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The Astrolabe
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The Astrolabe
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Topic: The Astrolabe (Read 3839 times)
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Bianca
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Posts: 41646
Re: The Astrolabe
«
Reply #30
on:
August 28, 2007, 03:53:01 pm »
An astrolabe derivative where the circular astrolabe is reduced to a quadrant was described in 1288 by Profat Tibbon of Montpelior. Few examples of astrolabe quadrants, commonly called the "quadrans novus" (new quadrant) survive, but many treatises on its construction and use were published. A form of astrolabe quadrant was quite popular in the Ottoman Empire until the early 20th century. Several quadrants using the stereographic projection were introduced in the 17th century. The most popular was devised by Edmund Gunter (1581-1626) in 1618. Gunter's quadrant was quite easy to use in comparison to the older quadrans novus and become widely used.
The Astrolabe Quadrant
The earliest known description of an astrolabe reduced to a quadrant with no moving parts was in 1288 by Jacob ben Mahir ibn Tibbon (1236-1304), more widely known by his Latin name of Prophatius Judaeus or Profeit Tibbon. Tibbon's treatise was quickly improved by Peter Nightingale whose account received wide distribution. The instrument was quickly named the quadrans novus (new quadrant) to differentiate it from the traditional quadrant or quadrans vetus (old quadrant).
The basic idea behind the idea of the quadrans novus is that the stereographic projection that defines the components of a planispheric astrolabe is just as valid if the astrolabe parts are folded into a single quadrant. The result is an instrument that can perform many of the functions of a standard astrolabe at much lower cost, but without the intuitive representation of the sky provided by the rotating rete,.
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Last Edit: August 28, 2007, 03:54:32 pm by Bianca2001
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Bianca
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Posts: 41646
Re: The Astrolabe
«
Reply #31
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August 28, 2007, 03:55:47 pm »
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Bianca
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Re: The Astrolabe
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Reply #32
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August 28, 2007, 03:57:10 pm »
It is not clear how popular the astrolabe quadrant became as few examples survive. There were, however, a number of treatises on the instrument so there is reason to believe that many were made, perhaps of cardboard or wood. Further, it is possible that brass quadrants were not as highly prized as true astrolabes due to their simplicity and were recycled into other instruments. The astrolabe quadrant was more popular in the Ottoman Empire from the 17th century until the early 20th century
The astrolabe quadrant in the picture is not a specific instrument but contains the arcs and scales that can be considered typical on a 14th or 15th century quadrans novus. The arcs reproduce the ecliptic and horizons of a circular astrolabe folded over the East-West line and then folded again about the meridian. Additional arcs are provided to determine the unequal hours and to find the sines and cosines of angles. The astrolabe quadrant is equipped with with a thread (usually of silk) with one end attached at the north pole and a weight on the other end. A bead or pearl can be slid up and down the thread to mark positions on the face of the instrument. The thread can be moved to any position on the face of the quadrant to simulate the rotation of the astrolabe rete.
The scales around the circular margin show the Sun's position in the zodiac, degrees and hours. The scales across the top are for solving trigonometry problems and a declination scale is along the meridian.
Many astrolabe functions are performed in the same way on the astrolabe quadrant as they are on a circular astrolabe. However, finding the time from the Sun's altitude requires the solution of an equation. All of the steps required to solve the equation can be performed on the astrolabe quadrant, but the procedure is somewhat involved.
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Bianca
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Re: The Astrolabe
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Reply #33
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August 28, 2007, 03:58:53 pm »
The Astrolabe to Modern Times
The use of the astrolabe declined in the last half of the 17th century. The invention of the pendulum clock made clocks much more reliable, and more specialized and accurate scientific devices, such as the telescope, became available. Astrolabe production continued into the 19th century, particularly in the Arab world. Much like sundials, any instruments made today are for curiosity or fun, although the educational value of the astrolabe is still appreciated.
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Bianca
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Re: The Astrolabe
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Reply #34
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August 28, 2007, 04:02:26 pm »
A S T R O L O G Y
Astrology has had a major influence on the history and development of astronomy.
The ancient astronomers were motivated to measure the positions of the stars and planets and to keep track of eclipses for astrological reasons (the word "horoscope" is from the Latin horoscopium which, in turn, is from the Greek words for "boundary" (the horizon) and "target" (object of observation).
Their terminology, measurements and techniques were the foundation for the astronomical knowledge that eventually evolved.
Many old astrolabes had astrological features that would allow the user to determine horoscopes.
Creating a horoscope requires knowledge of the positions of the planets and the ecliptic for a certain date and time. The astrologer interprets the aspects to advise his client.
The astrolabe was a convenient way to determine a horoscope because much astrological stress was placed on the position of the ecliptic. Of particular interest were the ecliptic degree on the eastern horizon (the ascendant), the ecliptic degree on the western horizon (the descendent) and the ecliptic degree on the meridian (the degree of mid-heaven). In use, the astrolabe is set to the time and date of interest (birth, death, coronation, etc.) and the ecliptic degrees are read directly.
http://astrolabes.org/history.htm
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Last Edit: August 28, 2007, 04:03:30 pm by Bianca2001
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