The astrolabe, from the Greek for star taker, is a handheld brass model of the sky that tells time, finds latitude, surveys heights, and predicts the solstice. Its base, the mater, holds interchangeable plates called tympans engraved for specific latitudes using stereographic projection, and over them turns the rete, an open star map whose pointers were shaped like leaves, hands, dogs’ heads, or dragons. You hang it plumb, sight a star such as Sirius with the alidade on the back, turn the rete to match its altitude, and read the time off the rim. The idea grew from Greek armillary spheres, and the popular claim that Hypatia invented it is a myth: her pupil Synesius only wrote that she taught him to build one, and Ptolemy’s Almagest describes a 3D armillary sphere instead.

The earliest manuals are John Philoponus’s 6th century Greek treatise and Severus Sebokht’s Syriac text describing brass instruments. In the Islamic world, Muhammad al-Fazari built the first, al-Sufi wrote 386 chapters on over a thousand uses, and it was used for prayer times, the qibla, and Ramadan. It reached India, where the Jain astronomer Mahendra Suri wrote a Sanskrit manual in 1370 calling it the king of instruments, and Europe through Gerbert of Aurillac, later Pope Sylvester II. Mariners stripped it to a heavy, open ring for sighting latitude at sea.

• Sharaf al-Din al-Tusi built a linear astrolabe that was just a marked rod and plumb line.
• Ibn al-Sarraj’s 14th century universal astrolabe worked at any latitude without swapping plates.
• Geoffrey Chaucer wrote an astrolabe treatise in plain English for his 10 year old son Lewis.
• Early astronomical clocks such as Richard of Wallingford’s and the Prague Orloj are mechanized astrolabes.
• Ulysse Nardin and Christiaan van der Klaauw still make wristwatches with astrolabe dials.