|theodolite: surveying instrument for measuring horizontal angles, and perhaps vertical angles, see article on the theodolite.
|theodolite and sundial: an instrument combining the functions of a sundial and a theodolite.
|throne: part of an astrolabe, connecting the instrument to the suspension shackle, see article on the astrolabe.
|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.
|triangulation instrument: instrument with three jointed arms with scales for surveying or range-finding, see article on the triangulation instrument.
|trigonus: triangular element in the type of altitude dial known as the 'organum Ptolemai'.
|tripod base: base of a tripod (three-legged support) on which to stand an instrument.
|tripod legs: legs of a tripod (three-legged support) on which to stand an instrument.
|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.
|tympanum: Latin name (plural tympana) for a latitude plate of an astrolabe.
|unequal hours: system of hour reckoning, see article on time and date.
|universal dial: a sundial that can be used in any latitude.
|universal projection: a type of projection of the celestial sphere appropriate for use in any latitude.
|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.
|vertical dial: sundial where the hour lines are marked on a vertical surface, see article on the sundial.
|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.
|volvelle: a device which rotates; usually referring to one or more discs which turn within a circular scale.
|wedge: the part of an astrolabe which secures the pin; also called the 'horse'.
|wegweiser: a circle marked with the points of the compass and a rotating index.
|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.
|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.
|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.
|zenith: the point on the celestial sphere directly above the observer.
|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.
|zodiacal calendar: pair of circular scales, one of the signs and degrees of the zodiac, the other of the days and months of the year, which yields the date of the sun's position in its annual cycle. Commonly included on the back of an astrolabe.
|zodiacal sign: one of the star constellations included in the zodiac.