planisphere

Planisphere is a circular, portable sky map that shows the visible stars for a given time and latitude. It consists of two overlapping disks: a fixed base plate that represents the observer’s horizon and a rotating star map overlay. The overlay is marked with dates or months around the edge and with hours or sidereal time on the inner scale. To use it, the observer selects the proper latitude on the base, then rotates the overlay so that the current date or season aligns with the current time. The window reveals the portion of the sky visible at that moment, allowing the user to identify bright stars and the shapes of constellations.

Planispheres are commonly used by amateur astronomers, students, and navigators to learn the sky, plan observing sessions, or practice celestial navigation. They come in northern- and southern-hemisphere versions, and many models are adjustable to a range of latitudes. Materials range from cardboard and paper to durable plastic, with some versions including markings for planets or deep-sky objects.

The planisphere has historical roots in the long tradition of star charts and spherical astronomy. While older forms existed in various cultures, the modern educational planisphere became widespread in the 19th and 20th centuries and remains a common introductory tool for learning the constellations and their seasons. In contemporary use, digital planispheres and apps have supplemented or replaced printed devices, but the basic principle—matching a time- and latitude-dependent map to the real sky—remains the same.