Planisphere

In astronomy, a planisphere (/ˈpleɪ.nɪˌsfɪər, ˈplæn.ɪ-/) is a star chart analog computing instrument in the form of two adjustable disks that rotate on a common pivot.

The night sky that one sees from the Earth depends on whether the observer is in the northern or southern hemispheres and the latitude.

Users hold the planisphere above their head with the eastern and western horizons correctly aligned to match the chart to actual star positions.

The word planisphere (Latin planisphaerium) was originally used in the second century by Claudius Ptolemy to describe the representation of a spherical Earth by a map drawn in the plane.

This usage continued into the Renaissance: for example Gerardus Mercator described his 1569 world map as a planisphere.

In this article the word describes the representation of the star-filled celestial sphere on a flat disc.

The shapes of the constellations are proportionally correct in a straight line from the centre outwards, but at right angles to this direction (parallel to the declination circles) there is considerable distortion.

For observers at moderate latitudes, who can see the sky near the celestial pole of their hemisphere better than that nearer the horizon, this may be a good reason to prefer a planisphere made with the polar azimuthal equidistant projection method.

However, most of the time the part of the sky near the horizon will not show many stars, due to hills, woods, buildings or just because of the thickness of the atmosphere we look through.

The changing positions of planets, asteroids or comets in terms of these coordinates can be looked up in annual astronomical guides, and these enable planisphere users to find them in the sky.

Some planispheres have a declination feature printed on the upper disc, along the line connecting north and south on the horizon.