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accessory: used here to refer to any associated parts of an instrument such as tripod base and legs.
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age of the moon: number of days that have passed since the previous new moon.
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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.
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almucantars: lines of equal altitude on a projection of the heavens based on the local horizon.
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altazimuth theodolite: theodolite for measuring altitude and azimuth simultaneously, see article on the theodolite.
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altitude: angle above and perpendicular to the horizon, or an angular elevation above the horizontal direction in the same azimuth.
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altitude dial: sundial measuring time from the altitude of the sun.
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armillary sphere: set of rings corresponding to the circles of the celestial sphere, see article on the armillary sphere.
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