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The Astrolabe

The most widely used astronomical instrument of the Middle Ages, the astrolabe is a hand-held device for measuring the height of the sun or a star above the horizon. This is combined with a series of movable plates that can be used for solving graphically otherwise complex geometrical problems of astronomy or astrology. The astrolabe ordinarily consists of a brass body into which are inserted a number of circular brass plates. A decorative extension at the top connects with a suspension for holding the device, and on the back of the instrument a movable sighting bar called an alidade is pivoted on the central axis. The large variety of astrolabes makes generalizations difficult, but the great majority range between seven and thirty centimeters in diameter.

The outermost circular plate, called the rete ("net"), serves as a star chart; the metal is cut away to leave an open network with a few dozen stars marked by points in the grillwork pattern. The apparent rotation of the heavens about the earth can be modeled by the rotation of the rete about its center (the north celestial pole). Although the stars remain fixed in their relative positions on the celestial sphere, turning with it once a day, the sun moves eastward through the zodiac along the elliptic once a year. Hence, instead of a fixed pointer for the sun, the rete contains an eccentric circle that is the projection of the elliptic, and the user must locate the sun on that circle by means of a calendar relating the day of the year to the sun's longitude along the elliptic. On some European astrolabes this information is provided by a graph on the back of the instrument.

Held fixed underneath the rete there is generally a horizon tablet, on which is inscribed a series of partial or complete circles, one within the other but not quite concentric, representing the horizon and successive circles of equal angular altitude (almuncantars), on up to the point directly overhead (zenith). By rotating the rete with respect to the horizon plate, it is possible to establish the relation of the stars or the elliptic to the horizon for any specified moment, or conversely, from the given positions of the sun or stars, to find the time. Since the relation of the stars to the horizon depends on the observer's latitude, an ordinary horizon tablet depends on the observer's latitude, an ordinary horizon tablet works for only a single latitude. Generally, however, three, four or five interchangeable plates provide for a series of different latitudes, thus making the astrolabe much less geographically restricted.

The astrolabe owes its simplicity as a mathematical instrument to the stereographic projection, whose fundamental property causes all circles on the celestial sphere to project as circles on the face on the astrolabe. This allows the instrument to serve as an analog computer for spherical astronomy. A typical astrological problem, simply soluble with an astrolabe, is: given the date, time and latitude, find the ascendant (that is, the part of the elliptic just rising).

The circumference of the astrolabe is calibrated in degrees, so that by taking a sight of the sun or other object with the alidade, it is possible to measure its angular height above the horizon (the altitude). A typical astronomical problem would be to find the time of day. The user first lines up the movable alidade with the sun and reads off its altitude from the scale on the rim of the astrolabe. Turning the device over, the user then rotates the rete until the appropriate points on the sun's position on the elliptic circle circle lies on the almuncantar corresponding to the observed altitude. The line from the center of the astrolabe through the sun's position to the of the instrument then gives the time, with noon at the top, midnight at the bottom, 6 p.m. to the right and 6 a.m. to the left.

The back side of the astrolabe includes a variety of graphical devices for determining trigonometric functions. Islamic astrolabes often display graphs for finding the direction of Mecca. European astrolabes, in contrast, frequently have graphs for finding the position o the sun, or giving the maximum altitude of the sun as a function of season and place. Inside the astrolabe, underneath the horizon tablets, Islamic instruments often have a gazetteer giving latitudes and longitudes of twenty or thirty towns, and, sometimes, azimuthal directions, or qiblas, for Mecca.

Besides the instrument just described, there exist also a few representatives of the universal astrolabe, which is designed to eliminate the use of specific plates for selected latitudes. There is also the spherical astrolabe, represented by a single complete specimen in the Museum of the History of Science at Oxford. A large observing instrument with multiple revolving rings was described by Ptolemy as an "astrolabon" but generally today that device is called an armillary sphere.

Although the astrolabe was presumably known in Greek antiquity, the oldest known account of its construction was written in the 6th century by John Philoponos of Alexandria, and the earliest surviving examples are Islamic astrolabes from the 10th century.

The single most comprehensive compendium on the astrolabe is R.T. Gunter, The Astrolabes of the World (1932, repr. 1976)

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Last modified: Sun Dec 27, 1998 / Jeremiah Genest