The twenty-four solar terms divide the ecliptic into equal arcs of fifteen degrees, and this one falls at 30 degrees of the sun's apparent longitude. It is a major term, a zhongqi, one of the twelve that fall in the even positions of the twenty-four. The major terms are the ones that anchor the lunar months, and they alternate strictly with the sectional terms, so a zhong is always followed by a jie fifteen degrees later.
The system travelled with the Chinese calendar. Japan, Korea and Vietnam all adopted the twenty-four terms and kept their own readings of the same names, and the terms remain in use in each. In 2016 the twenty-four solar terms were inscribed on the UNESCO Representative List of the Intangible Cultural Heritage of Humanity, on the ground that they remain a living framework for agricultural and everyday practice rather than an obsolete scheme.
Guyu, grain rain, closes the spring group of six terms and is the major term of the third month. Its name belongs to the agricultural family among the twenty-four, alongside the two terms of grain in early summer, and these names are the clearest evidence that the system was built as a farming instrument. A lunisolar calendar cannot tell a farmer when to sow, because its months drift against the sun; the solar terms can, because they do not.