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My tryst with Astrolabe

 Although I teach Physics, my first rendezvous with astrolabe was few years ago through philately. I got a Spanish postage stamp with the image of astronomer Al-Zarqali / Arzachel, beside his astrolabe. Then I got a Syrian stamp on which astronomer Sibt al-Mardini who was also a muwaqqit (the officer in-charge for regulation and maintenance of the clocks and with communicating the correct times of prayer to the muezzin) was busy doing some observations on his astrolabe. It is pertinent to mention that during golden age of Islamic science, the muwaqqits were astronomers well versed in Ilm al Miqat or the study of astronomical timekeeping.

Another Syrian stamp which was released at the occasion of 2nd International symposium for the history of Arabic Science clearly shows an astrolabe. I became fascinated by its beauty and came to know about two libraries and a museum in India where medieval scientific instruments including astrolabes are preserved. A small collection of unique medieval scientific instruments are preserved in Khuda Bakhsh Oriental Public Library at Patna. Another collection of instruments are preserved at Rampur Raza Library and few instruments are available at Salar Jung Museum, Hyderabad. All these instruments have been catalogued by great Sanskrit scholar R S Sarma. To know more about astrolabes I bought the catalogue of instruments at Rampur Raza Library and album of scientific instruments preserved at the Khuda Bakhsh Oriental Public Library. The album has been edited by Imtiaz Ahmed on the basis of a paper by R S Sarma entitled ‘A brief Introduction to the Astronomical Instruments preserved in Khuda Bakhsh Library, Patna’ published in Khuda Bakhsh Library journal in the year 1999.

Last year I had the opportunity to see the astrolabes and other instruments at Khuda Bakhsh Oriental Public Library. The astrolabe is an ancient astronomical instrument/ analogue computer. Astrolabe is a very versatile instrument. It was first designed by Greeks to measure the altitude of a heavenly body. It was used to ascertain position of celestial bodies like sun, moon, planets and stars in the sky. It was also used for measuring height and distances in land surveys. It can also be used to simulate the motion of heavenly bodies at any locality and time. In fact it can also be used to measure time. The design, manufacture and applications of astrolabes were improved by Arabs in middle ages. It was used to calculate the Qibla and to ascertain the times for Salah. Afterwards astrolabe was adopted for navigational purposes.

Several types of astrolabes have been made since antiquity. The most popular type is the planispheric astrolabe, on which the celestial sphere is projected onto the plane of the equator. A typical astrolabe was made of brass and was about 6-10 inches in diameter, although much larger and smaller ones were made. An astrolabe consists of a disk, called the mater (mother), which is deep enough to hold one or more flat plates called tympans, or climates. A tympan is made for a specific latitude and is engraved with a stereographic projection of circles denoting azimuth and altitude and representing the portion of the celestial sphere above the local horizon. The rim of the mater is typically graduated into hours of time, degrees of arc. Above the mater and tympan, the rete or ankabut, a framework bearing a projection of the ecliptic plane and several pointers indicating the positions of the brightest stars, is free to rotate. The rete, representing the sky, functions as a star chart. When it is rotated, the stars and the ecliptic move over the projection of the coordinates on the tympan. One complete rotation corresponds to the passage of a day.

On the back of the mater there is often engraved a number of scales that are useful in the astrolabe's various applications; these vary from designer to designer, but might include curves for time conversions, a calendar for converting the day of the month to the sun's position on the ecliptic, trigonometric scales, and a graduation of 360 degrees around the back edge. The al ihda or alidade is attached to the back face. When the astrolabe is held vertically, the alidade can be rotated and the sun or a star sighted along its length, so that its altitude in degrees can be read from the graduated edge of the astrolabe. The concept of astrolabe was introduced in India by noted polymath Alberuni through his writings. The medieval rulers Firoz Shah Tughlaq and Humayun patronized astronomy. During Mughal period some finest astrolabes were manufactured in Lahore.

Astrolabes have now been replaced by sextants, GPS and other sophisticated digital devices but their study and handling leads to insights into basic measurement, geometry, stereographic projection, astronomy, geography and design and application of instruments. There are about thousand Islamic astrolabes that survive in various museums throughout world. They are pieces of beauty and craftsmanship for beholders. Not only men but women also excelled in designing of astrolabes. Mirium al Ijli from Aleppo was a famous designer and crafter of intricate astrolabes that’s why she is also known as Mirium Asturlabi.

Dr Seemin Rubab is Associate Professor, Physics, NIT, Srinagar.