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The Sloane Astrolabe
England, around AD 1300
One of the earliest European
astrolabes
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.
R.T. Gunter, The astrolabes of the world-1 (Oxford, 1932)
A. MacGregor (ed.), Sir Hans Sloane, collector, sc (London, The British Museum Press, 1994)
J. Robinson, Masterpieces: Medieval Art (London, British Museum Press, 2008)
K. Sloan (ed.), Enlightenment. Discovering the (London, The British Museum Press, 2003)
F.A.B. Ward, Catalogue of European scientif (London, The British Museum Press, 1981)
R.T. Gunther, The astrolabes of the world (Oxford University Press, 1932)