Ancient Origins

The basic idea behind the planisphere was used in ancient Rome. The architect and engineer Vitruvius, writing around 27 B.C., described a star map engraved on a solid plate and a horizon mask that rotated over it to show the risings and settings of celestial bodies. A water clock turned the mask once a day to keep up with the sky. Nearly two centuries later, Claudius Ptolemy analyzed the map projections used for such devices in his treatise Planisphaerium.

By the 4th century A.D. a version known as the planispheric astrolabe was in use. Its star map was a skeletal metal framework sliding over a solid plate engraved with the observer's horizon. Medieval Arabs and Persians refined the astrolabe to a peak of versatility and beauty. Some of these ornate "mathematical jewels" made their way to Europe, where they were prized as almost magical. "All the conclusions that have been found, or might be found in so noble an instrument as an astrolabe, are not known perfectly to any mortal man in this region," wrote Geoffrey Chaucer in 1391. By the end of the Middle Ages, astrolabes were the universally recognized trademark of astronomers and astrologers.

Astrolabes were commonly used to sight on the Sun and stars to tell time. The invention of accurate clocks allowed this procedure to be reversed. If you knew the time, you could use this kind of device to find stars. And that is how planispheres have been employed ever since.