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ASTROLABE (from Gr. liar pop, star, a...

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Originally appearing in Volume V02, Page 795 of the 1911 Encyclopedia Britannica.
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ASTROLABE (from Gr. liar pop,
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star, and Aa(3eiv, to take)
  , an instrument used not only for stellar, but for solar and lunar altitude-taking . The principle of the astrolabe is explained in fig . 2 . There were two kinds,—spherical and planispheric . P in the
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plane of the circle, the angle B 0 D will be the angle sub-tended by the two
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objects P and Q at the eye . The earliest forms were " armillae " and spherical . Gradually, from Eratosthenes to Tycho, Hipparchus playing the most important
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part among ancient astronomers, the complex astrolabe was evolved, large specimens being among the chief observa- -
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ASTROLOGY 795 tory
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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
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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-tube, might be considered a
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complete astrolabe . Tycho's armillae were astrolabes . In fact the
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modern
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equatorial, and the altitude and azimuth circle are astrolabes in the strictest and
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oldest meaning of the
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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 .

1 (

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Plate) . This originated in the East, and was in early use in India,
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Persia and
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Arabia, and was introduced into
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Europe by the
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Arabs, who had perfected it —perhaps as early as A.D . 700 . It combines the planisphere and armillae of Hipparchus and others, and the
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theodolite of Theon, and was usually of brass, varying in diameter from a couple of inches to a
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foot or more . It was used for taking the altitudes of sun, moon and stars; for calculating latitude; for determining the points of the compass, and 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 jewel." Nevertheless it passed out of use, because incapable of any
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great precision . (2) The mariner's astrolabe, fig . 3, was adapted from that of astronomers by Martin Behaim, c . 1480 . This was the instrument used by 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 John Hadley's quadrant of 1731 .

End of Article: ASTROLABE (from Gr. liar pop, star, and Aa(3eiv, to take)
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