© Duane A. Cline 1999
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Navigation: The Astrolabe The astrolabe was an instrument used for the taking of altitudes of heavenly bodies, from which time and latitude were
deductible. The planispheric astrolabe, to which the name is now commonly restricted, is believed to have been a Greek
instrument and may have been invented as early as 240 B.C. With a history of 2,000 years it may claim to be the oldest
scientific instrument in the world. The Arabic scholars and mathematicians developed the astrolabe into a precise
instrument, which was capable of indicating time and the placement of heavenly bodies. 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. Seamen from about 1480 to the middle of the 18th century, relied largely upon such instruments and tables of the sun's
declination for finding latitude. To reduce wind resistance, the size of the astrolabe was kept down to 5 or 6 inches in diameter. However, the English
navigators preferred larger ones (6 or 7 inches in diameter) and a wider space between the sighting vanes on the alidade. By
the early seventeenth century the mariner's astrolabe generally had two quadrants divided into 90-degrees -- as in the
example on the enclosed worksheet. 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. You will find a worksheet for making an astrolabe enclosed in this packet. You will need to make certain that you have the
following supplies on hand when you begin the project. MATERIALS NEEDED: Assemble the astrolabe to resemble the sketch. 
Last modified October 7, 1999
by
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