Clocks, Sundial

Sundials are horological devices that indicate the time of day using the shadow cast by the apparent position of the sun.

Sundials

A sundial is a horological or time-measuring device that tells the time of day (referred to as local or civil time today) when direct sunlight shines by the apparent position of the Sun in the sky.

In simplest form, a sundial consists of a flat plate (the dial) and a gnomon, which casts a shadow onto the dial. As the motion of Sun changes through the sky, the shadow aligns with different hour-lines, which are marked on the dial to indicate the local time. The gnomon may be a rod, wire, or decorated metal casting or arrow. The gnomon casts a shadow to show the time. The style is the time-telling edge of the gnomon, and it must be aligned parallel to the axis of the Earth’s rotation for the sundial to be accurate throughout the year.

Figure 1: Sundial