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Armillary sphere with astrononomical clock Credit: Chris Bainbridge
The external parts of an armillary consist of a number of brass rings that are meant to represent different things. The equinoctial 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 is divided into 12 signs, and each sign into 30 degrees, and also into the months and days of the year so that the degree or point of the ecliptic in which the sun is stands over that day in the circle of months. The tropic of Cancer, touches the ecliptic at the beginning of Cancer, and the tropic of Capricorn, touches the ecliptic at the beginning of Capricorn, each 23½ is degrees from the equinoctial circle. The Arctic Circle, and the Antarctic Circle, each 23½ degrees from its respective pole. The equinoctial colure, passes through the north and south poles of the heaven and through the equinoctial points Aries and Libra, in the ecliptic. The solstitial colure passes 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. This is for showing the declination of the sun, moon, and stars; and each quarter of the latter, from the ecliptic, to its poles, therefore showing the latitude of the stars.
The very complex armillary is thought to have been invented by Eratosthenes some time between 276 and 194 B.C. Normally, a ball is put in the middle of the armillary to represent the Earth—the Sun for modern astronomers—to demonstrate the motion of the stars in the heavens. Before the invention of the telescope in the 17th century the armillary was the primary tool of the astronomer. In its simplest form, consisting of a ring fixed in the plane of the equator, the armillary is one of the most ancient of astronomical instruments. Later 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 to indicate the sun’s positions, in combination with angular divisions. When several rings or circles were combined representing the great circles of the heavens, the instrument became an armillary sphere.
The armillary was used by such great ancient astronomers as Hipparcos, Eratosthenes, and Ptolemy in their early ventures in to the study of the heavens. There is a great article about the armillary here. We have a great article here on Universe Today about the armillary. Astronomy Cast offers a good episode about the constellations and how to find them in the night sky.
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