Armillary Sphere

The armillary sphere is an instrument that models the celestial sphere with respect to the horizon of an observer. It is made up of rings (known in Latin as armillae) representing the great circles on the celestial sphere such as the horizon, the celestial equator, the colures and the ecliptic. They can be adjusted to any latitude and can be used either as an instrument of observation or as a tool of demonstration.

The larger examples with finely divided scales on the rings were used for measuring celestial positions in ecliptic or in equatorial co-ordinates. The earliest known description of such an observational armillary sphere was given around 150 A.D. by Claudius Ptolemy in his Almagest. Ptolemy's instrument, which is known as a zodiacal armillary sphere, measured co-ordinates with respect to the ecliptic (zodiac); i.e. the rings were calibrated to read ecliptic latitudes and longitudes. Islamic and Renaissance astronomers also employed equatorial armillary spheres that were designed to measure co-ordinates with respect to the celestial equator, i.e. in right ascension and in declination. Both types of armillaries were built and used in the 1580s by the Danish astronomer Tycho Brahe at his observatory Uraniborg on the isle of Hven.

Smaller versions were popular as a model of the universe and for demonstrating the principles of spherical astronomy and the motions of the heavenly bodies. Demonstrational armillary spheres would include additional rings delineating the tropics and the polar circles as well as a central earth surrounded by a set of nested rings representing the sun, the moon and the planets.

F. Nolte,"Die Armillarsph?re", Abhandlungen zur Geschichte der Naturwissenschaften und der Medizin, heft 2 (1922); reprinted in F. Sezgin (ed.), Arabische Instrumente in orientalistischen Studien, (6 vols, Frankfurt am Main, 1991), vol. 4, pp. 162-214; H. Raeder, E. Str?mgren and B. Str?mgren, Tycho Brahe's Description of his Instruments and Scientific Work as given in Astronomiae Instauratae Mechanica (Wandesburgi 1598) (Copenhagen, 1946); D. J. de Solla Price,"A Collection of Armillary Spheres and other Antique Scientific Instruments", Annals of Science, 10 (1954), 172-87; S. Schechner Genuth,"Armillary Sphere", in R. Bud and D. J. Warner (eds.), Instruments of Science: An Historical Encyclopedia (New York and London, 1998), pp. 28-31.

Robert van Gent
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