astrolabe

The astrolabe was the most frequently used astronomical instrument of the Middle Ages and early modern times. It served astronomical and surveying observation for nearly 2,000 years. The astrolabe could measure the altitude of the sun, the moon and the stars over the horizon, as well as the altitude of objects over the horizon (mountains, towers) and depths (wells, valleys); it was often employed in estimating the distance between the observer and the observed object. It also measured time, and featured various scales, charts and diagrams for reading astronomical and astrological data. In the 16th and 17th century, its essential use was in navigation at sea. Moreover, it was employed as a demonstration instrument.