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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.
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azimuth: the arc of the horizon between the meridian and the great circle passing through the zenith and an observed body.
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azimuth dial: sundial depending on measurement of horizontal angles, see article on the sundial.
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Babylonian hours: system of hour reckoning, see article on time and date.
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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.
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