The Armillary Sphere

A drawing of a demonstrational armillary sphere. A drawing of a demonstrational armillary sphere, from Libros del saber de astronomia del rey D. Alfonso X De Castilla.

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Armillary spheres can be divided into two main categories: the observational armillary, as used by Ptolemy and Tycho Brahe; and the demonstrational instrument. Both types consist of a number of rings (Latin: armillae) which are arranged so as to model the circles of the celestial sphere. Typically, armillary spheres used for observation were larger and possessed fewer rings than those which served as demonstrational instruments; this made them more accurate and easier to use. Often the rings of demonstrational armillaries, like those of the observational spheres, were divided, and some incorporated sights which could be used to orient the instrument appropriately.

The armillary sphere described by Ptolemy was a zodiacal instrument of six rings, designed to determine the locations of celestial objects according to the ecliptic co-ordinate system. The sphere was first set to the appropriate latitude by ensuring that its outer fixed meridian ring was perpendicular to the horizon, and parallel to the actual meridian - in other words aligned along a North-South line. The orientation of the instrument was establishing by sighting on a celestial object (the sun or a star) whose position in the ecliptic - that is, its celestial longitude - was known. Two rings, one a divided ecliptic ring, and one corresponding to the latitude component, were used for this task. An inner latitude ring was also divided; nesting within it was another ring to which diametrically opposed sights had been fitted, and which could turn within the latitude ring. By using these sights to observe a celestial body, the position of the body could be determined in ecliptic co-ordinates by reading off the longitude from the divided ecliptic ring and the latitude from the position of the far sight north or south of the ecliptic on the divided latitude ring.

Recommended Reading

Ptolemy, Almagest, especially Book V, Chapter I. Translated by G. Toomer, Ptolemy's Almagest, London 1984

S. Schechner Genuth, 'Armillary Sphere', in R. Bud & D. Warner (eds.), Instruments of Science: An Historical Encyclopedia, New York & London 1998, pp. 28-31

Full Bibliography