Astrolabe
A working model of the medieval astrolabe — the most sophisticated analog computer of the ancient world. Drag the golden rete to rotate the star map across the altitude-azimuth grid. For over a thousand years, this single instrument could tell time, find star positions, determine latitude, and predict sunrise.
About this experiment
The astrolabe is widely considered the most important scientific instrument of the medieval world. Its earliest known form dates to around 150 BC, attributed to Hipparchus, the Greek astronomer who developed stereographic projection — the mathematical technique at the heart of the device. The astrolabe was perfected during the Islamic Golden Age by scholars such as al-Fazari (8th century), al-Khwarizmi (9th century), and the extraordinary polymath Mariam al-Ijliyya, a 10th-century woman who crafted astrolabes in Aleppo. From the Islamic world, the instrument spread to medieval Europe, where it became essential to navigation, astronomy, and timekeeping.
The genius of the astrolabe is stereographic projection: the entire dome of the sky, with its circles of altitude and azimuth, is mathematically "flattened" onto a disk as seen from the south celestial pole. This projection has a remarkable property — it maps circles on the sphere to circles on the plane, preserving shapes locally. The tympan (the fixed plate) shows the local sky coordinates for a particular latitude, while the rete (the rotating overlay) carries the positions of bright stars and the ecliptic. By rotating the rete to match the current sidereal time, every star's altitude and azimuth can be read directly.
An astrolabe could do what would otherwise require complex trigonometric calculations: tell local time from star observations, predict sunrise and sunset, find the qibla direction toward Mecca, determine astrological houses, survey land, and even estimate the height of buildings. Chaucer wrote a treatise on its use in 1391 — one of the oldest technical manuals in English. The instrument remained in active use for over 1,500 years, only gradually superseded by the sextant and mechanical clocks in the 18th century.