The astrolabe was a key astronomical instrument during the Middle Ages. It can be used to tell the time, measure the heights of stars and buildings, and for many other calculations and observations. It works because it is a 2-dimensional representation of the 3-dimensional Universe as understood by the medieval astronomer - with the Earth at the centre of the apparent motion of the Sun.
This large brass instrument (Image 1), probably made in the 14th century, is typical of medieval English astrolabes. It has been engraved for the latitude 52 degrees. This is the latitude of London, so it is reasonable to suppose that it was intended for use in the capital.
The rete, a cut-out plate with pointers showing the position of stars, rotates over the projection. Most of the 41 pointers here are labelled with the names of stars, many of them in Arabic, reflecting the Arab influences on medieval European astronomy. One is in the shape of a bird but is not labelled - it is probably marking the beak of the constellation Corvus (the crow) (Image 2).
On the back of the astrolabe are concentric circles for calculations relating to the calendar. As well as a zodiac calendar, there is also a calendar in which saints' days and feast days are marked, providing a handy reference for a medieval religious man. We know that monks, friars and clerics were among those who owned astrolabes in the Middle Ages. This doesn't necessarily mean that this astrolabe was made for a religious man - these kinds of scales would also be useful for a politician or a merchant, since the medieval calendar was organised around saints' days and religious festivals.
Catherine Eagleton, 'A 14th-century English astrolabe', Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge, 2008 [http://www.hps.cam.ac.uk/whipple/explore/astronomy/amedievalastrolabe/, accessed 23 May 2013]