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An astrolabe is an astronomical instrument once used widely to measure stars or planets in order to determine latitude and time, primarily for navigational purposes. The original meaning of the word in Greek is "star-taker." The astrolabe was probably invented by astronomers in the second century B.C.

At least two forms of the astrolabe have existed. The older form, known as the planispheric astrolabe, consists of two circular metal disks, one representing Earth and the other, the celestial sphere at some particular location (latitude) on the Earth's surface. The first of these disks, called the plate or tympan, is fixed in position on a supporting disk known as the mater. It shows the great circles of altitude at the given latitude. Any given plate can be removed and replaced by a plate for some other altitude with the appropriate markings for that latitude.

The second of the disks, called the rete or spider, is attached to the plate and the mater by a metal pin through its center. The metal disk that makes up the rete is primarily cut out so that it consists of a complex series of curved lines ending in points. The points indicate the location of particular stars in the celestial sphere. The rete can be rotated around the central pin to show the position of stars at various times of the day or night, as indicated by markings along the circumference of the mater.

To use the astrolabe, an observer hangs the instrument from a metal ring attached at the top of the mater. A sighting device on the back of the astrolabe, the alidade, is then lined up with some specific star in the sky. As the alidade is moved to locate the star, the rete on the front of the astrolabe is also pivoted to provide the correct setting of the celestial sphere for the given time of day. That time of day can then be read directly off the mater.

A much simpler form of the astrolabe was invented in about the fifteenth century by Portuguese navigators. It consisted only of the mater and the alidade, suspended from a ring attached to the mater. The alidade was used to determine the elevation of a star above the horizon and, thus, the latitude of the ship's position. This form of the astrolabe, known as the mariner's astrolabe, later evolved into the instrument known as the sextant.

More elaborate forms of the mariner's astrolabe were later developed and are still used for some specialized purposes. One of these, known as the impersonal astrolabe, was invented by the French astronomer André Danjon (1890-1967). The modern prismatic astrolabe is based on Danjon's concept. In this form of the astrolabe, two light rays from the same star are passed through a prism, one directly and one after reflection from the surface of a pool of mercury. The star is observed as it rises (or sets) in the sky. During most of this period, the two light rays passing through the prism are out of phase Galileo's astrolabe. Photo Researchers, Inc. Reproduced by permission. with each other. At some point, the specific latitude for which the astrolabe is designed is attained and the two star images coincide with each other, giving the star's precise location at that moment.

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