
pillar dial: type of altitude dial, see article on the sundial.

pin gnomon: the part of a sundial which casts the shadow, where this part is in the form of a pin.

plane table: type of surveying instrument, see article on the plane table.

plane table alidade: an alidade specially adapted for use with a plane table.

planetary hours: system of hour reckoning, see article on time and date.

planetary temperaments: astrological character of the fixed stars; as assigned in classical astrology, each star had a nature and effect similar to one or more of the planets.

planisphere: a representation of a spherical body on a flat surface, commonly a map of the earth or of the heavens.

planispheric astrolabe: astronomical instrument based on a planispheric projection of the heavens, see article on the astrolabe.

plate: part of an astrolabe with a projection of altitude and azimuth lines on to the equatorial plane, see article on the astrolabe.

plumb level: device for determining a horizontal level or an angle of elevation by a plumb line or plummet.

plumb line: a suspended thread with a weight at its end, indicating the vertical.

plummet: a form of plumb line in which the 'line' and weight are a single rigid piece.

polar dial: type of sundial, see article on the sundial.

polyhedral dial: sundial with hour lines on various faces of a solid figure, see article on the sundial.

prime vertical: celestial great circle passing through the east and west points and the zenith.

primum mobile: instrument for finding the sines and versed signs of angles, see article on the primum mobile.

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.
