5.2 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Armillary sphere | 1/5 | https://en.wikipedia.org/wiki/Armillary_sphere | reference | science, encyclopedia | 2026-05-05T09:40:04.003517+00:00 | kb-cron |
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. With the Earth as center, an armillary sphere is known as Ptolemaic. With the Sun as center, it is known as Copernican. The flag of Portugal features an armillary sphere. The armillary sphere is also featured in Portuguese heraldry, associated with the Portuguese discoveries during the Age of Exploration. Manuel I of Portugal, for example, took it as one of his symbols where it appeared on his standard, and on early Chinese export ceramics made for the Portuguese court. In the flag of the Empire of Brazil, the armillary sphere is also featured. 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.
== Description and use ==
The exterior parts of this machine are a compages [or framework] of brass rings, which represent the principal circles of the heavens:
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. The ecliptic B, which is divided into 12 signs, and each sign into 30 degrees, and also into the months and days of the year, in such a manner that the degree or point of the ecliptic on which the sun appears, on any given day, stands over that day in the circle of months. 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 circle 231⁄2 degrees from the equinoctial circle. The Arctic Circle E, and the Antarctic Circle F, each circle 231⁄2 degrees from its respective pole at N and S. The equinoctial colure G, passing through the north and south poles of the heavens at N and S, and through the equinoctial points in Aries and Libra, in the ecliptic. The solstitial colure H, passing through the poles of the heavens, and through the solstitial points in Cancer and Capricorn, in the ecliptic. Each quarter of the equinoctial colure is divided into 90 degrees, from the equinoctial to the poles of the world, for showing the declination of the sun, moon, and stars; and each quarter of the solstitial colure, from the ecliptic as e and f, to its poles b and d, for showing the latitude of the stars. In the north pole of the ecliptic is a nut b, to which is fixed one end of the quadrantal wire. To the other end is a small sun Y, which is carried around the ecliptic B—B, by turning the nut. In the south pole of the ecliptic is a pin d, on which another quadrantal wire is situated, with a small moon Ζ upon it, which may be moved around by hand. A mechanism causes the moon to move in an orbit which crosses the ecliptic at an angle of 51⁄3 degrees, to opposite points called the lunar nodes, and allows for shifting these points backward in the ecliptic, as the lunar nodes shift in the heavens. Within these circular rings is a small terrestrial globe I, fixed on an axis K, which extends from the north and south poles of the globe at n and s, to those of the celestial sphere at N and S. On this axis the flat celestial meridian L is fixed, which may be set directly over the meridian of any place on the globe, so as to keep over the same meridian upon it. This flat meridian is graduated the same way as the brass meridian of the common globe, and its use is much the same. To this globe is fitted the movable horizon M, so as to turn upon the two strong wires proceeding from its east and west points to the globe and entering the globe at the opposite points off its equator, which is a movable brass ring set into the globe in a groove all around its equator. The globe may be turned by hand within this ring, so as to place any given meridian upon it, directly under the celestial meridian L. The horizon is divided into 360 degrees all around its outermost edge, within which are the points of the compass, for showing the amplitude of the sun and the moon, both in degrees and points. The celestial meridian L passes through two notches in the north and south points of the horizon, as in a common globe: if the globe is turned around, the horizon and meridian turn with it. At the south pole of the sphere is a circle of 25 hours, fixed to the rings. On the axis is an index which goes around that circle, if the globe is turned around its axis.