The Astrolabe

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Written by Richard Covington
 
The astrolabe below —shown in working condition and dismantled—was made in Damascus around 1230 by “al-Sarraj the muezzin,” according to an inscription. On the mater, the brass plate onto which the other parts fit (at center in photograph below), is a geographical gazetteer of 38 localities, displaying longitude, latitude and direction of prayer for each. It has four different plates for use in different latitudes, plus a replacement. At the bottom is the skeletal rete on which the positions of heavenly bodies are represented by star pointers. 


NATIONAL MARITIME MUSEUM, LONDON
Gingerly, Fuat Sezgin takes the gleaming brass astrolabe out of its display case and hands it to me. “Don’t drop it,” the science historian warns with an elfin grin. From the 38 astrolabes in the Institute for the History of Arabic– Islamic Science at Johann Wolfgang Goethe University in Frankfurt, I’ve selected a copy of an elegantly designed model constructed in Muslim Seville in the 13th century. This movable enigma is 16.5 centimeters (6½") in diameter, about the size of a dessert plate, and six millimeters (¼") thick. Front and back are crawling with etched circles, arcs, Arabic lettering and numerals, zodiac signs and dials within dials festooned with tiny hooks and pointers—a beautiful but terrifying astronomy exam.

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“See that light?” the professor asks, pointing to a ceiling fixture. “Hold the astrolabe up to the light, look along the pivoting ruler on the back and line it up with the light, which is your star,” he explains. “Where the ruler crosses a scale that circles the back rim of the instrument, the number shows the altitude, in degrees, of that star above the horizon. You take that measurement and the sun’s celestial longitude, using the separate calendar scale on the back, match them up with the star’s altitude and the sun’s coordinates on the front of the astrolabe, and you can determine the name of the star and its location.”

“Got it?” asks Sezgin, as he replaces the instrument in its case. “It just takes some practice,” he adds, with a confidence I am very far from sharing.

After hours poring over explanations, watching demonstrations on the Web site of the Institute and Museum of the History of Science in Florence, Italy and fiddling with “The Electrical Astrolabe,” a whiz-bang computer simulation created by James Morrison, a retired software engineer from Delaware, I’m still a tenderfoot. But I can now report I know my way around the astrolabe well enough to tell time and even locate a few stars with it.

Based on an ancient Greek concept, the astrolabe is the salient emblem of Muslim science. The 10th-century astronomer Abd al-Rahman al-Sufi claimed it had a thousand uses—a bit of poetic exaggeration, of course. The instrument served chiefly to pinpoint stars; predict sunrises, sunsets and prayer times; find the qibla (the direction for prayer toward Makkah); survey land; and cast horoscopes. A simplified version, known as the mariner’s astrolabe, was used for navigation.

An endearing but unlikely Islamic legend has it that the second-century Alexandrian astronomer Ptolemy conjured up the astrolabe when he dropped the celestial globe he was studying while riding a donkey. The donkey stepped on the globe and flattened it, inspiring Ptolemy to reproduce the three-dimensional sky on a two-dimensional plane.

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In fact, an earlier Greek astronomer named Hipparchus from Nicaea (present-day Iznik in Turkey) wrote about the concept of stereographic projection around 150 BC. Although ancient Greek scientists probably created astrolabes, none has survived. The oldest instrument extant, designed by Nastulus in Baghdad in about 927, is now part of Kuwait’s national collection.

The most complete collection of astrolabes in the world, with some 136 instruments, is at Oxford University’s Museum of the History of Science; the UK’s National Maritime Museum at Greenwich has around 70. In the US, Chicago’s Adler Planetarium, the Smithsonian’s Museum of American History and Harvard University each has extensive collections on display.

Some astrolabes are incomparable works of art. In medieval workshops in Baghdad, Aleppo, Cairo, Toledo, Seville, Istanbul and Lahore—and later in 16th-century Augsburg and Nuremberg in Germany and Louvain in Belgium—metalworkers fashioned pieces of incredible finesse, precision and occasional whimsy, with star pointers shaped like birds’ beaks, dogs’ heads, even court jesters. One of the most exquisite astrolabes in the Frankfurt museum is a copy of a 17th-century Persian design swirling with filigree ornamentation and incised with geographical coordinates for 46 cities between Baghdad and Balkh in northern Afghanistan.

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The word astrolabe is a Greek–Arabic hybrid that literally means “star-holder,” an apt description for a device that indicates the positions of the stars, sun, moon and planets. Essentially, it is a map of the heavens, depicting the apparent movements of celestial bodies in terms of celestial latitudes and longitudes, combined with slide rule-like features that allow calculation.

