|astrological houses: 12-part division of the heavens centred on the celestial pole, also called 'houses of heaven', see article on the astrolabe.
|astrological instrument: an instrument whose primary use was for astrological purposes, such as casting horoscopes, rather than for observational astronomy.
|astronomical clock: a clock incorporating astronomical instruments such as an astrolabe or armillary sphere.
|astronomical compendium: several instruments compiled into one device, see article on the astronomical compendium.
|astronomical ring dial: a portable astronomical instrument comprising three circles, one to be aligned with the equator, one with the meridian and the third to indicate right ascension and declination. It can be used for astronomical measurement or for telling the time.
|azimuth: the arc of the horizon between the meridian and the great circle passing through the zenith and an observed body.
|azimuth dial: sundial depending on measurement of horizontal angles, see article on the sundial.
|Babylonian hours: system of hour reckoning, see article on time and date.
|baseline: measured distance between two positions used as the observing stations for a triangulation survey; angles measured from this line serve to locate the other points in the survey.
|Butterfield dial: type of sundial adjustable for latitude, whose gnomon incorporates a latitude pointer often in the form of a bird. This dial type is associated with the 17th-century Paris-based English maker Michael Butterfield.
|calendar: system of reckoning dates, see article on time and date.
|calendar-zodiac scale: scales relating the sun's position in the zodiac to the date.
|caliper: instrument used to take dimensions. One type has two curved legs whose endpoints are used to determine the diameters of spheres.
|Cardan suspension: universal mounting attributed to the 16th-century Italian philosopher and physician Girolamo Cardano which, by means of gimbals, allows the supported part to remain horizontal irrespective of the orientation of the rest of the instrument.
|cardinal points: the main directions on the mariner's compass, north, south, east and west.
|carpenter's rule: measuring ruler carrying one or more scales for calculating the volume of timber or the surface area of board or planking.
|celestial globe: globe marked with the positions of the stars.
|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.
|celestial poles: diametrically opposite points of the celestial sphere about which the heavens are observed to rotate once a day.
|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.
|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.
|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.