The astrolabe was a tool used by explorers to help them figure out where they were. It worked by using the positions of the stars or sun. This is different from the sailor's astrolabe which was used on ships to calculate their distance above and below the equator by measuring the distance of the sun and stars above the horizon.
An early astrolabe was made in the Hellenistic world in 150 BC. It is often attributed to Hipparchus. A marriage of the planisphere and dioptre, the astrolabe was a calculator able to work out many kinds of problems in astronomy. Theon of Alexandria wrote a complex book on the astrolabe. But Lewis (2001) says that Ptolemy used an astrolabe to make the astronomy observations recorded in the Tetrabiblos.
- Evans (1998:155) "The astrolabe was in fact an invention of the ancient Greeks."
Krebs & Krebs (2003:56) "It is generally accepted that Greek astrologers, in either the first or second centuries BC, invented the astrolabe, an instrument that measures the altitude of stars and planets above the horizon. Some historians attribute its invention to Hipparchus"