Astrolabe quadrant from Canterbury

Canterbury, England
Probably AD 1388

An extremely rare astronomical instrument from medieval Canterbury

This extremely rare instrument is likely to have belonged to a travelling scholar who may have lost it whilst in Canterbury – maybe on pilgrimage. It remained in the ground undisturbed until it was discovered in 2005 during an excavation at the House of St Agnes in St Dunstan's Street.

Astrolabe quadrants are amongst the most sophisticated instruments ever made before the invention of the modern computer. They combine the mathematical and astronomical features of an astrolabe with a much smaller size. They were thus an extremely handy tool for their owners enabling them to establish the time of day and to carry out a range of calculations based on the position of the sun, such as the length of day. The side of the quadrant with the rotating disc, called a volvelle, has tables that enable the user to calculate the date of Easter. This is very important as this prominent Christian feast is not fixed in the calendar and its date changes every year. The wings of the rotating eagle correlate the relevant data.

Astrolabe quadrants are extremely rare and fewer than 10 medieval examples have been recorded. The instrument was first described in the West by Jacob ben Machir ibn Tibbon of Montpellier (about 1236-1305). He was a member of a family that originated from Granada who was renowned for translations from Arabic into Hebrew. Astrolabe quadrants are thus uniquely placed to explain the link between the Islamic World and Christian Europe and the role of Jewish scholars.

Examination of the scales on the instrument allows us to date the astrolabe quadrant to 1388 and to firmly place its origins in England. This makes the Canterbury quadrant the only example of certain English origin. It is also the first ever occurrence of such an instrument having been found during an archaeological dig – scientific instruments are generally handed down from generation to generation or found in attics, they are hardly ever found in the ground.