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 Glossary
 A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | 178 to 227 of 227 glossary terms
 sector: calculating instrument using pairs of lines on the faces of two hinged arms, see article on the sector. sexagenarium: an instrument in the form of a volvelle for planetary calculations. shadow square: a form of geometrical quadrant, where two sides of a square are divided into equal parts and a plumb line or alidade from the opposite corner is used for measuring angles in terms of ratios (that is, tangents). Where a plumb line is used, one side has a pair of fixed sights. The name comes from the fact that a measurement of the altitude of the sun, expressed as a ratio, applied to the length of the shadow cast by an upright structure, yields its height. See also under astrolabe. sight: a device through which an object of interest can be viewed. Sights appear on surveying and astronomical instruments and in a more specialised form in the gunner's sight. simple theodolite: surveying instrument for measuring horizontal angles, see article on the theodolite. sinical quadrant: a quarter of a circle with a scale of degrees at its circumference which carries a pattern of criss-crossing vertical and horizontal lines. The ratio of the length of a given line to the quadrant's radius gives the sine or cosine of the corresponding angle. solar time: time measured directly from the position of the sun, see article on the sundial. solstice: either of two points on the ecliptic where the sun achieves its maximum declination. The term also refers to the two dates when the sun reaches this position in its annual cycle. The winter solstice corresponds to the shortest day, the summer solstice to the longest. Near these points the sun's declination changes only slowly, hence the etymological meaning of solstice as 'standing sun'. star pointer: point marking the position of a star, see article on the astrolabe. string gnomon: the part of a sundial which casts the shadow, where this part is in the form of a taut string. sun and moon dial: sundial that can also tell the time from the shadow cast by the moon, see article on the sundial. sundial: instrument of many different forms for finding the time from the position of the sun, usually by measuring the position of a shadow, see article on the sundial. sundial and dividers: a compound instrument combining the functions of both a sundial and dividers. surveying and gunnery instrument: compound instrument combining functions required in the closely-associated fields of surveying and gunnery, such as measuring bearings and finding ranges. surveying compass: magnetic compass with fixed sights used for measuring horizontal bearings by magnetic azimuth. See also circumferentor. surveying instrument: surveyors used instruments to take bearings, elevations, distances and other associated measures, as well as, for example, laying out plans and drawing topographical surveys. Included are such instruments as the surveying compass, surveying quadrant, surveying rod, surveying staff, surveyor's cross, surveyor's line, surveyor's square and theodolite. Surveying instruments also include compound instruments embodying several such functions. surveying instruments: see surveying instrument. surveying quadrant: quadrant used for land surveying, usually for measuring altitude and azimuth. surveying rod: an instrument used by surveyors for taking distances and other associated measurements. surveying staff: an instrument used in surveying to measure elevations. surveying staffs: see surveying staff. surveyor's cross: instrument for establishing right-angled lines of sight, by means either of open sights set at the ends of an equal-arm cross or slits in a cylinder. surveyor's line: a long string or rope used to measure distances for surveying. surveyor's square: another name for a surveyor's cross. 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.
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