What is a sundial?

Sundial instrument
   Sundial is an instrument for measuring time by means of the motion of the sun's shadow. Before clocks and watches became common it was in general use as a timekeeper. The art of constructing sundials to suit any place and situation was an important branch of mathematical study.
   A sundial consists of two parts: the stile, or gnomon, usually the edge of a plate of metal, made parallel to the earth's axis, and pointing toward the pole; and the dial plane, on which are marked the several hours of the day, their halves, quarters, etc. Sundials receive various names, according to the positions they are constructed to occupy. When the sundial is level it is called a horizontal sundial; when perpendicular to that plane, a vertical sundial An equinoctial sundial is one whose plane is parallel to the equator. There are others. the south sundial, north sundial, east sundial, west sundial, polar sundial, and declining sundial.