ASTROLABE, an instrument used for the taking of altitudes of heavenly bodies, from which time and latitude are deducible.
The planispheric astrolabe, to which the name is now commonly restricted, is believed to have been a Greek instrument invented by Hipparchus (15o B.c.), or even by Apollonius of Perga (c. 240 B.C.) . It has recently been revived by Prof. Jenkin, of Oxford, as a useful educational instrument, so with a history of 2,000 years it may claim to be the oldest scientific instrument in the world, and has played a correspondingly important part in the history of civilization.
In its most usual form it consists of an evenly-balanced circle or disc of metal or wood, hung by a ring and provided with a rotatable alidade or diametral rule with sights, turning within a circle of degrees for measuring the altitudes of sun or stars. Seamen from the time of Martin Behaim (c. 1480) to the middle of the 18th century, when the astrolabe and cross staff were superseded as navigational instruments by Hadley's quadrant (see SEXTANT), relied largely upon s ich instruments and tables of the sun's declination for finding their latitude.
On the back is a circular map of the stars, the rete, beautifully designed in fretwork cut from a sheet of metal, with named pointers to show the positions of the brighter stars relatively to one another and to a zodiac circle showing the sun's position for every day of the year. Lying below the rete are one or more interchangeable plates engraved with circles of altitude or almucantars.
To obtain the time, first measure the altitude of the sun, then, having noted the sun's position for the day in the zodiac circle, rotate the rete until the sun's position coincides with a circle on the plate corresponding to the observed altitude. A line drawn through this point of coincidence and the centre of the instru ment to a marginal circle of hours shows the time.
Among the accessories often introduced in the earlier astro labes were "shadow scales," for simple surveying, measuring heights and distances; calendar scales showing the sun's place in the zodiac for every day of the year ; magnetic compasses, usual in instruments of the i 6th and 17th centuries, which thereby became useful to surveyors as circumferentors; and lastly, various lines and tables of use to astrologers. The principal varieties of astrolabes in use in the different countries of Europe and of the nearer East may be studied in the Lewis Evans collection in Oxford. The finest English examples, a great 2 f t. astrolabe by Cole, of 1575, and a seaman's astrolabe by Elias Allen (1616) belong to St. Andrews university.