Armillary sphere

An armillary sphere (variations are known as spherical astrolabe, armilla, or armil) is a model of objects in the sky (in the celestial sphere), consisting of a spherical framework of rings, centred on Earth, 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.

The exterior parts of this machine are a compages [or framework] of brass rings, which represent the principal circles of the heavens.

The Greek astronomer Hipparchus (c. 190 – c. 120 BCE) credited Eratosthenes (276 –194 BCE) as the inventor of the armillary sphere. The name of this device comes ultimately 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. Before the advent of the European telescope in the 17th century, the armillary sphere was the prime instrument of all astronomers in determining celestial positions.

 Description and use of the armillary sphere

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 its 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 degrees 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, for showing the latitude of the stars.

Throughout Chinese history, astronomers have created celestial globes (Chinese: 浑象) to assist the observation of the stars. The Chinese also used the armillary sphere in aiding calendrical computations and calculations. Chinese ideas of astronomy and astronomical instruments became known in Korea as well, where further advancements were also made.

According to Needham, the earliest development of the armillary sphere in China goes back to the astronomers Shi Shen and Gan De in the 4th century BCE, as they were equipped with a primitive single-ring armillary instrument.^[6] This would have allowed them to measure the north polar distance (declination) a measurement that gave the position in a xiu (right ascension).^[6] Needham's 4th century dating, however, is rejected by British sinologist Christopher Cullen who traces the beginnings of these devices to the 1st century BCE.^[7]

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