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Astrolabe
AstrolabeThe astrolabe is an ancient astronomical instrument, dating back more than 2,000 years, used to observe the positions of the stars. With modifications it has also been used for time-keeping, navigation, and surveying. Astrolabes depict the visual reference points of stars on the night sky as a function of time. As such, an observer can also set the time to predict the visible star pattern expected. The most common type of astrolabe, the planispheric astrolabe, consists of a star map (the rete) engraved on a round sheet of metal. With regard to the rete, only the angular relationship of the stars needs to be accurate to ensure proper functioning of the astrolabe. A metal ring is moved across the map to represent the position of the local horizon. An outer ring is adjusted to allow for the apparent rotation of the stars around the North Star, using prominent stars as reference points. Astrolabes were forerunners of mechanical clocks, and looked somewhat like watches. With a set of tables, the observer could determine the day and hour for a fixed location by the position of the stars. With the addition of a sighting-rule, called an alidade, an astrolabe could be used as a surveying instrument. The rule could be moved across a scale to measure elevation. Navigational astrolabes marked celestial altitudes (the altitude in degrees above the horizon). Although there is evidence to support the assertion that ancient Greek culture had astrolabes, it is certain that the Arabs perfected and made regular use of the astrolabe. With the clear desert sky at their constant disposal, the Arab people excelled in astronomy and used the stars to navigate across the seas of sand . Regular use of astrolabes continued into the 1800s. The newer prismatic astrolabe continues to be used for precision surveying. Modern versions of stellar charts and bowls with adjustable time and date markings on sliding rings are based upon earlier astrolabe construction and design principles. See also Celestial sphere: The apparent movements of the Sun, Moon, planets, and stars; History of exploration I (Ancient and classical); History of exploration II (Age of exploration) |
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"Astrolabe." World of Earth Science. 2003. Encyclopedia.com. 28 May. 2012 <http://www.encyclopedia.com>. "Astrolabe." World of Earth Science. 2003. Encyclopedia.com. (May 28, 2012). http://www.encyclopedia.com/doc/1G2-3437800039.html "Astrolabe." World of Earth Science. 2003. Retrieved May 28, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3437800039.html |
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astrolabe
astrolabe, from the Greek astron, star, and labin, to take. The planispheric astrolabe was perhaps the earliest and certainly the most important of instruments to be used by astronomers. The earliest surviving description of its use dates to the 7th century. Chaucer, who seems to have possessed one, wrote an explanation of its use for his little son Lowys. The instrument is, in effect, a highly complex planisphere which rotates over a brass plate on which are engraved azimuths and altitudes as seen at the observer's latitude. The hours of the day, the days of the year, and the signs of the zodiac are set on the rim. Set to the right time and date the bearings and altitudes of the fixed stars on the under plate will be correct. Data for the sun and planets had to be set at the time of observation.
On the back of the instrument is a sight rule, the alidade, which is the basis of the seaman's astrolabe, a simple instrument introduced during the 15th century, probably by the Portuguese, for measuring the altitude of celestial bodies at sea. It consisted essentially of a heavy graduated ring of brass fitted with the sighting device at its centre. The only engraving necessary was of two scales running from 0° to 90° so that both ends of the alidade could be read. Like the planispheric astrolabe the instrument was suspended by the thumb or possibly by a short line from a shackle at the top of the ring so that it hung vertically. The alidade was then turned about its axis, so that the sun or star could be sighted along it, and the altitude read. The earliest recorded use of the mariner's astrolabe was by Diogo d'Azambuja in his exploration by sea down the west coast of Africa in 1481. After 1550 Portuguese mariners' astrolabes were graduated to read the zenith distance of the body rather than the altitude. It would seem from contemporary accounts that English seamen at any rate found the astrolabe more convenient to use when the sun stood high in the sky, but preferred the cross-staff when it was at lower altitudes. |
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"astrolabe." The Oxford Companion to Ships and the Sea. 2006. Encyclopedia.com. 28 May. 2012 <http://www.encyclopedia.com>. "astrolabe." The Oxford Companion to Ships and the Sea. 2006. Encyclopedia.com. (May 28, 2012). http://www.encyclopedia.com/doc/1O225-astrolabe.html "astrolabe." The Oxford Companion to Ships and the Sea. 2006. Retrieved May 28, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O225-astrolabe.html |
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astrolabe
astrolabe , instrument probably used originally for measuring the altitudes of heavenly bodies and for determining their positions and movements. Although its origin is ancient and obscure, its invention is frequently ascribed either to Hipparchus or to Apollonius of Perga. For many centuries it was used by both astronomers and navigators. A simple astrolabe consisted of a disk of wood or metal with the circumference marked off in degrees. It was suspended by an attached ring. Pivoted at the center of the disk was a movable pointer called by Arab astronomers the alidade. By sighting with the alidade and taking readings of its position on the graduated circle, angular distances could be determined. Mariners, if sufficiently skilled in navigation, could use the astrolabe to determine latitude, longitude, and time of day and as an aid in making other calculations. It was much used on voyages of discovery in the 15th cent. and was important until the invention of the sextant in the 18th cent. The more elaborate astrolabes bore a star map (the planisphere, a circular map, was added by Hipparchus), a zodiacal circle, and various other useful or decorative devices. |
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"astrolabe." The Columbia Encyclopedia, 6th ed.. 2011. Encyclopedia.com. 28 May. 2012 <http://www.encyclopedia.com>. "astrolabe." The Columbia Encyclopedia, 6th ed.. 2011. Encyclopedia.com. (May 28, 2012). http://www.encyclopedia.com/doc/1E1-astrolab.html "astrolabe." The Columbia Encyclopedia, 6th ed.. 2011. Retrieved May 28, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1E1-astrolab.html |
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astrolabe
astrolabe Early astronomical instrument for showing the appearance of the celestial sphere at a given moment and for determining the altitude of celestial bodies. The basic form consisted of two concentric disks, one with a star map and one with a scale of angles around its rim, joined and pivoted at their centres (rather like a modern planisphere), with a sighting device attached. Astrolabes were used from the time of the ancient Greeks until the 17th century for navigation, measuring time, and terrestrial measurement of height and angles.
