When did the twenty-four solar terms begin?

Solar terms refer to the twenty-four solar terms and climate, which is a calendar invented in ancient Central Plains of China to guide farming. In ancient China, the sundial was measured by earth gauge, and the longest sundial was defined as "solstice" (also known as long solstice, long solstice and summer solstice), and the shortest sundial was defined as "short solstice" (also known as short solstice and winter solstice). The length of day and night in spring and autumn is equal, so it is defined as "vernal equinox" and "autumnal equinox". There were only four solar terms in Shang Dynasty, and it developed to eight in Zhou Dynasty, and it didn't become twenty-four solar terms until the Western Han Dynasty.

Preface to Historical Records Taishi Gong

The meridian of the sun is 3 15 degrees. Beginning of spring is the first of the 24 solar terms. In China, it usually starts around February 4th of the Gregorian calendar. It means to begin to enter the spring, "yang and thorns, everything is spring." After beginning of spring, everything was revived and full of vitality, and the four seasons began.

Concepts such as yin and yang, four seasons, eight places, twelve degrees and twenty-four solar terms are mentioned in Six Classics.