What is the working principle of a sundial?

Sundial timing principle: During the day, the shadows cast by the objects illuminated by the sun are constantly changing. First, the length of the shadow changes, and the shadow is the longest in the morning. As time goes on, the shadow becomes shorter and longer after noon; Then the direction of the shadow is changing. In the northern hemisphere, the shadow in the morning is in the west, the shadow at noon is in the north, and the shadow at night is in the east.

In principle, it can be timed according to the length or direction of the shadow, but it is more convenient to time according to the direction of the shadow. Therefore, the timing is usually based on the position of the shadow. Because the sundial must rely on sunlight, it can't be used on cloudy days and nights. Therefore, it is not enough to use the sundial to time, and other kinds of timers, such as water clocks, are needed to cooperate.

Extended data:

Humans have used sundials for a long time. The ancient Babylonians began to use it 6,000 years ago, and China in the Zhou Dynasty 3,000 years ago. The sundial can display not only the time of day, but also the solar terms and months. Of course, its shortcomings are also obvious. It is huge and cannot be used when there is no sunshine, such as cloudy days and cloudy nights.

The earliest reliable record in China is the short shadow level (horizontal sundial) invented by Yuan Chong in 594 AD (14th year of Sui Dynasty) mentioned in Tian Wenzhi. The clear record of the equatorial sundial was first seen in the shadow picture described in Volume 2 of Zeng Minxing's "Lonely Wake Magazine" in the Southern Song Dynasty.

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