Today, all kinds of clocks and watches give people the convenience for calculation of time. But in ancient China without clocks and watches, how did people calculate the time?

Initially, most people identified the time according to the position of the sun, the moon and stars in the sky. However, such method was not much accurate. Afterward, by watching the sun, someone designed a kind of time-counter to identify the time, i. e. the sundial. In Qin and Han dynasties, the sundial had become popular among people. The sundial is a round plate, whose surface is carved with marks indicating 12 hours, with a copper needles erected in the center of the sundial's surface. Under the sun, the shadow of the copper needle moves slowly on the surface with the movement of the sun, that is, when the shadow of the copper neelde moves to a certain mark, it is the time that the mark indicates. In this way, the calculation of time is more accurate.

But when it is cloudy or at night, the sundial doesn't function. Later, some people resorted to the method of dropping water or sand to calculate the time, and thereby invented a new tool "copper kettle clepsydra".

Copper kettle clepsydra was also called "clepsydra" or "kettle clepsydra". The earliest clepsydra was a copper pot holding water with a small hole at the bottom and a pole with scales inserting in the center. When the water dropped through the small hole, people would determine the time by the scale on the pole with the decline of the water level. However, the accuracy of this method of calculation the time through clepsydra still unsatisfactory.

As clepsydra was passed on from generation to generation, it gradually evolved into a set of four pots, which were placed in order on a four-level wooden stand. The one on the highest level is called "the Sun Pot", and the other threee post below it are named "the Moon Pot", "the Star Pot" and "the Water receiving Pot" respectively. The Sun Pot, the Moon Pot and the Star Pot all have a hole at the botton so that water can drop through and the Water-receiving Pot has a gauge inside. The water drops from the Sun Pot into the Moon Pot and then into the Star Pot and finally into the Water-receiving Pot. As more and more water drops into the Water-receiving Pot, the gauge gradually rises due to the buoyancy of water. Thus people could identify the time by observing the every scales of the gauge emerging above the water. The more accurate it is to calculate the time. Now, the four-level clepsydra of Yuan and Qing Dynasties are preserved respectively in Museum of Chinese History and Palace Museum in Beijing for the exhibition.

Sundial and copper kettle clepsydra are the crystallization of the ancient Chinese wisdom and creativity. They were not only show us how the ancient Chinese calulated times, but also provide precious materials for research on the development of science and technology in ancient China.