The astrolabe is an observation and calculation instrument with many uses: using the height of a heavenly bodily in relation to the horizon as a reference point, it enabled navigators, astronomers and astrologers to determine their location, the time, the hours of sunrise and sunset or the date as well as to measure inaccessible distances. The astrolabe consists of a disc, known as the mater, on which the map of the world appears, with an outer rim, called the limb, graduated in degrees. An open-work circle, the rete, indicates the positions of the brightest stars. A rotating arm, the alidade, is used to take measurements. The school of Alexandria probably developed the idea of the astrolabe in the third century BC, but it did not enter into widespread use until around the eighth century. Astronomers and navigators found it especially helpful. The Flemish instrument-maker Rennerus Arsenius, grandson of the mathematician Gemma Frisius, whose workshop was in Leuven, built this very beautiful piece.