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Moveable quadrant

AST0972
Telescopes

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Object ID AST0972
Description This 84cm (33 inches) movable quadrant once belonging to the Greenwich Hospital Schools (whose buildings are now occupied by the National Maritime Museum). The school had an active astronomical observatory between 1852 and 1865. The quadrant later belonged to a former assistant at the Royal Observatory, Mr P. J. Melotte who presented it to the museum in 1953. A movable quadrant is, like a mural quadrant, a telescope mounted on a quarter circle frame which is then mounted on a stand allowing it to point in any direction. This feature is unlike a mural quadrant which is fixed along a north-south line. The telescope is pivoted at what would be the centre of the circle while the circle's circumference carries a scale from 0 to 90 degrees. This allows the observer to measure the angular height of the star he or she is observing. The origional telescope for this object is missing.The stand is made up of a quarter circle with a radius of 0.84m. The plumbline is missing. This is a scaled down version of Bird's 8ft mural quadrant installed at the Royal Observatory in 1750. Bird's original quadrant had been so successful it inspired orders from observatories across Europe. A feature common to all these quadrants is the two scales, the inner dividing the quarter circle into 90 degrees the outer dividing it into 96 divisions. The 96 unit scale was created by George Graham for his original 8ft quadrant to simplify the process of accurate division. 96 can be continuously divided by 2 making it possible to use geometry to ensure their accuracy. This was at a time when the divisions had to be done by hand. There was then a table to convert this scale into the more conventional 90 degree scale. In practice, both scales were read.
Date made circa 1750

Artist/Maker Bird, John
Place made London, England
Credit National Maritime Museum, Greenwich, London
Parts
  • Moveable quadrant (AST0972)
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