The astrolabe, a pocket sky that could tell time, latitude and prayer direction
Long before calculators, a brass disc etched with wires and cutouts could find the height of a star, identify planets, tell the hour and work out where on Earth you stood. The Greeks called it the star-taker. In the tenth century one astronomer catalogued more than 1,000 uses for it.
An astrolabe is essentially a flattened armillary sphere, a model of the visible dome of the sky pressed onto a plate. Theon of Alexandria wrote a detailed treatise on it in late antiquity. His daughter Hypatia is often credited with inventing it, but that rests on a misreading: her pupil Synesius wrote that she taught him to build one, not that she devised it. Whether Ptolemy used one remains disputed. The oldest treatise that survives is John Philoponus's Greek work from around 550, and a Mesopotamian bishop, Severus Sebokht, writing in Syriac, described brass examples a century later.
Astronomers in the Islamic world transformed the device. They added angular scales and circles for compass bearings along the horizon. It helped set daily prayer times, locate the direction of Mecca and fix the lunar calendar that governs Ramadan. ʿAbd al-Raḥmān al-Ṣūfī devoted 386 chapters to it, and the oldest surviving instrument dates from 927–928 CE. Inventors kept varying it: a spherical version, a simple graduated rod known as the staff of al-Tusi, and in 1235 a geared mechanical model by Abi Bakr of Isfahan.
Europe learned it partly through Gerbert of Aurillac, the future Pope Sylvester II, who probably carried it north of the Pyrenees before the eleventh century. Geoffrey Chaucer wrote a treatise on it for his son. Portuguese sailors under Prince Henry used a simplified version to read latitude at sea, since metal instruments were accurate but heavy.
India gave it a grander title. In 1370 the Jain astronomer Mahendra Sūri, backed by the sultan Firuz Shah Tughlaq, wrote the first Sanskrit manual and called the instrument Yantrarāja, the king of astronomical instruments.
Source: Astrolabe
