A planisphere is a star chart instrument designed to help observers to locate stars and constellations. The two discs rotate and can be aligned to show the visible stars on any day of the year and at any given time.

A planisphere of this type is specific to one location, but it never goes out of date. This example was produced by Philips for an observer near London in 1900.

The planisphere originated in the 2nd century CE and was first used by Ptolemy, the Greco-Egyptian mathematician and astronomer, to provide a flat representation of the night sky. Planispheres are still used today as a learning aid by amateur stargazers.

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