
octant: the eighth part of a circle or an instrument measuring up to 45 degrees.

old quadrant: type of horary quadrant, see article on the quadrant.

opposition: two planets 180? from each other in the ecliptic are said to be in opposition. This relative position had important astrological significance.

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

orthographic planisphere: flat map produced by orthographic projection.

pedometer: device worn by a walker to measure distance by counting paces.

pendant sundial: a form of sundial suspended by a cord and possibly hung around the neck.

perpetual calendar: calendar covering a number of years, see article on time and date.

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.

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

protractor: instrument for setting out and measuring degrees.
