Sky Map · The Ancient Sky Computer
A disc of brass that told the time, tracked the calendar and mapped the stars — a thousand years before the first computer. Most of us will never hold one; here you can watch it work.
Drag the rete to turn the sky · tap any part to learn what it is · tap any star to dive into the Deep Sky viewer.
Tap a part to light it up on the instrument above. An astrolabe is a stack: a dished base holds a plate engraved for one latitude, and an openwork star map spins on top.
It flattens the dome of the sky onto a disc. Using stereographic projection — drawing every point of the sky as seen from the south celestial pole — circles in the sky stay circles on the metal. That one trick is why a flat plate can stand in for the round heavens.
The plate is your world; the rete is the sky. The fixed plate is engraved with the things that never move for you: your horizon, rings of equal altitude (the almucantars) climbing to the point overhead, and lines of compass direction (azimuth). The rete carries the things that do move: the ring of the ecliptic (the Sun’s yearly path, divided into the zodiac) and pointers to the brightest stars.
You set it to a moment by turning the rete. Rotate the star map until it matches the real sky — put a known star over its measured altitude, or the Sun’s place on the ecliptic onto the horizon at sunset. Now the disc mirrors the heavens, and you can read straight off it: which stars are up, how high, when a star will rise, the length of the night, and the hour itself. For a thousand years this was the pocket computer of astronomers, navigators and timekeepers.
Positions from real astronomy · star pointers at J2000 coordinates · a Sky Map recreation.