Armillary sphere

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Armillary sphere
Armillary sphere

An armillary sphere (variations known as a spherical astrolabe, armilla, or armil) is a model of the celestial sphere, invented by the ancient Greek Eratosthenes in 255 BC. Its name comes from the Latin armilla (circle, bracelet), since it has a skeleton made of graduated metal circles linking the poles and representing the equator, the ecliptic, meridians and parallels. Usually a ball representing the Earth or, later, the Sun is placed in its center. It is used to demonstrate the motion of the stars around the Earth. The Chinese during the 1st century BC (Western Han Dynasty) also invented the armillary sphere, while the 2nd century Chinese astronomer Zhang Heng is credited as the world's first to apply motive power (using hydraulics) in rotating his armillary sphere.

In its simplest form, consisting of a ring fixed in the plane of the equator, the armilla is one of the most ancient of astronomical instruments. Slightly developed, it was crossed by another ring fixed in the plane of the meridian. The first was an equinoctial, the second a solstitial armilla. Shadows were used as indices of the sun's positions, in combinations with angular divisions. When several rings or circles were combined representing the great circles of the heavens, the instrument became an armillary sphere.

Eratosthenes used most probably a solstitial armilla for measuring the obliquity of the ecliptic. Hipparchus probably used an armillary sphere of four rings. Ptolemy describes his instrument in the Syntaxis (book v. chap. i.), and it is of great interest as an example of the armillary sphere passing into the spherical astrolabe. It consisted of a graduated circle inside which another could slide, carrying to small tubes diametrically opposite, the instrument being kept vertical by a plumb-line.

Armillary spheres were developed by the Greeks and were used as teaching tools already in the 3rd century B.C.. In larger and more precise forms they were also used as observational instruments. No material advance was made on Ptolemy's instrument until Tycho Brahe, whose elaborate armillary spheres passing into astrolabes are figured in his Astronomiae Instauratae Mechanica. Armillary spheres became popular again in the Late Middle Ages; the Danish astronomer Tycho Brahe (1546-1601) constructed several of such instruments.

Renaissance scientists and public figures often had their portraits painted showing them with one hand on an armillary sphere, which represented the height of wisdom and knowledge.

Armillary spheres were among the first complex mechanical devices. Their development led to many improvements in techniques and design of all mechanical devices.

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.

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.

Contents

[edit] The description and use of the armillary sphere

Armillary sphere diagram
Armillary sphere diagram

The exterior parts of this machine are a compages of brass rings, which represent the principal circles of the heaven.

  1. 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 his right ascension in time.
  2. 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 in which the sun is, on any given day, stands over that day in the circle of months.
  3. 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 23½ degrees from the equinoctial circle.
  4. The Arctic Circle E, and the Antarctic Circle F, each 23½ from its respective pole at N and S.
  5. 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.
  6. The solstitial colure H, passing through the poles of the heaven, and through the solstitial points Cancer and Capricorn, in the ecliptic. Each quarter of the former of these colures 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 latter, from the ecliptic as e and f, to its poles b and d, f or 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, and to the other end a small sun Ψ, which is carried round the ecliptic B—B, by turning the nut : and in the south pole of the ecliptic is a pin d, on which is another quadrantal wire, with a small moon Ζ upon it, which may be moved round by hand : but there is a particular contrivance for causing the moon to move in an orbit which crosses the ecliptic at an angle of 5⅓ degrees, in to opposite points called the moon's nodes; and also for shifting these points backward in the ecliptic, as the moon's nodes shift in the heaven.

Within these circular rings is a small terrestrial globe J, 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 is fixed the flat celestial meridian L L, 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: both here, if the globe be turned round, the horizon and meridian turn with it. At the south pole of the sphere is a circle of 25 hours, fixed to the rings, and on the axis is an index which goes round that circle, if the globe be turned round its axis.

The whole fabric is supported on a pedestal N, and may be elevated or depressed upon the joint O, to any number of degrees from 0 to 90, by means of the arc P, which is fixed in the strong brass arm Q, and slides in the upright piece R, in which is a screw at r, to fix it at any proper elevation.

In the box T are two wheels (as in Dr Long's sphere) and two pinions, whose axes come out at V and U; either of which may be turned by the small winch W. When the winch is put upon the axis V, and turn backward, the terrestrial globe, with its horizon and celestial meridian, keep at rest; and the whole sphere of circles turns round from east, by south, to west, carrying the sun Y, and moon Z, round the same way, and causing them to rise above and set below the horizon. But when the winch is put upon the axis U, and turned forward, the sphere with the sun and moon keep at rest; and the earth, with its horizon and meridian, turn round from horizon to the sun and moon, to which these bodies came when the earth kept at rest, and they were carried round it; showing that they rise and set in the same points of the horizon, and at the same times in the hour circle, whether the motion be in the earth or in the heaven. If the earthly globe be turned, the hour-index goes round its hour-circle; but if the sphere be turned, the hour-circle goes round below the index.

And so, by this construction, the machine is equally fitted to show either the real motion of the earth, or the apparent motion of the heaven.

To rectify the sphere for use, first slacken the screw rin the upright stem R, and taking hold of the arm Q, move it up or down until the given degree of latitude for any place be at the side of the stem R; and then the axis of the sphere will be properly elevated, so as to stand parallel to the axis of the world, if the machine be set north and south by a small compass: this done, count the latitude from the north pole, upon the celestial meridian L, down towards the north notch of the horizon, and set the horizon to that latitude; then, turn the nut b until the sun Y comes to the given day of the year in the ecliptic, and the sun will be at its proper place for that day: find the place of the moon's ascending node, and also the place of the moon, by an Ephemeris, and set them right accordingly: lastly, turn the winch W, until either the sun comes to the meridian L, or until the meridian comes to the sun (according as you want the sphere or earth to move) and set the hour-index to the XXI, marked noon, and the whole machine will be rectified. — Then turn the winch, and observe when the sun or moon rise and set in the horizon, and the hour-index will show the times thereof for the given day.

[edit] Development in East Asia

Celestial globe from Qing Dynasty
Celestial globe from Qing Dynasty

Throughout Chinese history, astronomers have created celestial globes to assist the observation of the stars.

The earliest celestial globe was dated back to 52 BC in the West Han dynasty by the astronomers Geng Shou-chang (耿壽昌) and Luo-xia Hong (落下閎). The first water powered celestial globe was created by Zhang Heng(张衡) in the East Han dynasty.

Then Li Chun-feng (李淳風) of the Tang dynasty created one in 633 AD with three spherical layers to calibrate multiple aspects of astronomical observations.

In 723 AD, Tang dynasty Buddhist monk Yi-xing (一行) and government official Liang Ling-zan (梁令瓚) combined Zhang Heng's water powered celestial globe with an escapement device. The result was allegedly the world's first water powered mechanical clock.

Jang Yeong-sil, a Korean inventor, was ordered by King Sejong the Great of Joseon to build an armillary sphere. The sphere, built in 1433 was named Honcheonui (혼천의).

The Honcheonsigye, an armillary sphere activated by a working clock mechanism was built by the Korean astronomer Song Iyeong in 1669. It is highly valued in term of clock-making technology and is the only remaining astronomical clock from the Joseon Dynasty.

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[edit] External links

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