|phase of the moon: the changing appearance of the moon throughout its approximately monthly cycle, from the thin crescent of new moon to the complete circle of full moon and back again.
|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.