Using this projection the distances between the declination circles are enlarged in such a way that the shapes of the constellations remain correct. The stereographic projection solves this problem while introducing another. One notable planisphere using azimuthal equidistant projection addresses this issue by printing a northern view on one side and the southern view on the other, thus reducing the distance charted from the center outward. If we study the famous constellation of Orion in this projection and compare this to the real Orion, we can clearly see this distortion. That distortion will be worse as the distance to the pole gets greater. 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. Using this projection the sky is charted centered on one of the celestial poles (polar), while circles of equal declination (for instance 60°, 30°, 0° (the celestial equator), −30°, and −60°) lie equidistant from each other and from the poles (equidistant). One such method is the polar azimuthal equidistant projection. For planispheres there are two major methods in use, leaving the choice with the designer. Planispheres, like all charts, are made using a certain projection method. Since the planisphere shows the celestial sphere in a printed flat, there is always considerable distortion. The first star chart to have the name "planisphere" was made in 1624 by Jacob Bartsch.īartsch was the son-in-law of Johannes Kepler, discoverer of Kepler's laws of planetary motion. In this article the word describes the representation of the star-filled celestial sphere on the plane. This usage continued into the Renaissance: for example Gerardus Mercator described his 1569 world map as a planisphere. 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. National Library of Wales MS 735C, Aberystwyth. Users hold the planisphere above their head with the eastern and western horizons correctly aligned to match the chart to actual star positions. The portion of the star chart visible in the window then represents (with a distortion because it is a flat surface representing a spherical volume) the distribution of stars in the sky at that moment for the planisphere's designed location. The disk and overlay are adjusted so that the observer's local time of day on the overlay corresponds to that day's date on the star chart disc. The window is marked to show the direction of the eastern and western horizons. A full twelve months of calendar dates are marked on the rim of the starchart. Planispheres only show the stars visible from the observer's latitude stars below the horizon are not included.Ī complete twenty-four-hour time cycle is marked on the rim of the overlay. Planisphere makers will usually offer them in a number of versions for different latitudes. A planisphere window is designed for a particular latitude and will be accurate enough for a certain band either side of that. The night sky that one sees from the Earth depends on whether the observer is in the northern or southern hemispheres and the latitude. The star chart contains the brightest stars, constellations and (possibly) deep-sky objects visible from a particular latitude on Earth. The chart and overlay are mounted so that they are free to rotate about a common axis. The term planisphere contrasts with armillary sphere, where the celestial sphere is represented by a three-dimensional framework of rings.Ī planisphere consists of a circular star chart attached at its center to an opaque circular overlay that has a clear elliptical window or hole so that only a portion of the sky map will be visible in the window or hole area at any given time. The astrolabe, an instrument that has its origins in Hellenistic astronomy, is a predecessor of the modern planisphere. It is an instrument to assist in learning how to recognize stars and constellations. It can be adjusted to display the visible stars for any time and date. ɪ-/) is a star chart analog computing instrument in the form of two adjustable disks that rotate on a common pivot. In astronomy, a planisphere ( / ˈ p l eɪ. JSTOR ( June 2008) ( Learn how and when to remove this template message).Unsourced material may be challenged and removed. Please help improve this article by adding citations to reliable sources. This article needs additional citations for verification.
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