Sundial

A sundial is a device that tells the time of day by the apparent position of the Sun in the sky. In the narrowest sense of the word it consists of a flat plate (the dial) and a gnomon which casts a shadow onto the dial. As the sun appears to move across the sky, the shadow aligns with different hour-lines which are marked on the dial to indicate the time of day. The style is the time-telling edge of the gnomon, though a single point or nodus may be used. The gnomon casts a broad shadow; the shadow of the style shows the time. The gnomon may be a rod, a wire or an elaborately decorated metal casting. The style must be parallel to the axis of the Earth's rotation for the sundial to be accurate throughout the year. The style's angle from horizontal is equal to the sundial's geographical latitude.

In a broader sense a sundial is any device that uses the sun's altitude or azimuth (or both) to show the time. In addition to their time-telling function, sundials are valued as decorative objects, as literary metaphors and as objects of mathematical study.

It is common for inexpensive mass-produced decorative sundials to have incorrectly aligned gnomons and hour-lines, which cannot be adjusted to tell correct time.

There are several different types of sundials. Some sundials 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. Sundials employ many types of gnomon. The gnomon may be fixed or moved according to the season. It may be oriented vertically, horizontally, aligned with the Earth's axis, or oriented in an altogether different direction determined by mathematics.

Given that sundials use light to indicate time, a line of light may be formed by allowing the sun's rays through a thin slit or focusing them through a cylindrical lens. A spot of light may be formed by allowing the sun's rays to pass through a small hole or by reflecting them from a small circular mirror.

Sundials also may use many types of surfaces to receive the light or shadow. Planes are the most common surface, but partial spheres, cylinders, cones and other shapes have been used for greater accuracy or beauty.

...
Wikipedia