palaeography (the study of writing) and codicology (the study of books).

Medieval calendars stood at the crossroad of science and religion, Jewish as well as Christian, and were central to medieval culture, commerce, and daily life. A major AHRC-funded project ‘Medieval Monographs on the Jewish Calendar’, which ran from October to 2008 to March 2013, explored the place of calendars in medieval culture and what they tell us about medieval notions of faith, time and science.

Professor Sacha Stern of University College London led the project, working with post-doctoral research assistants Dr Israel Sandman and Dr Ilana Wartenberg. Funded by an award of £732,243, they studied three neglected 12th-century books on the Jewish calendar by the Hebrew authors Abraham bar Hiyya and Abraham ibn Ezra (of Spanish origin) and Jacob bn Samson (of northern France). They studied and compared a large number of manuscripts held in libraries around the world, edited the texts, and worked to translate and explain them in a new edition. They also ran a series of thematic workshops, with the participation of international experts, and a conference open free of charge to the public on ‘Time, Astronomy, and Calendars in the Jewish tradition’.

“These texts have been well-known for centuries, but they’ve never been properly read in recent times,” says Professor Stern. At least two of the works were written in exactly the same year, 1123, but nobody knows why. “It’s really strange. This year doesn’t have anything particularly significant about it. The writers had very different agendas and they didn’t seem to know about each other.”

Just one incomplete copy of Jacob ben Samson’s monograph has survived, in a manuscript in the Bodleian Library in Oxford, and has never been published to this day. The other works are preserved in numerous manuscript copies from a wide variety of provenances. Not all were easy to read, and some were prone to scribal errors. There were also problems with the terminology: scientific vocabulary was not yet established, so Stern and his team had to determine why the author chose a particular word and what he meant by it. Mathematical errors introduced in the copying process also had to be identified and explained. “There’s a real mathematical dimension to these works”, says Professor Stern. “A lot of calculations are involved, as well as a good deal of astronomy. The Jewish calendar tracks the moon and sun far more precisely, for example, than the Christian calendar.”

The works contain multiple references to the Christian and Islamic calendars. “Christians were increasingly recognising that the Jewish calendar was more accurate. They began looking at it as a model for calendar reform, which eventually led to the reform of the Christian calendar in 1582,” explains Professor Stern. “Today’s Gregorian calendar is partly based on an examination of the Jewish calendar. The Christian date of Easter is, in theory, very similar to the date of the Jewish Passover, but they have drifted apart.”

‘Astrolabes in Medieval Jewish Society’ is another closely-linked project. An AHRC research grant of £319,688 has enabled principal investigator Professor Charles Burnett of the Warburg Institute, University of London, and co-investigator Dr Stephen Johnston of the Museum of the History of Science in Oxford, with Dr Josefina Rodriguez Arribas as the researcher, to locate, examine and catalogue astrolabes that were made or possessed by Jews in the Middle Ages. Combining the resources of libraries and archives all around the world and based at the Warburg Institute, the researcher is studying texts about their construction and use, including the writings of Abraham ibn Ezra, who introduced the subject into Hebrew. However, Jews were writing treatises on the astrolabe until the 17th century, so the project is dealing with six centuries of texts, diagrams of astrolabes, and actual instruments.

The astrolabe was the most sophisticated scientific instrument of the middle ages. It was designed mainly to simplify long and complex astronomical calculations, but it was also used in observation (e.g., checking the positions of the sun, the moon, and the stars) and to update and check astronomical tables. Astrologers and magicians made frequent use of it as well.

“It’s a 2D projection of the 3D celestial sphere,” says Professor Burnett. Turn it correctly and you’ll see which stars should appear in the sky on a given date – so long as your astrolabe works in your geographical location, for “you need a plate that corresponds to your latitude.”

It was also a highly symbolical object, for the medieval understanding of the universe was contained in an ensemble of metal pieces the diameters of which were frequently no more than 7 inches. Holding an astrolabe was like holding the universe in one’s hand. Knowledge of astrolabes was an essential part of the science of the stars. Burnett adds: “They had some exotic value, but everyone knew about them. The 12th-century Christian scholar Peter Abelard even named his son Astrolapsus.”

The researcher, Josefina Rodriguez, has compiled a database of around 20 astrolabes with inscriptions in Hebrew lettering – which was also used to write other languages including Arabic, Catalan and Italian– and more than 100 Hebrew manuscripts relating to their manufacture and usage. She has collated the information and images, gathering them into a monograph on the astrolabe’s place in medieval Jewish society and an illustrated catalogue.

So far, the search for astrolabes has taken the researcher to many parts of the world, including to Palermo, Cracow, Paris, Munich, Berlin, and Toledo (with trips to Boston, Chicago, Washington, Tel Aviv and Cairo planned). Four of the instruments are in museums and private collections in England. “Astrolabes with Hebrew markings are rare, and some feature Hebrew letters that have been used to write in other languages, such as Arabic,” says Professor Burnett. “We’re still hoping to locate more of them.”

The project is shedding light on medieval society as a whole, for astrolabes were used in astronomy, astrology, land surveying, calendar, telling time, religion, and magic. Astrolabes were the jewel of the crown of medieval science, the mere mention of the name of ‘astrolabe’ brought the prestige of the best knowledge available in the Middle Ages.

“Some historians of the astrolabe have dealt with it entirely from a mathematical point of view, looking at the positions of stars, but we’re also looking at the social roles,” says Professor Burnett. “This is an instrument that belongs to a particular time and place, and it’s a window into that society.”

For further information, please go to the UCL project website.

More information on the Jewish astrolabe is available on the Warburg Institute website.

More information on Jewish astrolabes is also available on a project blog.

By Anne Wollenberg

Image 1: Astrolabe in Hebrew and Judaeo-Arabic, private collector, Paris, possibly 14th c., 12,7 cms diameter.

Image 2: Astrolabe in Judaeo-Arabic (Arabic in Hebrew script), Khalili collection, London, possibly 1300, 18,4 cms diameter.

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