A sundial uses the movement of the sun to keep track of time. There are several varieties, but one of the most basic consists of a thin rod or triangle -- called a gnomon -- and a flat circular or rectangular plate with hour graduations. The shadow cast by the gnomon tells the time.
From the time the sun rises until the time it sets, the gnomon casts a shadow on the face of the sundial. The shadow is longest at dawn and at twilight, and it gradually moves around the face as the sun moves. The face is graduated in such a way that the shadow falls on the number that corresponds to the time of day.
Latitude and Longitude
A sundial that works in Canada won't work in Mexico; because of the different latitudes, the sun's path is different in each place. In addition, a sundial situated at the extremity of a time zone -- for example, one in Maine -- won't give the true time for that zone. It's off by four minutes for every degree it is displaced from the reference longitude of the time zone. Each sundial must therefore be calibrated for a specific location.
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Time of Year
The shadow cast by the gnomon changes height from summer to winter; it is longer in the winter, when the sun is lower in the sky. Consequently, sundials often have two sets of numbers -- an inner set for summer and an outer set for winter.