ca. AD 400: The astrolabe is an astronomical instrument used for manifold applications. These include measurements of angles (e.g. culmination heights of stars), positions of celestial bodies, determination of time at a given latitude and vice-versa, and triangulation. Thus, it has been an essential tool for localisation during expeditions and for navigation. As a predecessor of the modern planisphere, it demonstrates the daily and annual motion of the sky. The astrolabe is made up of a main disk of brass and one or more hollow plates which are pivoted against each other. The main disk is engraved with lines that form important coordinate grids. The hollow plate is constructed for a specific latitude and shows the portion of the celestial sphere which is above the local horizon at a given time. Some astrolabes have a narrow rule or label which rotates over the upper disk. It is marked with a scale of declinations. The backside has another rule (Alhidade) in addition, which provides a bearing device to measure angles.

The principles of construction were known in antiquity. Depending on sources the invention is attributed to Eratosthenes (284-202 BC) or to Hypatia (ca. AD 370-415), a mathematician living in Alexandria. Her father, Theon of Alexandria (ca. 335-405), wrote a detailed treatise on the astrolabe. Two centuries before, Ptolemy might have used a very rudimentary astrolabe to make astronomical observations. In the medieval Islamic World, the device was further improved and elaborated to its perfection. The first one to build is the 8th century Persian astronomer Muhammad al-Fazari (died in 796 or 806). The earliest surviving astrolabe is dated AD 927/8. The knowledge about this device came via Spain (Andalusia) into the Latin speaking Europe during the 12th century. From the 15th century through approximately 1650 it was the most popular astronomical instrument. Then, it was substituted by specialised and more precise equipment.