Planisphere

A planisphere is a star finding instrument made of two adjustable disks sharing a common pivot, an analog computer in chart form that a user turns to the current date and time so a window in the upper disk reveals only the stars visible from a given location at that moment. It descends from the astrolabe, itself rooted in Hellenistic astronomy, and Claudius Ptolemy used the word planisphere in the second century AD for a flat representation of the celestial sphere, though the first star chart to carry the name was made in 1624 by Jacob Bartsch, Johannes Kepler's son in law. Unlike the three dimensional ring framework of an armillary sphere, the planisphere compresses the celestial sphere onto a flat, portable device, making it a practical tool for learning to recognize constellations and bright stars.