What is a shadow clock/sundial?

 A sundial/shadow clock is a instrument that indicates the time of the day by the shadow, cast on a surface marked to show hours or fractions of hours, of an object on which the sun's rays fall. There are different types of shadow clocks. Some shadow clocks use a shadow or the edge of a shadow while others use a line or spot of light to indicate the time. The shadow-casting object, known as a Gnomon, may be a long thin rod, or other object with a sharp tip or a straight edge. The largest sundial in the world, was constructed in c.1724 in Jaipur, India, and covers almost one acre(.4 hectare) and has a gnomon over 100 ft (30 m) high surmounted by an observatory. We can precisely measure a sundial between sunrise and sunset.