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( Planetarium)
A planetarium is a theatre built primarily for presenting educational and entertaining shows about astronomy and the night sky, or for training in celestial navigation. A dominant feature of most planetariums is the large dome-shaped projection screen onto which scenes of stars, planets and other celestial objects can be made to appear and move realistically to simulate the complex 'motions of the heavens'. The celestial scenes can be created using a wide variety of technologies, for example precision-engineered 'star balls' that combine optical and electro-mechanical technology, slide projector, video and fulldome projector systems, and lasers. Whatever technologies are used, the objective is normally to link them together to provide an accurate relative motion of the sky. Typical systems can be set to display the sky at any point in time, past or present, and often to show the night sky as it would appear from any point of latitude on Earth. Planetariums have become well-nigh ubiquitous, with some privately owned. A rough estimate is that in the United States there is one planetarium per 100,000 population, ranging in size from the Hayden Planetarium's 20-meter dome seating 430 people, to three-meter inflatable portable domes where children sit on the floor. Such portable planetariums serve education programs outside of the permanent installations of museums and science centers. See also the Timeline of planetariums for specific dates and events in the historical influences on and development of planetariums. The small size of typical 18th century orreries limited their impact, and towards the end of that century a number of educators attempted some larger scale simulations of the heavens. The efforts of Adam Walker (1730-1821) and his sons are noteworthy in their attempts to fuse theatrical illusions with educational aspirations. Walker's Eidouranion was the heart of his public lectures or theatrical presentations. Walker's son describes this "Elaborate Machine" as "twenty feet high, and twenty-seven in diameter it stands vertically before the spectators, and its globes are so large, that they are distinctly seen in the most distant parts of the Theatre. Every Planet and Satellite seems suspended in space, without any support; performing their annual and diurnal revolutions without any apparent cause". Other lecturers promoted their own devices R E Lloyd advertised his Dioastrodoxon, or Grand Transparent Orrery, and by 1825 William Kitchener was offering his Ouranologia, which was 42&_160;feet (13&_160;m) in diameter. These devices most probably sacrificed astronomical accuracy for crowd-pleasing spectacle and sensational and awe-provoking imagery.
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