messuring the sun with the astrolabe
The principles of the astrolabe projection were known before 150 B.C., and true astrolabes were made before A.D. 400. The astrolabe was highly developed in the Islamic world by 800 and was introduced to Europe from Islamic Spain (al-Andalus) in the early 12th century.
ingerly, 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. “That one’s stunning, but it sure is complicated,” he says with frank, but less than reassuring, cheeriness. “I’ll just run you through the basics.” “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. 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. 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. Calculating the height of an object as a proportion of your distance from it, using an astrolabe.
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.
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