A sundial is an device that used to measure time by the position of the Sun. It is called “rigui” in Chinese. In ancient China, it was used by Chinese people to count hours during the sunny days. When the sun moves across the sky, it casts shadows from its Gnomon, a thin rod or a sharp, onto a flat calibrated dial. The lines marked on the dial indicate the hours of the day. At different time, the different hour-lines will be marked on the dial. Hence, people in ancient China could easily know the time via reading the lines from the sundial.
According to historical records, the flat horizontal dial plate, the earliest sundial of China, was invented in 574 AD. It is one of the most important inventions in ancient China. Today, you still have chance to appreciate this crystallization of wisdom in some places of China, such as the Palace Museum at the Forbidden City.