
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: aidememoire 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: 12part 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.
