|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.