The fate of a rare fourteenth-century astrolabe quadrant — a kind of medieval pocket calculator — hangs in the balance while the British Museum in London tries to raise £350,000 (US$700,000) to acquire it before its deferred release to a private buyer. The palm-sized brass device can be used to determine the time of day from the position of the Sun and to calculate the date of Easter, among other functions.

The simplicity of the instrument suggests that, in the Middle Ages, some sections of English society were surprisingly literate in basic mathematics and astronomy. Most surviving astrolabes, whether standard disc-shaped devices or quarter-circle quadrants, are elaborate instruments capable of astrological calculations that require specialist academic knowledge. This one is a stripped-down, everyday item — something that a cleric or a merchant could have carried for convenient time-keeping. “You had to know some astronomy to work one of these devices,” says Jim Bennett, director of the Museum of the History of Science in Oxford, UK. It suggests, he says, that people of the time “had a closer astronomical awareness than we do now”.

Found in 2005 at Canterbury, UK, the quadrant has been dated to about 1388, around the time that Geoffrey Chaucer began to write The Canterbury Tales. Chaucer was highly informed about astronomy and astrology. In 1391, he wrote a treatise on astrolabes that became the standard reference text for several centuries. The Canterbury quadrant was excavated from beneath a series of clay floors on the site of an old inn, just outside the city's walls on the main road towards London. Perhaps it was lost at the inn by a merchant travelling to or from Canterbury, rather like Chaucer's pilgrims.

Palm-sized 'calculator' for medieval pockets. Credit: MUSEUMS, LIBRARIES AND ARCHIVES COUNCIL

The instrument was initially put up for sale in 2007 by the London auctioneers Bonhams, and was expected to fetch £60,000–£100,000. Subsequent private dealings led to an agreed sale at more than three times the original figure. The identities of the dealer and the latest buyer have not been made public.

Because of the cultural importance of the astrolabe quadrant, Britain's culture minister, Margaret Hodge, was persuaded by a review committee to defer granting an export licence for it until June 2008 to give the British Museum time to try to buy the instrument for its new medieval gallery. Such deferrals are more usually applied to works of fine art than to scientific items.

The existence of this practical device in Chaucer's time sheds new light on his astrolabe treatise. His dedication of the book to his son had left scholars bemused. It seemed hard to believe that a young person would have understood how to use an astrolabe. The Canterbury quadrant “supports the idea that Chaucer could write such a treatise at a popular level”, Bennett explains. “It suggests that this kind of knowledge wasn't too arcane or academic.”