|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.
|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.
|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.
|climates: division into latitude zones of the part of the earth known to ancient geographers, used by Ptolemy.
|clinometer: an instrument for measuring inclination.
|common hours: system of hour reckoning, see article on time and date.
|compass: instrument indicating directions by a magnetic needle, see article on compass.
|compass card: card or paper with a printed, drawn or painted compass rose, usually carried above the magnetic needle of a compass.
|compass dial: form of azimuth dial.
|compass rose: diagram of the points of the compass, often decorated and coloured. North is usually indicated by a fleur-de-lys.
|compasses: instrument for drawing circles. see article on drawing instruments.
|compendium: several instruments compiled into one device; also see article on astronomical compendium.
|conjunction: two planets having the same position (in longitude) in the ecliptic are said to be in conjunction. This relative position had important astrological significance.
|cross-staff: instrument for measuring angles between distant objects, mostly altitudes, see article on cross-staff.
|cross-staffs: see article on cross-staff
|crucifix dial: form of polyhedral sundial in the shape of a crucifix, see article on sundial.