|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.
|cylinder dial: type of altitude dial, see article on the sundial.
|declination: angle above or below the celestial equator.
|declinatory: instrument used in constructing a sundial, to find the declination of a plane.
|degrees: the conventional divisions of a circle, one being the 360th part of a circle, and thus the units used to measure and express angles.
|diagonals: lines used in the subdivision of divisions of an arc, also known as transversals. Each diagonal line is drawn across the division from its beginning to its end, and is divided linearly either by dots or by concentric arcs. In this way the linear division of a line can be used for the angular subdivision of an arc.