Astrolabe: the sky that fitted in the hand

Today we pick up a mobile phone to check the time and open GPS to find our location. A thousand years ago, an astronomer did both with a round brass plate that fitted in the palm. It was called the astrolabe. It was invented in Greece, but between the eighth and eleventh centuries CE, scholars in Baghdad, Iran and Andalusia gave it the form in which it remained the most trusted companion of astronomers for centuries.

A Greek invention, a new home

According to the World History Encyclopedia, the astrolabe was probably invented by the Greek astronomer Hipparchus of the second century BCE. Ptolemy later described the method of projecting the celestial sphere onto a flat surface, on which the instrument is based. In the sixth century CE, John Philoponus wrote a treatise on it.

According to the historian G J Toomer in the Dictionary of Scientific Biography, treatises on it existed in Syriac even before any were written in Arabic. The knowledge thus reached Baghdad through several languages.

How the astrolabe works

According to the World History Encyclopedia, the main part of the astrolabe is a deep brass dish called the “mater”. A plate is placed in it, engraved with lines of the sky for a particular latitude. Fine astrolabes came with several plates for different latitudes. Above it sits a rotating, net-like part called the “rete”. Its pointed tips mark the positions of stars, and it turns on a central pin. On the back is a rotating bar called the “alidade”, used to sight the sun or a star.

The principle was to measure the altitude of the sun or a star with the alidade, then turn the rete to set the instrument to the sky of that moment. According to the World History Encyclopedia, the astrolabe was used to find the time, the altitude of the sun and stars, and the latitude of a place. It also measured the height of tall objects such as mountains, so surveyors used it too.

The first makers in Baghdad

In the Biographical Encyclopedia of Astronomers, Kim Plofker writes that the eighth-century astronomer al-Fazari is said to have been the first person in the Muslim world to build a plane astrolabe. He was attached to the court of Caliph al-Mansur (754 to 775). Only the title of his book on the use of the astrolabe survives.

The work spread in the ninth century. According to Toomer, al-Khwarizmi, the author of algebra, also wrote two books on the astrolabe, one on its construction and one on its use.

According to Monica Rius in the same encyclopedia, the oldest surviving astrolabe in the world today was made by a craftsman named Nastulus. It bears the date 315 AH, which corresponds to 927 or 928 CE, and is in the Museum of Islamic Art in Kuwait.

Al-Sufi’s huge book

In the tenth century, Abd al-Rahman al-Sufi of Iran wrote about the instrument in exceptional detail. According to Paul Kunitzsch in the same encyclopedia, he was born in Rayy in 903 CE and died in 986. His best-known work is the Book on the Constellations, written around 964.

According to the database of the Max Planck Institute in Berlin (ISMI), al-Sufi wrote a treatise on the use of the astrolabe. One version has 16 books and 1,760 chapters and is dedicated to Adud al-Dawla, the ruler of the time. A shorter version of the same treatise has 170 chapters.

One plate for every latitude

The biggest weakness of the ordinary astrolabe was that it needed a separate plate for each latitude. In the eleventh century, a craftsman in the Andalusian city of Toledo found a solution. According to Roser Puig, Abu Ishaq Ibrahim al-Zarqali was a famous instrument maker who taught himself astronomy. He made a plate whose very name was “al-safiha al-mushtaraka li-jami al-uruz”, meaning “the plate common to all latitudes”. He made one such instrument in 1048 or 1049 CE for the ruler of Toledo. He died in Córdoba on 15 October 1100, and his treatises on instruments were translated into Latin and Hebrew.

According to the World History Encyclopedia, knowledge of the astrolabe reached Europe through Andalusia from about 1000 CE. The English poet Geoffrey Chaucer wrote a treatise on its use for his son Lewis around 1391.

The astrolabe makers of Lahore

The subcontinent also produced great craftsmen of this art. Royal Museums Greenwich in London holds an astrolabe made between 1641 and 1642 CE by Muhammad Muqim of Lahore. According to the museum, he was the grandson of the famous astrolabe maker Ilahdad and is counted among the best instrument makers of seventeenth-century Lahore. So far, 37 astrolabes made by him have been recorded.

Today the astrolabe adorns museums, but its lesson has not grown old: measure the sky carefully and it tells both the time and the place. A student under the clear sky of Quetta who builds a simple instrument from cardboard and thread and measures the altitude of a star would be repeating the same thousand-year-old tradition.