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A sundial is an instrument for measuring time, using the position of the sun in the sky. A sundial consists of a face similar to a clock face, and a gnomon, which casts shadows on the face.

The sun's apparent motion across the sky depends on the latitude of the observer and the time of the year. To measure uniform increments of time, these factors must be compensated for. The most common compensation is to angle the gnomon to be parallel to the polar axis of the earth, which requires tilting the gnomon at an angle equal to the latitude of the sundial and pointing it to the nearer pole (north in the northern hemisphere or south in the southern hemisphere). If the face is perpendicular to the gnomon, then equal increments of time can be marked off by marking equal angles on the face. If the face is flat on the ground, then the angles must be adjusted to obtain equal time increments.

A sundial has several advantages and several disadvantages. When properly constructed, it is able to determine local solar time moderately accurately, and is not dependent on possibly inaccurate physical processes. However, it does not work at night, or when the sun is obscured by clouds. Away from the equator, the apparent motion of the sun varies through the year, requiring corrections for greater accuracy.