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"astrolabe." World Encyclopedia. 2005. Encyclopedia.com. 28 May. 2012 <http://www.encyclopedia.com>. "astrolabe." World Encyclopedia. 2005. Encyclopedia.com. (May 28, 2012). http://www.encyclopedia.com/doc/1O142-astrolabe.html "astrolabe." World Encyclopedia. 2005. Retrieved May 28, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O142-astrolabe.html |
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astrolabe
astrolabe An ancient device for measuring the altitudes of stars, like a simple sextant. A basic astrolabe consists of a disk hung vertically, with a sight (the alidade) which is pivoted so that it may be pointed at a chosen star. The local time can then be read off the face (the tablet) of the astrolabe. Different tablets can be used for different latitudes. Sophisticated modern versions are used for high-precision measurements of star positions; see Danjon Astrolabe; Prismatic Astrolabe.
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"astrolabe." A Dictionary of Astronomy. 1997. Encyclopedia.com. 28 May. 2012 <http://www.encyclopedia.com>. "astrolabe." A Dictionary of Astronomy. 1997. Encyclopedia.com. (May 28, 2012). http://www.encyclopedia.com/doc/1O80-astrolabe.html "astrolabe." A Dictionary of Astronomy. 1997. Retrieved May 28, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O80-astrolabe.html |
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astrolabe
astrolabe an instrument used to make astronomical measurements, typically of the altitudes of celestial bodies, and in navigation for calculating latitude, before the development of the sextant. In its basic form (known from classical times) it consists of a disc with the edge marked in degrees and a pivoted pointer. Recorded from late Middle English, the word comes ultimately (via Old French and medieval Latin) from Greek astrolabos ‘star-taking’.
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ELIZABETH KNOWLES. "astrolabe." The Oxford Dictionary of Phrase and Fable. 2006. Encyclopedia.com. 28 May. 2012 <http://www.encyclopedia.com>. ELIZABETH KNOWLES. "astrolabe." The Oxford Dictionary of Phrase and Fable. 2006. Encyclopedia.com. (May 28, 2012). http://www.encyclopedia.com/doc/1O214-astrolabe.html ELIZABETH KNOWLES. "astrolabe." The Oxford Dictionary of Phrase and Fable. 2006. Retrieved May 28, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O214-astrolabe.html |
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astrolabe
as·tro·labe / ˈastrəˌlāb/ • n. chiefly hist. an instrument formerly used to make astronomical measurements, typically of the altitudes of celestial bodies, and in navigation for calculating latitude, before the development of the sextant. |
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"astrolabe." The Oxford Pocket Dictionary of Current English. 2009. Encyclopedia.com. 28 May. 2012 <http://www.encyclopedia.com>. "astrolabe." The Oxford Pocket Dictionary of Current English. 2009. Encyclopedia.com. (May 28, 2012). http://www.encyclopedia.com/doc/1O999-astrolabe.html "astrolabe." The Oxford Pocket Dictionary of Current English. 2009. Retrieved May 28, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O999-astrolabe.html |
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astrolabe
astrolabe instrument used for taking altitudes and solving astronomical problems. XIV. — OF. astrelabe — medL. astrolabium — Gr. astrólabon, sb. use of n. of adj. astrólabos ‘star-taking’, f. ástron STAR + *lab-, base of lambánein take.
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T. F. HOAD. "astrolabe." The Concise Oxford Dictionary of English Etymology. 1996. Encyclopedia.com. 28 May. 2012 <http://www.encyclopedia.com>. T. F. HOAD. "astrolabe." The Concise Oxford Dictionary of English Etymology. 1996. Encyclopedia.com. (May 28, 2012). http://www.encyclopedia.com/doc/1O27-astrolabe.html T. F. HOAD. "astrolabe." The Concise Oxford Dictionary of English Etymology. 1996. Retrieved May 28, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O27-astrolabe.html |
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astrolabe
astrolabe
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"astrolabe." Oxford Dictionary of Rhymes. 2007. Encyclopedia.com. 28 May. 2012 <http://www.encyclopedia.com>. "astrolabe." Oxford Dictionary of Rhymes. 2007. Encyclopedia.com. (May 28, 2012). http://www.encyclopedia.com/doc/1O233-astrolabe.html "astrolabe." Oxford Dictionary of Rhymes. 2007. Retrieved May 28, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O233-astrolabe.html |
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