At the time of Geoffrey Chaucer, who wrote about the astrolabe for his son around 1391, the instrument was already an important measuring tool for calculating the position of the sun in relation to the planets and the stars on any given day. Astrolabes had been in use for over 2,000 years by the 14th Century and remained in use until 1650 when navigation instrumentation evolved into the sextant that we know today.

...
Invented by the ancient Greeks, the astrolabe has been used by astronomers and navigators for centuries.

Origin and Uses of the Astrolabe

The astrolabe originated in ancient Greece in about 200 B.C. and was rapidly adopted by civilizations throughout the Middle East that recognized its potential as an accurate astronomical measuring device. Its uses were many, including defining the time of day, the positions of the planets and plotting horoscopes. It could also be used for forecasting the positions of the sun and the moon in relation to the earth and was invaluable to mariners in plotting a course, using the stars as direction finders.

Islamic Traditions

James Morrison, writing on the history of the astrolabe on his website "The Astrolabe," discusses its use in the ancient Islamic world for defining prayer times and for plotting the direction of Mecca. Antique astrolabes that date back to the 10th Century show engraved marks that indicated plotting points for just such uses. Astronomy and astrology were considered fundamental to Islamic culture, and the astrolabe was an accurate scientific instrument capable of providing reliable readings.

Europe

The astrolabe's accuracy and versatility made it a valuable possession, and by the 11th Century its popularity was spreading from North Africa through Spain and into central Europe. Symbols marked on it for Western religion replaced those of Islam, providing a way of predicting saints' days and the calendar feast days, such as Easter and Christmas. The year, months and weeks were calculable, compared to previous civilizations in which time was based on days and seasons.

Seafaring

Now that stars could be plotted on a map and courses set by their positions, navigation became more of an exact science. Maps could be more accurately drawn and trade more easily carried out between countries separated by oceans. Exploration by sea brought civilizations together in productive and positive ways, as well as provided opportunities for exploitation and war. In the same way that the Internet has brought the world even closer together, the astrolabe pushed boundaries and horizons, making the entire globe navigable.