
nonius: a way of subdividing a quadrant of a circle to provide fine discrimination of parts of degrees, comparable in purpose to the later vernier scale. The name comes from the latinized surname of the 16thcentury Portuguese mathematician Pedro Nu?ez.

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

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.
