The astrolabe was an instrument used for the taking of altitudes of heavenly bodies, from which time and latitude were deductible.  For fifteen centuries it was the ultimate astronomical instrument. The astrolabe was probably invented by the Greeks, possibly as early as 240 B.C.  

Ptolemy (2nd century AD) refers to the mathematics used to construct the astrolabe, and Theon of Alexandria (4th century AD) wrote the first treatise on this versatile instrument. In the middle ages, Arabic astronomers preserved and added to the knowledge of the Greeks. By the eleventh century, astrolabes were known in Europe through trade with the Arabs.  With a history of 2,000 years it may claim to be the oldest scientific instrument in the world.

Navigating with the Astrolabe

The astrolabe could be used to solve many problems. It was used as a timepiece that could tell time both during the day and night, a surveying tool to measure distances and make more accurate maps, and a practical tool for all sorts of astronomical calculations. However, such a valuable tool was not constructed easily and was often ornately decorated to illustrate an instrument maker's skill.  

In its most usual form, the mariner's astrolabe consists of an evenly balanced circle or disk of metal, hung by a ring and provided with a rotatable alidade with sights. The alidade, turning within the circle of degrees marked on the outer edge, was used for measuring the altitudes of the sun or stars.  

For taking a sight it was suspended by a thread or line -- not between the thumb and fingers. The navigator used the pair of sighting holes in the vanes of the alidade for shooting a star. To shoot the sun, the navigator would hold the astrolabe in such a manner as to allow the sun's rays to pass through the upper vane, turning the alidade until the small beam of light fell on the hole of the lower vane