The so-called Sloane Astrolabe is one of the oldest and most enigmatic mathematical instruments in the medieval collection of the British Museum. Its size and stunning design, encompassing animals and mythical beasts in delightful detail, make it intriguing to behold before one even considers its function.
This astrolabe is named after Sir Hans Sloane, as it forms part of his collection which became the basis of the British Museum in 1753. The object is made of brass, but the application of lacquer at some point in the past has resulted in the darker colour of most of the components.
Astrolabes are basically two-dimensional maps of the three-dimensional celestial sphere (the most important stars and the main celestial circles, such as the tropics, the ecliptic and the celestial equator are projected onto a flat plane). This construction process, which was known to the ancient Greeks, is similar to that used in the production of a map. Astrolabes are among the most sophisticated instruments made before the invention of the computer, enabling the user to determine the time in different hour systems at day and night, to establish heights and angles, and to facilitate the casting of horoscopes. As the astrolabe is capable of determining the time of prayer according to the rules that govern every Muslim, it was especially popular in the Islamic world.
Every astrolabe can be used at a variety of latitudes for which the initial flat discs, the so-called 'plates' are engraved. The Sloane Astrolabe has three plates that are laid out for six different latitudes between 48 degrees 30 minutes and 54 degrees, (for most places in Europe). Only one city is mentioned, Lundoniarum (London), marked on the plate for 52 degrees. The back is highly decorated with interwoven mythical animals and foliate scrolls. A number of calendrical scales are engraved on it and enable the user to calculate the dates of the movable feasts, and to read the names of 48 saints. It is marked with the names of three saints of particular English significance - Dunstan, celebrated on the 19 May, Augustine of Canterbury on the 26 May, and Edmund, celebrated on the 20 November. These names, together with the plate marked for London and the overall design make an English origin for this astrolabe most likely.
It is totally unclear for whom this instrument was made or who the maker was. Its unusual size and the precision in execution suggest, however, that it was made by a skilled craftsman for a wealthy client.