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.
