Composition and basic working principle of astronomical instrument standard table

A long time ago, people found that houses, trees and other things would produce shadows under the sunlight, and the changes of these shadows had certain rules. So he erected a pole or pillar on the flat ground and observed the change of the shadow. This pole or pillar is called a "watch"; You can know the time by measuring the length and direction of the shadow with a ruler. Later, I found that the shadow of the table was always cast to the north at noon, so I laid a ruler made of slate on the ground, perpendicular to the vertical table. One end of the ruler is connected to the foot of the table, and the other end extends to the true north direction. This ruler made of slate is called a reed. 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 learned that the shadow at noon in a day was the shortest, but also concluded that the shadow from summer to the sun was the shortest at noon in a year. At noon from winter solstice, the sun shines obliquely and the shadow is the longest. So the ancients used the length of the noon shadow to determine the solar terms and the length of a 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. No wonder the ancients in China have long known that a year is equal to more than 365 days.

On the existing stargazing platform in Dengfeng, Henan Province, the 40-foot-high platform and 128-foot-long ruler are also a huge standard.

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