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The nomenclature of an armillary sphere is based on the terminology of the following astronomical coordinate systems (see table below). Equinoctial: refers to the celestial equator. The term equinoctial refers to the fact that the vernal and autumnal equinoxes occur when the sun passes through this great circle. The celestial equator is a projector of the earth’s equator onto the celestial sphere. The hour markings of the armillary sundial are inscribed on the inner surface of this ring. An armillary sphere such as this one might well have been used for education, aiding understanding of the 3-dimensional geometry of the celestial sphere. This makes sense, as many medieval and early-modern texts in basic astronomy refer to or assume the use of armillaries as models of the heavens. Medieval illustrations even suggest that a 3-dimensional visual aid might well have been a necessary companion to texts on the celestial sphere, such as the 13th-century De sphaera by Sacrobosco, which was a widely used university textbook. The Beijing Capital International Airport Terminal 3 features a large armillary sphere metal sculpture as an exhibit of Chinese inventions for international and domestic visitors. The tropic of Cancer C, touching the ecliptic at the beginning of Cancer in e, and the tropic of Capricorn D, touching the ecliptic at the beginning of Capricorn in f; each 23 1⁄ 2 degrees from the equinoctial circle.
About it rotate a set of rings representing the heavens – the celestial sphere – with one complete revolution approximating 24 hours. De Rose, L.: La sfera armillare di Solunto, in XVIII Seminario di Archeoastronomia, 19–20 marzo, Sestri Ponente, Genova (2016)
Armillary spheres were among the first complex mechanical devices. Their development led to many improvements in techniques and design of all mechanical devices. Renaissance scientists and public figures often had their portraits painted showing them with one hand on an armillary sphere, which represented the zenith of wisdom and knowledge. The sphere is mounted at the celestial poles that define the axis of rotation, and its structure includes an equatorial ring and, parallel to this, two smaller rings representing the Tropics of Cancer and Capricorn to the north and south, respectively. Williams, Henry Smith (2004). A History Of Science. Whitefish, MT: Kessinger Publishing. ISBN 1-4191-0163-3.
In Renaissance Europe, the armillary sphere was amongst the first complex mechanical devices. Public figures often had their portraits painted showing them with one hand on an armillary sphere which represented the height of wisdom and knowledge. Kern, Ralf: Wissenschaftliche Instrumente in ihrer Zeit. Vom 15. – 19. Jahrhundert. Verlag der Buchhandlung Walther König 2010, ISBN 978-3-86560-772-0 There is no evidence for the Hellenistic origin of the spherical astrolabe, but rather evidence so far available suggests that it may have been an early but distinctly Islamic development with no Greek antecedents." Suspended within a ring, the sphere sits in a cradle mounted on an elegant tripod stand. The position of the ring can be adjusted for latitude within the cradle. At the top of the stand is another ring representing the celestial horizon. As the sphere rotates, anything above the horizon ring at any one time will, in theory, be visible in the heavens; anything below will be hidden.The armillary sphere survives as useful for teaching, and may be described as a skeleton celestial globe, the series of rings representing the great circles of the heavens, and revolving on an axis within a horizon. With the earth as center such a sphere is known as Ptolemaic; with the sun as center, as Copernican. [1] Savage-Smith, Emilie (1985), Islamicate Celestial Globes: Their History, Construction, and Use, Smithsonian Institution Press, Washington, D.C.
Historians disagree about the origin and the use of demonstrational armillary spheres, partly because the available evidence is difficult to interpret. A representation of an armillary sphere is present in the modern flag of Portugal and has been a national symbol since the reign of Manuel I. The metal rings, usually made of brass or bronze, are referred to as armilla, which is Latin for bracelet. Armilla were prestigious armbands awarded to Roman soldiers for distinguished gallantry.Zenith: in the horizon coordinate system, it is the position directly overhead on the celestial sphere. The zenith’s altitude is 90 o. Elements of the general description incorporate text from the Encyclopædia Britannica First Edition (1771).
Museum numbers W.34-1939 to W.37-1938, a centre table, tripod table, armillary sphere and gate-leg table, were bought for the Museum by the National Art Collections Fund from the estate of Percival Griffiths (1861-1937). The choice was left to the Museum. A mirror, W.39-1938 was also given by his widow, Mrs Percival Griffiths, in his memory. Percival Griffiths, who lived at Sandridgebury, Hertfordshire, formed a collection of English furniture dating from the late 17th and early 18th century under the guidance of R.W. Symonds, the furniture historian and prolific writer on English furniture. The equinoctial colure G, passing through the north and south poles of the heaven at N and S, and through the equinoctial points Aries and Libra, in the ecliptic. Art historian Jessica Stewart writes that in the 17th century, rings such as the above specimen were “used by astronomers to study and make calculations. These pieces of jewelry were considered tokens of knowledge. Inscriptions or zodiac symbols were often used as decorative elements on the bands.” An armillary sphere can be turned into an armillary sundial by setting its celestial polar axis equal to the latitude of the sundial’s location and directed at the North Celestial Pole. The sphere’s Horizon ring is fixed parallel to the terrestrial horizon. And finally the meridian ring called the Solstitial Colure, which passes through the celestial poles and the two solstices, is rotated to the vertical position so as to coincide with the local meridian of the sundial’s location. Terminology of astronomical coordinate systems The equinoctial A, which is divided into 360 degrees (beginning at its intersection with the ecliptic in Aries) for showing the sun's right ascension in degrees; and also into 24 hours, for showing its right ascension in time.
Inscriptions or zodiac symbols were often used as decorative elements on the bands.
Darlington, Oscar G. "Gerbert, the Teacher," The American Historical Review (Volume 52, Number 3, 1947): 456–476. An armillary sphere (variations are known as spherical astrolabe, armilla, or armil) is a model of objects in the sky (on the celestial sphere), consisting of a spherical framework of rings, centered on Earth or the Sun, that represent lines of celestial longitude and latitude and other astronomically important features, such as the ecliptic. As such, it differs from a celestial globe, which is a smooth sphere whose principal purpose is to map the constellations. It was invented separately, in ancient China possibly as early as the 4th century BC and ancient Greece during the 3rd century BC, with later uses in the Islamic world and Medieval Europe.
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