The Chaucer astrolabe, preserved in the British Museum, remains the earliest dated astrolabe made in Europe. It is of the type described by the poet Geoffrey Chaucer in his A Treatise on the Astrolabe, written circa 1390. That text is considered the earliest technical manual written in English. No specific surviving astrolabe has been identified as the one used by Chaucer.
An astronomical instrument used for observing planetary movements, the astrolabe was indispensable for navigation. Brass astrolabes, a type of analog calculator, were developed in the medieval Islamic world, and were also used to determine the location of the Kaaba in Mecca, in which direction all Muslims face during prayer. Planispheric, or flat, astrolabes, were more common than the linear or spherical types. In planispheric astrolabes the celestial sphere was drawn on a flat surface and represented on one plate.
The earliest surviving dated astrolabe of the planispheric type dates from 927 or 928. Coincidentally it is also preserved in London, at the School of Oriental and African Studies at the University of London.