An astrolabe is an elaborate instrument for measuring angles of slope. They were used by astronomers, navigators and astrologers for predicting the position of stars and planets and for determining local time or latitude.
Astrolabes show how the sky looks at a specific place at a given time. An astrolabe consists of a disk which is deep enough to hold one or more flat plates called tympans. The rim of the disk is usually graduated into hours of time or degrees of arc.
The tympan is made for a specific latitude and is engraved with circles representing the portion of the sky above the local horizon. Above the tympan, a framework showing a projection of the ecliptic plane and several pointers indicating the positions of the brightest stars, is free to rotate. These pointers may be simple points or artistic shapes, such as balls, stars, snakes, hands or leaves.
Astrolabes such as these were too complex to used for marine navigation. A different device, called a “mariner’s astrolabe”, which was simply a ring marked in degrees, was used for for measuring celestial altitudes at sea.
Astrolabes were usually made of brass and were about 15 centimetres in diameter, although they could be much larger or much smaller. After the invention of printing, cheap astrolabes made of paper were produced. Very few of these have survived.
The astrolabe was invented in ancient Greece around 150 BC. Its invention is often attributed to Hipparchus, who also discovered the precession of the equinoxes and was influential in the development of trigonometry.
Astrolabes continued to be used in the Greek and Roman world. Theon of Alexandria wrote a detailed treatise on the astrolabe in about 390 AD and in 550 AD the Christian philosopher, John Philoponus, wrote a treatise on the astrolabe which is the earliest extant Greek treatise on the instrument.
The astrolabe was introduced to the Islamic world in the eighth and ninth centuries through translations of the Greek texts. It was important to Muslims because it enabled them to determine times for prayers and helped find the direction of Mecca.
There are stylistic differences between astrolabes from different Muslim societies, with those from India being relatively simple while Persian astrolabes became quite complex.
The astrolabe was introduced to Europe in Moorish Spain in the 10th century but did not become widespread throughout Europe until the 14th century. Early European manufacturing centered around Nuremberg and Augsburg in Germany and in Paris.
The use of the astrolabe declined in the last half of the 17th century with the invention of the pendulum clock and the telescope. Astrolabe production has continued, particularly in the Arab world, as a curiosity or educational item.