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triangulation instrument: instrument with three jointed arms with scales for surveying or range-finding, see article on the triangulation instrument.
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trigonus: triangular element in the type of altitude dial known as the 'organum Ptolemai'.
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tripod base: base of a tripod (three-legged support) on which to stand an instrument.
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tripod legs: legs of a tripod (three-legged support) on which to stand an instrument.
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tropic of Capricorn: line of geographical latitude and corresponding line of declination coinciding with the sun's position at the autumnal equinox, its most southerly position in the sky.
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tympanum: Latin name (plural tympana) for a latitude plate of an astrolabe.
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unequal hours: system of hour reckoning, see article on time and date.
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universal dial: a sundial that can be used in any latitude.
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universal projection: a type of projection of the celestial sphere appropriate for use in any latitude.
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vane: upright piece, in this context usually forming a sight and often pierced with a pinhole, a slit, or a slot with a vertical wire.
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