Although there are spherical astrolabes, the most common is the flat, or planispheric, astrolabe, which consists of four parts. A plate, or tympanum, representing the sky fits into a larger base plate, the mater (“mother” in Latin), which is calibrated in degrees (and sometimes also in hours) around the rim. The rete (“net” in Latin) is a large openwork disk with star pointers; a circle showing the sun’s annual path, the ecliptic, is engraved on the rete. (Some astrolabes were also fitted with a clock-like hand on the front called the rule.) On the back is another ruler, the alidade, which pivots on a brass pin that passes through the center of the mater, the tympanum and the rete. All the parts can pivot concentrically in relation to each other.

Because the sky looks different according to one’s location on Earth, a person in Baghdad sees constellations in different positions than someone in Cairo, Córdoba or Toledo, for instance, on any given night. To take this shifting sky into account, observers used different tympanum plates for different latitudes. Some planispheric astrolabes were equipped with as many as nine interchangeable plates for latitudes ranging from Zaragosa to Ghana and Sri Lanka. But in 11th-century Toledo, Ibrahim al-Zarqali (known in the West as Azarchel) perfected the safîha, a universal astrolabe with only one plate that was capable of making readings at any latitude.

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Generally speaking, if you know the time, you can locate virtually any celestial body using the astrolabe. Conversely, if you know the coordinates of the sun or stars, you can tell the time. Say you want to predict the time that sunrise will occur on a certain date. You locate that date on the circular calendar engraved on the back of the astrolabe, line it up with the edge of the alidade and read off the coordinates for the sun’s celestial longitude on that date. Then you rotate the rule on the front of the astrolabe so that it crosses that longitude marked on the small ecliptic circle on the rete. You then rotate the rule and the rete together until they intersect on the eastern horizon shown on the astrolabe. You see that the rule crosses a time marked on the rim of the mater: That is the time of sunrise on the date you selected.

Many astrolabes also had “shadow squares” engraved on their backs to enable the observer to measure the height of buildings, trees, mountains and so on. For example, if you know how far you are from the base of a tower, you hold up the instrument and sight the top of the tower along the alidade. Where the alidade crosses the shadow square, you read off the number on the vertical scale as a ratio to the horizontal scale. Using this ratio, you can calculate the height of the tower as a proportion of your horizontal distance from it.

Brought to Europe through Muslim Spain around the 13th century, astrolabes remained popular until the 17th century, when they were supplanted by pendulum clocks and telescopes. The 14th-century poet Geoffrey Chaucer wrote the first treatise in English on astrolabes to teach his 10-year-old son, Lewis, about astronomy. The instruments remain handy devices for understanding time and the heavens, whether you use a cardboard astrolabe or a computer-simulated one.


http://www.saudiaramcoworld.com/issue/200703/the.astrolabe.a.user.s.guide.htm

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Galleries at the Institute for the History of Arab–Islamic Science are packed with reproductions of astronomical and navigational instruments. 






Paris-based author Richard Covington writes about culture, history and science for Smithsonian, The International Herald Tribune, U.S. News & World Report and the London Sunday Times. His e-mail is richard peacecovington@gmail.com.

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                      I B R A H I M    i b n   S A I D    A L - W A Z Z A N  A N D   T H E   S A F I H A


 


Bab Mardum Mosque, Toledo



This short article is taken from the full article which is available here as a PDF file

http://www.muslimheritage.com/features/default.cfm?ArticleID=523




The glory of Toledo in the 11th century was the development of exact sciences. Here can be found the mathematicians al-Waqqadi and al-Tugibi; the geometers Ibn al-Attar and Ibn Hamis who were also astronomers. Amongst these illustrious figures was Muhammad ibn Al-Safar, who in 1029 made an astrolabe which, centuries after, was found in the Sprenger collection. It was later transferred to the Staatsbibliothek of Berlin and can today be located in the Westdeutsche Bibliothek of Marburg (still in Germany). Here could be found Ibn al-Bagunis and Ibn Wafid and here could be seen the rising star of Muslim scholarship, the young al-Zarqali. Said Al-Andalusi (al-Tulaytuli) (of Toledo) (1029-1070) has left us important information on this subject in his Tabaqat al-Umam (The Classification of Nations) which has been studied by illustrious historians

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Ibrahim ibn Said Al-Wazzan (1085 C.E.) is known to have been a prominent instrument maker working in Valencia and Toledo. From 1085 onwards his son Muhammad worked with him. He made at least six astrolabes, one of which he constructed in Toledo in June 1067 and which centuries later became part of the collection of D. Faustino de Borbon before ending up in the Archaelogical Museum of Madrid (there is an electrotype of it in the London Science Museum). The following year, he made another astrolabe also in Toledo, which now is in the Lewis Evans Collection in the Museum of the History of Science, Oxford. His four other astrolabes are today scattered in different museums, mostly in Rome at the Museo Astronomico and the Museo Kircheriano. In 1085, with his son, he made a celestial globe with its stand (Kursi) for the Vizier Abu Issa B. Labbun (the minister at the Toledo court mentioned above). The globe was formerly in the Belluomini Collection, but is now in the Museo di storia della Scienza in Florence. Tracing the works of this remarkable instrument maker and giving him due credit through raising awareness of his achievements is highly necessary. There is a vast but widely scattered bibliography mainly in Spanish that can be researched through.

