I stand straight in the sun and think of myself as a pole. In the morning, the shadow points to the northwest, in the afternoon, the shadow points to the northeast, and the shortest shadow points to the north at noon.

At local noon, the shadow must face due north, and then it can be judged according to the direction of the shadow: if the shadow faces northwest, it means that the sun is in the southeast; if the shadow is at 15 degrees west longitude, it means that the sun is at 15 degrees east longitude, and the running speed of the sun is 15 degrees/hour, then it must be local time1hour.

So just measure the angle between the shadow and the north direction and multiply it by 4, which is the time difference between that time and 12. If you subtract the shadow from the local time in the west, if the direction of the shadow is in the east, you can add 12.

Extended data:

The ancients used a sundial to tell the time through the shadow of the sun. Guibiao is the oldest timekeeping instrument in China, and its actual invention date has not been verified. However, it can be known that the use of ghost watch has developed considerably in Shang and Zhou Dynasties, and it is an important timing method.

A watch is a timekeeping instrument with the sun as the observation target, and its structure is quite simple. The so-called watch is developed from the watch, and then changed from performance to sundial. At first, the "watch" was just a bamboo pole or a stone pillar standing upright on the ground, which is also commonly known as "measuring the sun with the shadow of a pole". We use the change of polar shadow to understand the change of time.

In daily life, ancient people found that the shadow of an object irradiated by the sun would change its direction and length with the position of the sun, and there were certain rules.