See also:plane of the circle, the
See also:angle B 0 D will be the angle sub-tended by the two
See also:objects P and Q at the
See also:eye . The earliest forms were " armillae " and spherical . Gradually, from Eratosthenes to Tycho,
See also:Hipparchus playing the most important
See also:part among
See also:ancient astronomers, the complex astrolabe was evolved, large specimens being among the chief observa- -
See also:ASTROLOGY 795 tory
See also:instruments of the 15th, 16th and even r 7th centuries; while small ones were in use among travellers and learned men, not only for astronomical, but for astrological and topographical purposes . Nearly every one of the modem instruments used for the observations of
See also:physical astronomy is a part of the perfected astrolabe . A collection of circles such as is the armillary sphere, if each circle were fitted with a view-
See also:tube, might be considered a
See also:complete astrolabe . Tycho's armillae were astrolabes . In fact the
See also:equatorial, and the altitude and
See also:azimuth circle are astrolabes in the strictest and
See also:oldest meaning of the
See also:term; and Tycho in one of his astrolabes came so near the modern equatorial that it may be taken as the first of the kind . The two forms of the planispheric astrolabe most widely known and used in the r5th, 16th and even 17th centuries were: (r) the portable astrolabe shown in fig .
See also:Plate) . This originated in the East, and was in early use in India,
See also:Persia and
See also:Arabia, and was introduced into
See also:Europe by the
See also:Arabs, who had perfected it —perhaps as early as A.D . 700 . It combines the planisphere and armillae of Hipparchus and others, and the
See also:theodolite of Theon, and was usually of brass, varying in diameter from a couple of inches to a
See also:foot or more . It was used for taking the altitudes of
See also:moon and stars; for calculating latitude; for determining the points of the compass, and
See also:time; for ascertaining heights of mountains, &c.; and for construction of horo- scopes . The instrument was a marvel of convenience and ingenuity, and was called " the mathematical
See also:jewel." Nevertheless it passed out of use, because incapable of any
See also:great precision . (2) The mariner's astrolabe, fig . 3, was adapted from that of astronomers by
See also:Martin Behaim, c . 1480 . This was the instrument used by
See also:Columbus . With the tables of the sun's declination then available, he could calculate his latitude by meridian altitudes of the sun taken with his astrolabe . The mariner's astrolabe was superseded by
See also:John Hadley's quadrant of 1731 .
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