|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.