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theodolite: surveying instrument for measuring horizontal angles, and perhaps vertical angles, see article on the theodolite.
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theodolite and sundial: an instrument combining the functions of a sundial and a theodolite.
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throne: part of an astrolabe, connecting the instrument to the suspension shackle, see article on the astrolabe.
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triangulation: surveying technique involving the measurement of a baseline, the location of other stations by taking angles from either end, and perhaps the extension of the survey through the addition of further triangles.
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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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vertical dial: sundial where the hour lines are marked on a vertical surface, see article on the sundial.
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vertical disc dial: sundial where the hour lines are marked on a vertical surface in the form of a disc, see article on the sundial.
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volvelle: a device which rotates; usually referring to one or more discs which turn within a circular scale.
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wedge: the part of an astrolabe which secures the pin; also called the 'horse'.
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wegweiser: a circle marked with the points of the compass and a rotating index.
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wind names: a nomenclature for dividing an azimuth circle into degrees, where eight 45-degree scales are allocated to eight traditional named winds of the Mediterranean.
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wind rose: type of compass rose, where the directions are indicated by the names or initials of the traditional winds of the Mediterranean. May also refer to a whole instrument serving only to indicate the names of the wind directions.
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wind vane: flag-like vane mounted on a vertical post free to align itself with, and indicate the direction of, the wind. Also an instrument performing the same function.
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zenith: the point on the celestial sphere directly above the observer.
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zodiac: a band of 12 constellations of stars straddling the ecliptic; the ecliptic and zodiac are conventionally divided into these constellations, 30 degrees being allocated to each.
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