What is an Astrolabe?

Astrolabes are an ancient astronomical instrument. They were first used in ancient Greece, were extensively developed in the medieval Islamic world and became the key astronomical instrument of the western middle ages.

When mapping the heavens astronomers assume that the stars seen in the night sky are all at an equal distance from the earth, existing on the inside of an enormous sphere that has the earth at its centre. By using this model, they can create the two-dimensional representation of this celestial sphere seen on star charts and astrolabes, the starry equivalent of a map of the earth.

The front of the astrolabe shows a map of the night sky in the form of a rotating, net-like, rete. The rete was also sometimes described by medieval writers with its Arabic name, alhanthabuth or alancabut, meaning spider.

The spikes on the rete indicate the positions of individual stars. Sirius, also known as the Dog Star, is often represented by the head of the dog. The rete also shows the path of the sun against the background stars; this path is known as the ecliptic, and the constellations through which the sun passes on the ecliptic are the well-known signs of the Zodiac.

Beneath the rete sits a plate inscribed with a projection of sky above an observer at a given latitude: the plate could be removed and replaced with others calibrated for different latitudes if necessary. The plate and the rete nest inside the body (or womb) of the astrolabe, called the mater, which is inscribed on its outer edge, or limb, with scales of degrees and hours. Fixed to the centre of the astrolabe is a rotating rule used for taking readings.

On the back of the astrolabe is a rotating bar called the alidade or label, which is used to measure the altitude above the horizon of celestial bodies. This side of the instrument is divided into degrees for taking altitude measurements, and is also engraved with a calendar and divisions of the zodiac.

By using the data from both sides of the instrument and the measured positions of objects in the sky, the user of the astrolabe can calculate many facts about their position in time and space, including the hour of the day, the date, and their position on the earth’s surface.

Find out more about astrolabes and astronomy in the medieval period by investigating some of the manuscripts held in St John's College Library:

• Geoffrey Chaucer's Treatise on the Astrolabe
• Masha' allah, Compositio et operatio astrolabii
• Thomas Betson of Syon's map of the sky

Part of the front of a 15th-century French Astrolabe, Wh.0999, showing the rete and the plate beneath it. Image © Whipple Museum of the History of Science, University of Cambridge.

Part of the back of the same astrolabe. The letters L, S, and S near the edge stand for Libra, Scorpio, and Saggitarius. The letters S, O, N, and D stand for September, October, November, and December. Image © Whipple Museum of the History of Science, University of Cambridge.

Home | What is it? | Build it | Use it | More Information