Taiji in solar terms of Gaocheng stargazing platform

According to records, the standard table of Zhougong Shadow Measuring Station is used for observation (sun shadow, sun shadow measurement, verification season and calculation year).

Ghost table is an astronomical instrument for measuring the length of the sun shadow, which consists of two parts: ghost table and ghost table. Like the sundial, the sundial is an ancient astronomical instrument, which uses the shadow of the sun to measure. The so-called altimeter method, popularly speaking, is to erect a pole perpendicular to the ground and determine the seasonal change by observing and recording the change of its shadow at noon. A straight rod perpendicular to the ground is called a "table", and a scale horizontally placed on the ground and engraved with scales to measure the shadow length is called a "GUI". ?

As early as the 20th century BC, during the period of Taosi ruins, the standard table drawing method was used in the Central Plains of China. Scholars in the Han Dynasty also determined the "twenty-four solar terms" by using the length of the sun shadow in the standard table, determined the shortest day in the Yellow River basin as the winter solstice, took the winter solstice as the starting point of the "twenty-four solar terms", and divided the period from winter solstice to the next winter solstice into 24 segments (each segment is 15 days), and the number of days between each two solar terms was averaged. The ancients called this method "calming qi method" (also called "average time method"). Measure the winter solstice first, because the shadow is the longest in the winter solstice, and the shadow length changes most obviously in the adjacent days, which is more conducive to observation and recording. ? [ 1-2]

In this way, the standard table can be used not only to formulate festivals, but also to list the dates of 24 festivals in the future solar calendar and almanac, as an important basis for guiding the agricultural activities of the working people of the Han nationality. Mark the length of the shadow on a circle with a period of one year. The center of the circle is the standing point of the table, and the generated map is a Taiji diagram with a non-zero center. After eliminating the center point of zero radius, the Tai Chi map is the common 24 solar terms Tai Chi map.