Human’s Good Helper from Ancient Time on
A sundial is an instrument which measures time by the position of the sun. It casts a shadow on a flat surface marked with the hours of the day. As the position of the sun changes, the time indicated by the shadow changes.
Sundials in the form of obelisks (3,500BC) and shadow clocks (1,500BCE) are known from ancient Egypt, and were developed further by other cultures, including the Chinese, Greek, and Roman cultures. In ancient times, people found out that the direction and length of shadows cast by objects changed with the sun's position and four seasons. That was crucial to them in cultivating.
A standard sundial is installed at 45 degrees north to keep time accurately. However, a sundial designed for one latitude has to be adjusted for use at another latitude by titling its base so that its style or gnomon is parallel to the Earth’s axis of rotation, so that it points at the north celestial pole in the northern hemisphere, or the south celestial pole in the southern hemisphere.
The “shadow-maker” of the sundial is called a gnomon. The sun casts a shadow from the gnomon to a surface called the dial face. Most sundials indicate on the dial face by the shadow of a line in space called the style. For our sundial at the small square, visitors act as the style if they stand facing north or south in the central point of the dial face. It means they should be parallel to the Earth's axis of rotation. However, our sundial cannot be very accurate because of different figures of visitors.