How were the solar terms established in the Central Plains of ancient China?

According to legend, officials in Shao Hao in the Yellow Emperor era were all named after birds: Xuanniao Division (vernal equinox and autumnal equinox), Zhao Bosi Division (summer solstice and winter solstice), Qingniao Division (beginning of spring, Changxia) and Danniao Division (beginning of autumn, beginning of winter). Xuanniao is a swallow, which usually comes and goes at the vernal equinox. Zhao Bo is a shrike, which usually comes and goes in summer. Bluebirds are pigeons and geese, which usually start singing in spring and stop in long summer. Danniao is a pheasant, which usually comes and goes in autumn and starts in winter. Named after the officials in charge of separation, arrival, opening and customs clearance, it shows that in ancient times, the arrival and departure of migratory birds did have the experience of symbolizing the seasons. In Oracle Bone Inscriptions, the word "Nian" is the image of people bearing five grains, while the word "He" represents plants with drooping ears, so Shuowen said that "five grains are ripe for the year". This is exactly the same as the ancient Tibetan saying "Take cooked wheat as the beginning of a year" (The Biography of Tubo in the old Tang Dynasty) and the Li people's saying "Take cooked yam as the astronomical year" (Yuhuan in Taiping), both of which are traces left by the phenological era.