Appreciation of Chinese Studies "Celestial Instruments-Classic Table"

Celestial instruments-standard watches

Witty words; Punchline

The manometer is a simple and important astronomical instrument, which consists of two parts, namely a vertical manometer and a horizontal manometer. This scale is mainly used to determine the winter solstice, thus determining the length of the tropical year. In addition, we can also determine the direction and solar terms by observing the changes of surface shadows.

Classical explanation

A long time ago, people observed the changes of shadows cast by houses, trees and other objects in the sun, and learned that the changes of these shadows have certain rules. Later, people erected a post or pillar on the flat ground to study the changes of shadows. This column is called a watch. Take a ruler and measure the length and direction of the shadow, and you will know the correct time. It was found that the shadow of the table was always cast in the true north direction at noon, so people laid a ruler made of slate flat on the ground, perpendicular to the vertical table. One end of the ruler was connected with the table base, and the other end extended to the true north direction. This ruler made of slate is called "Gui". At noon, the shadow was cast on the slate, and the ancients could directly read the length of the shadow.

After long-term observation, the ancients not only knew that the noon shadow was the shortest, but also knew that the noon shadow was the shortest from summer to morning and the longest from winter to morning. Since then, the ancients used the length of the noon shadow to determine the solar terms and the length of the year. For example, the longest value of surface shadow is measured twice in a row, and the number of days between the two longest values is the length of one year. Therefore, the ancients in China have long known that there are more than 365 days in a year.