
projection: translation of a figure on to a plane or curved surface using straight lines in a systematic way. For example, a spherical surface can be projected on to a plane (the plane of projection) by means of straight lines drawn from all points on the surface to a certain defined point (the point of projection) and marking where they intersect the plane. In a stereographic projection, such as is used for the ordinary astrolabe, points on a containing circle are projected on to an equatorial plane from one pole; in an orthographic projection, such as is used in a Rojas design of universal astrolabe, the point of projection is at infinity and the projection lines are parallel.

proportional compasses: drawing instrument consisting of two legs each with points at either end; used for transferring dimensions in enlarged or reduced ratio.

proportional dividers: drawing instrument consisting of two legs each with points at either end; used for transferring dimensions in enlarged or reduced ratio.

proportional instrument: unusual instrument used to mechanically perform functions in trigonometry.

protractor: instrument for setting out and measuring degrees.

quadragesima: the 40 days of Lent or the first Sunday in Lent.

quadrans vetus: type of horary quadrant, see article on the quadrant.

quadrant: instrument based on a quarter of a circle, see article on the quadrant.

quadratum nauticum: diagram for finding course directions from latitudes and longitudes, see article on the astrolabe.

quatrefoil: decorative form with four leaves or petals.

radio latino: instrument for measuring angles in surveying and gunnery, see article on the radio latino.

ramming rods: long rods, often in wood, intended for use in gunnery and used to push the projectile inside a cannon, or to compress the powder in an arquebus.

reduction compass: a drafting instrument with two pivoting arms which sits parallel to a drawing surface, held up by its fixed points. The arms have no scales but can be divided at any given position by moveable points.

Regiomontanustype dial: design of portable altitude dial adjustable for any latitude.

rete: skeletal star map representing the rotation of the heavens on an astrolabe. The rete normally features a projection of the ecliptic and pointers for prominent stars, and can be rotated over a chosen latitude plate. see article on the astrolabe.

right ascension: angle parallel to the celestial equator, measured eastwards from the spring equinox.

ring dial: simple form of altitude dial in the shape of a ring, for use in one latitude.

rule (or ruler): an instrument of multifarious functions in the form of a straight rod of metal, wood or other material; a folding rule is an alternative form with two or more jointed legs.

Saints' days: anniversary days for Christian saints.

scaphe dial: sundial where the hour lines are marked on a concave hemisphere or part hemisphere.

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 crisscrossing 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 closelyassociated 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 rightangled lines of sight, by means either of open sights set at the ends of an equalarm 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 rangefinding, see article on the triangulation instrument.

trigonus: triangular element in the type of altitude dial known as the 'organum Ptolemai'.
