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Many instruments have been called 'astrolabes' through the centuries. The name comes from the Greek meaning 'star taker'. The instrument that is most commonly referred to by the name astrolabe is more properly called a planispheric astrolabe. It's an instrument that is essentially a computer -- or maybe more accurately a circular slide rule -- that displays the sky.

The astrolabe came in many different forms with the earliest existing examples coming from about the year 1000, though various descriptions exist that suggest they have existed much longer. A typical model would have a series of plates that each represent a single latitude. The plate shows the horizon and has markings to show how high something is above the horizon and it's position east and west (the azimuth). This is overlaid by a 'rete' (Latin for 'net') that has pointers for the positions of stars. By positioning a given star (or sun) at it's observed position. The astrolabe is then set for the condition of the sky at the time of the observation. It doesn't have the ability to show where the planets are, however. The position of the sun can be determined by from a calendar scale found on the back and then set with a rule on the front.

A properly set astrolabe can tell time (day or night), sunrise, sunset, twilight, astrological measurements, star rise & set times, among other readings. It can also be used for some surveying measurements such as measuring the height of a structure. Islamic astrolabes frequently had markings to determine prayer times and the direction to Mecca.

For detailed information on astrolabes, check out Jim Morrison's astrolabe site in the related links.

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15y ago

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