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A device of unique capabilities, the astrolabe remained the most popular of all astronomical instruments for centuries, writes
B. N. Goswamy
“And it is He who ordained the stars for you that you may be guided
thereby in the darkness of the land and the sea.” The Koran
I wonder how many of us really know what an astrolabe is. Till I came to learn of some splendid examples, I did not know much about the instrument either. True that I had seen one in use: in a Mughal painting showing the birth of prince Salim where a mullah-like figure is holding an astrolabe up to read from it, obviously to record the exact time of the prince’s birth. But my curiosity was truly aroused when I saw an old piece that came from the Arab world, for there was in it such precision, such beauty of execution. It shone like a golden disc, and its moveable parts, much like the calligraphic engraving on it, were exquisitely crafted. Scientific instrument that it was, it had the air of a great work of art. More of this later; first, however, something about astrolabes in general. To begin with, the word itself. It goes back to Greek in which ‘aster’ means a star, and ‘lambanein’ means ‘take, seize, catch, grasp, apprehend, determine, estimate’. Taken together, then, an astrolabe is a ‘star-taker’ or ‘star-catcher’. To proceed further and to understand the device, however, one has to quote, for exactitude of words is needed. “The astrolabe”, one reads — generally a brass disk with a knob at the top to hold it, or hang it, from — “was developed by ancient astronomers to measure the altitude and direction of celestial bodies over the horizon, calculate the seasons, the movements of the zodiac and to foretell eclipses”. In other words, this very ancient astronomical computer — the origins of it go back to the time of Hipparchus in Greece, close to 150 BC — was invented for solving problems relating to time and the position of the Sun and stars in the sky. Several types of astrolabes were made. By far, the most popular type was the planispheric astrolabe, on which “the celestial sphere is projected onto the plane of the equator. This is done by drawing the sky on the face of the astrolabe and marking it so that positions in the sky are easy to find. To use an astrolabe, you adjust the moveable components to a specific date and time. Once set, the entire sky, both visible and invisible, is represented on the face of the instrument.” This allowed a great many astronomical problems to be solved in a very visual way. Typically, the astrolabe was used for finding the time during the day or night, finding the time of a celestial event such as sunrise or sunset, and as a handy reference of celestial positions. Even though it is easy to associate astrolabes with navigation — for mariners were the ones whose lives on uncharted waters often depended upon it — the typical astrolabe was not originally conceived as a navigational instrument. Also, if it is a matter of measuring or telling time, the astrolabe could be seen as being related to devices such as the sundial, the sand-glass, the candle-clock, the weight-driven clock. But the old instruments were often used strictly for astrological purposes. And in the late medieval times, they were turned into one of the basic astronomy education tools at universities in Europe. The history of the astrolabe goes back, as noticed, to something like 2000 years. The principles of astrolabe projection were known by 150 BC, but true astrolabes were made by 400 AD. It was the Arabs, however, who kept refining the instrument: the inspiration must have come from the needs of those adventurous Arab navigators, and the words of the holy Koran. By the year 800, they had developed some truly refined instruments, and it was they — from Islamic Spain, specifically Andalusia — who introduced them to Europe. For the next 500 years, the astrolabe remained the most popular of all astronomical instruments and ruled the continent, as it were, as a device of unique capabilities. Even Geoffrey Chaucer, the great poet, was seduced by its charms, writing an elaborate piece on it in 1392. Great university towns in Europe — among them Nuremberg and Augsburg in Germany, and Louvain in France — turned into centers where these instruments were actually produced. With the passage of time, however, the astrolabe yielded place — after about 1650 or so — to even more specialised and accurate instruments: the sextant and the theodelite, for example. But it has occupied for all these centuries a place in the memory of men: a reminder of times gone by. For it was, as someone said, “the multipurpose laptop computer of our predecessors”. Long after they went out of use, astrolabes kept being collected, two of the largest collections now being in the Adler Planetarium and Astronomy Museum in Chicago, and in the Museum of the History of Science at Oxford. Romance has still not ebbed out of that device. To come back home: one knows only too well how well-studied the heavenly bodies were in ancient India, advances in astronomy being one of our great achievements. But, by all accounts, the first Indian treatise on the astrolabe came to be written only in 1370: by a Jain scholar, Mahendra Suri by name. In the Arab world, on the other hand, where astrolabes of a very high order had started being made as early as the nineth century, elaborate treatises were constantly being written. Some superb examples have come down, both of the instrument and the text, one of the most celebrated surviving astrolabes bearing the signature: “Hussain bin Ali”, and a date equivalent to 1309/10 AD. Considering its perfection, it has been spoken of as a “stunning symbiosis of applied mathematics and applied art”. To end these brief references to the history of the astrolabe with a romantic account, however, it comes from Canada. In May 1613, Samuel de Champlain, French explorer-cartographer, travelled up the Ottawa River, naturally carrying an astrolabe with him. Rapids came in the way, and Champlain and his men were forced to portage and to climb over and under fallen logs at one particularly difficult point by Green Lake. Here, according to several 19th century authors, Champlain lost his astrolabe. But 254 years later, the astrolabe was found: a 14-year-old farm boy named Edward Lee came upon it in 1867 while helping his father clear trees by that lake.
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