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celestial globe: globe marked with the positions of the stars.
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celestial planisphere: map of the heavens, produced by geometrical projection of a sphere on to a flat surface. A common example of the use of a form of planisphere is the rete of an astrolabe.
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celestial poles: diametrically opposite points of the celestial sphere about which the heavens are observed to rotate once a day.
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celestial sphere: sphere in the heavens on which the celestial motions are considered to be moving for the purpose of positional measurements from earth. It is far enough away for the earth's size to be negligible in relation to the distance of the stars, so that all observers can be considered to be at the centre of the celestial sphere.
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celestial spheres: system of concentric, rotating orbs centred on the earth, proposed by Aristotle and others as constituting the physical construction of the heavenly system of planets and stars.
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chalice dial: sundial where the hour lines are inscribed on the inside surface of a goblet. A second set of lines can be included to allow for refraction, when the goblet is full. See also scaphe dial.
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circumferentor: form of surveying compass with fixed sights, where bearings are read from the position of a magnetic compass, see also article on the theodolite.
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climates: division into latitude zones of the part of the earth known to ancient geographers, used by Ptolemy.
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clinometer: an instrument for measuring inclination.
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common hours: system of hour reckoning, see article on time and date.
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