Shadow length of twelve solar terms

As early as the Spring and Autumn Period and the Warring States Period, China was able to measure the length of the sun's shadow at noon with a soil gauge (a pole was erected on the plane), thus determining four solar terms: winter solstice, summer solstice, vernal equinox and autumn equinox. In a year, the shortest day of the noon soil return shadow is the summer solstice, the longest day is the winter solstice, and the shadow with moderate length is the vernal equinox or autumn equinox. The Book of History in the Spring and Autumn Period recorded the solar terms. The twenty-four solar terms are completely recorded in Huainanzi written by Liu An in the Western Han Dynasty. In ancient China, the lunar calendar was used to record time, and the solar calendar was used to divide the 24 solar terms of spring, summer, autumn and winter. Our ancestors called five days 1, and three days were called solar terms. The whole year was divided into 72 days out of 24 solar terms.