planisphere
Quick Summary
A planisphere is a representation of a sphere on a flat plane. Specifically, it is a polar projection of the celestial sphere and stars onto a plane. This projection allows for the display of celestial phenomena at any given time, often with adjustable components.1
A related instrument, the planispheric astrolabe, was an early scientific tool used for timekeeping and astronomical observations. It enabled astronomers to calculate the positions of the Sun and stars relative to the horizon and meridian. The astrolabe provided a flat image of the celestial sphere, including key circles like the ecliptic and tropics, functioning as a rudimentary analog computer.2
A nautical planisphere is a specific type used by navigators, representing the terrestrial globe on a plane. Astrolabes were used in celestial navigation from the Middle Ages and were eventually supplanted by sextants.32
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