|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.
|dial: instrument for telling time by the position of the sun (occasionally by the moon), see article on the sundial.
|dialling quadrant: instrument in the form of a quadrant used in the construction of a sundial.
|diptych dial: type of sundial which, in many cases, could be called an astronomical compendium, in the form of a folding tablet, usually in ivory, with at least a string gnomon and a horizontal dial on the inside, but often with other dials as well.
|disc dial: type of sundial belonging in the class of altitude dial.
|dividers: instrument with jointed legs for measuring and transferring distances, see article on the dividers.
|dominical letter: letter denoting the day of the first Sunday of the year, see article on time and date.
|drawing instruments: instruments used for drawing and constructing geometrical figures, see article on drawing instruments.
|Easter table: table for calculating the date of Easter, see article on time and date.
|ecliptic: line on the celestial sphere representing the annual path of the sun through the stars.
|epacts: numbers used in the calculation of Easter, see article on time and date.
|equal hours: system of hour reckoning, see article on time and date.