The Astrolabe

An astrolabe is a kind of inclinometer or clinometer, also known as a tilt meter, gradient meter or slope gauge (among other things). As its various names suggest, it is used for measuring things like the angle of a slope, or the angle of elevation of an object above the horizon. Astrolabes have been used since ancient times, both for navigation and for pinpointing the position of various celestial objects. The astrolabe is believed to have been invented in Greece in around 150 BCE, possibly by the Greek astronomer and mathematician Hipparchus of Nicaea (circa 190 - circa 120 BCE). The example shown below dates from 1326 CE, and is known as the Chaucer astrolabe. It is one of the earliest known examples of a European astrolabe, and can be seen at the British Museum.

The Chaucer astrolabe © Trustees of the British Museum

An astrolabe like the one shown above is typically made of brass, and consists of a number of moving parts. The main body of the instrument is a large disk called the mater (which is Latin for mother) which has a raised border so that it can accommodate a number of thin, flat, circular plates. The ring around the edge of the mater (called the limb) typically has marks representing degrees or hours (or both). Each circular plate (or tympanum) is marked with circles representing altitude and azimuth for a particular latitude. The instrument can be dismantled and reassembled so that the tympanum most closely representing the latitude at which it is being used is on top of the others, and can thus be seen.

On top of this assembly sits a circular framework called the rete, through which the tympanum immediately beneath it can be seen. This framework includes a circular element called the ecliptic, which represents the path followed by the Sun during the course of a year, and various pointers that represent the positions of some of the brightest stars. Above the rete is an object that looks like a clock hand, called the rule. The whole assembly is held together by a central pin, allowing the rule and rete to rotate over the tympanum. The back of the instrument is engraved with scales that are used for measuring angles. A swivel-mounted ring at the top of the astrolabe allows it to be suspended using a chord during use.

The astrolabe is a versatile instrument that can be used for many different purposes, including timekeeping during both the day and night, determining the latitude of the user, and calculating the positions of stars, planets and other celestial objects. Astrolabe design became more sophisticated and increasingly ornate over time. Indeed, some examples could probably be considered to be works of art in their own right. The functionality of the astrolabe was also extended, to the point where it could be used for literally hundreds of different purposes.

Many fine examples of astrolabes were produced in the Islamic world during a period stretching from the ninth to the thirteenth centuries CE. During this time the Islamic empire included Spain, most of North Africa and the Middle East, and in the East it extended as far as the borders of China and India. Islamic explorers used the instrument to study the heavens for the purposes of navigation, whilst Islamic scholars made similar observations in order to determine prayer times, and to find the direction in which Mecca lay. Much of the scientific knowledge of the Islamic world eventually found its way to Europe via Spain, and a number of notable examples of astrolabes were produced in Europe during the Renaissance period.