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An astrolabe is a device for measuring the celestial altitude of a celestial body.

It consists of a series of disks on which are shown the positions of prominent stars.

These are aligned with their positions on what can be seen of the celestial sphere, and hence obtain a reading of their altitudes.

Historical Note

The astrolabe dates to the time of Apollonius of Perga, although some sources credit it invention to Hipparchus of Nicaea.

Theon of Alexandria wrote a detailed treatise on the astrolabe.

It is supposed that Claudius Ptolemy may have used an astrolabe to provide the data for his Apotelesmatika.

Some authors attribute the invention of the astrolabe to Hypatia of Alexandria, based on a misinterpretation of a letter written by one of her students, but it is well established that it had been invented some $500$ years earlier.

Use of the astrolabe was adopted by the medieval Arab world, where its design was made considerably more sophisticated and intricate.

Linguistic Note

The word astrolabe derives ultimately from the Greek word astrolabos from astron (star) and lambanein (to take).

Hence it literally means star-taker.