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The 'Chaucer' astrolabe (London, British Museum)
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While a quadrant is listed among Richard II's chapel goods, no astrolabe is mentioned. This sophisticated instrument in the British Museum is nevertheless of exceptional interest in the context of Richard's court. This type of astrolabe has been linked to the poet, Geoffrey Chaucer (c. 1340-1400), who held important royal offices under Richard II.
An astrolabe is basically a two-dimensional map of the celestial sphere. The most important stars and the major circles in the sky are projected onto a metal sheet. This part of the astrolabe is called a rete, Latin for 'net' or 'web', since the cut-out piece at the front of the astrolabe resembles a spider's web. Underneath the rete are positioned a number of plates, allowing the instrument to be used in different latitudes. Rete and plates are housed in a hollowed out container, the 'mater', which carries special markings on the front and back. Attached to the back is a ruler, called the 'alidade', with two sighting vanes (small pierced metal protrusions on the edge of the ruler). Astrolabes are amongst the most sophisticated instruments ever made. The beginnings were in classical Greece at about the time of the birth of Christ and Muslim scholars developed the instrument further from the eighth century onwards. Astrolabes were known in western Europe from the tenth century. An astrolabe enables its user to tell the time during day and night with the help of the position of the sun and certain stars. It can also be used to prepare horoscopes and as a tool in surveying.
This particular astrolabe, dated 1326, resembles the instrument described in Geoffrey Chaucer's Treatise on the Astrolabe. The treatise is addressed to 'his little son, Lewis' and uses the year 1391 for its calculations. Like the astronomical and astrological references in Chaucer's Canterbury Tales, it demonstrates his unusually well-informed interest in contemporary science. The British Museum astrolabe is one of a group of instruments that show the features of the diagrams used in manuscripts of Chaucer's Treatise, but it is the only one securely dated. It has been suggested that other, undated astrolabes of a similar design were made much later, when owning 'an astrolabe like the one Chaucer had' became fashionable.
No astrolabe is listed in the treasure roll.