Gnomon is the most ancient astronomical instrument. It had been used as early as in the period during the Yin-Zhou Dynasties (about 3000 years ago) in China. The horizontal plate lying along the south-north direction is named "Gui" whereas the vertical pole standing straightly to the south of "Gui" is known as Biao. At noon time, Biao would project its shadow on the central line of Gui. The length of the shadow would be the longest on winter solstice and the shortest on summer solstice. Therefore one could determine the solstices and the rest of the Chinese twenty-four solar terms from the length of its changing shadows. In addition, according to the various periods of the shadow length, the number of days in a tropical year was also obtained (365.25days).

This instrument was cast in 1437 (the Ming Dynasty) and renovated in the Qing Dynasty (1644-1911).