|accessory: used here to refer to any associated parts of an instrument such as tripod base and legs.
|age of the moon: number of days that have passed since the previous new moon.
|alidade: straight rule with sights. An alidade could be used for surveying with a plane table but it also appears in a different form as part of an astrolabe.
|almucantars: lines of equal altitude on a projection of the heavens based on the local horizon.
|altazimuth theodolite: theodolite for measuring altitude and azimuth simultaneously, see article on the theodolite.
|altitude: angle above and perpendicular to the horizon, or an angular elevation above the horizontal direction in the same azimuth.
|altitude dial: sundial measuring time from the altitude of the sun.
|armillary sphere: set of rings corresponding to the circles of the celestial sphere, see article on the armillary sphere.
|aspectarium: aide-memoire which depicts the astrologically significant angles (or aspects) between the planets.
|astrolabe: astronomical instrument based on a planispheric projection of the heavens, see article on the astrolabe.
|astrolabe quadrant: quadrant that has many of the lines and functions of the ordinary astrolabe, projected for a single latitude.
|astrolabum catholicum: type of universal astrolabe, see article on the astrolabe.
|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.