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Toledo was the residence of a great number of able architects. Fath B. Ibrahim (Fl. 934; d. 1013) was known as al-Qashari, a scholar, pious man and architect who, although flourishing in the Caliph court of Cordova, was also credited for building two mosques in Toledo. He also restored the fortifications of Makkada and Waqqash. His contemporaries were the architects Musa. B. Ali, al-Banna (the constructor) and Saada, who erected the mosque of Bab Mardum known now as San Cristo de la Luz and is dated from the years 999-1000.

Al-Zarqali (Arzachel) (1029-1087) was a Spanish Muslim of a family of artisans who entered the services of Qadi Ibn Sa'id of Toledo as a maker of delicate instruments. He lived in Toledo until the city became insecure following Christian attacks.

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Al-Zarqali prepared the famous Toledan Tables, the original version of which were in Arabic are lost but two Latin versions have survived. Ptolemy's exaggerated estimate of the length of the Mediterranean Sea at 62o were first cut by al-Khwarizmi to 52o then probably by al-Zarqali to the near the correct value of 42o. Al-Zarqali's work was translated into Latin by Gerard of Cremona and was very popular for more than two centuries. All subsequent tables for different locations in the Christian West were based on al-Zarqali's measurements such as the tables of Marseilles, and his tables were also adapted to the meridians of London, Paris and Pisa. Robert of Chester's work was less a translation than an adaptation of the tables of al-Battani and al-Zarqali for the coordinates of London, 1149.

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Al-Zarqali was also a renowned instrument maker. As Barron Carra de Vaux tells us, he was given the surname `Al-Nekkach' which means the engraver of metals. According to established tradition he was a mechanic and metal craftsman who was very able with his hands. It was as an instrument maker that al-Zarqali entered the services of Qadi Ibn Said of Toledo. He was needed to make delicate instruments essential to continue astronomical observations begun in 1060, possibly by Yahia Ibn Abi Mansur. First Al-Zarqali built instruments for other scholars, but when they realised his great intellect, they became interested in him. As he told them that he was man of little learning, having never studied any science nor touched a book, they put him to task and made him study and learn, putting at his disposal the books he needed to educate himself. Two years later, in 1062, he became a member of the group and soon after its director. Al-Zarqali continued to make instruments requested by others but now began to invent his own and he also began to teach his own masters to the point that they soon began to follow his example. He invented and constructed an astrolabe - a safiha - about which he wrote a treatise out of which a whole literature developed. A Jew from Montpellier in France translated it into Latin; King Alfonso of Castile made two translations of it into Romance (Spanish) whilst Regiomontanus in the fifteenth century published a collection of problems on the `noble instrument of the safiha.' Around 1062 Al-Zarqali constructed the water clocks of Toledo, descriptions of which can be found in Al-Zuhri as conveyed to us in Spanish by Millas Vallicrosa and partly in English as in the following extract from Thomson:

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The clocks consisted of two basins, which filled with water or emptied according to the increasing or waning of the moon. At the moment when the new moon appeared on the horizon, water would begin to flow into the basins by means of subterranean pipes, so that there would be at day-break the fourth of a seventh part, and at the end of the day half a seventh part, of the water required to fill the basins. In this proportion the water would continue to flow until seven days and as many nights of the month had elapsed, by which time both basins would be half filled. The same process during the following seven days and nights would make the two basins quite full, at the same time that the moon was at its full. However, on the fifteenth night of the month, when the moon would begin to wane, the basins would also begin to lose every day and night half a seventh part of their water, until by the twenty-first of the month they would be half empty, and when the moon reached her twenty-ninth night not a drop of water would remain in them. It is worthy of remark that, should anyone go to any of the basins when they were not filled, and poured water into them with a view to quicken its filling, the basins would immediately absorb the additional water and retain no more than the just quantity; and, on the contrary, were anyone to try, when they were nearly filled, to extract any or the whole of their water, the moment the hands are raised, the basins would pour out sufficient water to fill the vacuum in an instant

The clocks were in use until 1133 when Ibn Zabara was given permission by Alfonso VII (the new Christian ruler) to see how they worked but he failed to reassemble them after dismantling them. With Al-Zarqali now dead, details of his techniques were lost.

http://www.muslimheritage.com/features/default.cfm?ArticleID=523

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ABOVE

 An image of an Islamic observatory, showing an astrolabe in use. From the Whipple Collection.