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( Planisphere)
A planisphere is a star chart analog computer in the form of two adjustable disks that rotate on a common pivot. It can be adjusted to display the visible stars for any time and date. It is an instrument to assist in learning how to recognize stars and constellations. The astrolabe is a predecessor of the modern planisphere. That instrument was known to the ancient Greeks. A planisphere consists of a circular star chart attached at its center to an opaque circular overlay that has a clear eliptical window or hole so that only a portion of the sky will be visible in the window or hole area at any given time. The chart and overlay are mounted so that they are free to rotate about a common pivot point at their centers. The star chart contains the brightest stars, constellations and (possibly) deep-sky objects visible from certain locations on Earth. That location is a band around the Earth centered on a certain (designed for) latitude within the northern or southern hemispheres. Since the night sky that one sees from the Earth depends on the observer's latitude, planisphere windows are designed for a range of particular latitudes and one should choose a planisphere for which the stated design latitude is the closest match to the latitude and hemisphere of the observer. A complete twenty-four hour time cycle is marked on the rim of the overlay. A full twelve months of calendar dates are marked on the rim of the starchart. The window is marked to show the direction of the eastern and western horizons.
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Planisphere Subcategories
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