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polar dial: type of sundial, see article on the sundial.
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polyhedral dial: sundial with hour lines on various faces of a solid figure, see article on the sundial.
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prime vertical: celestial great circle passing through the east and west points and the zenith.
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primum mobile: instrument for finding the sines and versed signs of angles, see article on the primum mobile.
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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.
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proportional compasses: drawing instrument consisting of two legs each with points at either end; used for transferring dimensions in enlarged or reduced ratio.
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proportional dividers: drawing instrument consisting of two legs each with points at either end; used for transferring dimensions in enlarged or reduced ratio.
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proportional instrument: unusual instrument used to mechanically perform functions in trigonometry.
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protractor: instrument for setting out and measuring degrees.
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quadragesima: the 40 days of Lent or the first Sunday in Lent.
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quadrans vetus: type of horary quadrant, see article on the quadrant.
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quadrant: instrument based on a quarter of a circle, see article on the quadrant.
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quadratum nauticum: diagram for finding course directions from latitudes and longitudes, see article on the astrolabe.
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quatrefoil: decorative form with four leaves or petals.
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radio latino: instrument for measuring angles in surveying and gunnery, see article on the radio latino.
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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.
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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.
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Regiomontanus-type dial: design of portable altitude dial adjustable for any latitude.
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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.
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right ascension: angle parallel to the celestial equator, measured eastwards from the spring equinox.
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