
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.
