Chūnfēn

Chunfen, the Spring Equinox, is the fourth solar term, at the sun's longitude of 0 degrees, the vernal point, around the twentieth of March. It is a principal (zhongqi) term and one of the two equinoxes, when day and night stand equal and the sun crosses the celestial equator going north. In Chinese cosmology it marks the balance point of yin and yang before the light grows dominant. The farming almanac treats it as the heart of the spring planting season, and old customs include the egg-balancing game and offerings to the sun. Being a solar marker anchored to the equinox itself, it falls near the same Gregorian date each year. These are the observances of the East Asian agricultural tradition, resting on the astronomical division of the tropical year into twenty-four equal solar arcs.

Chunfen, the Spring Equinox, falls at 0 degrees of ecliptic longitude, when the sun crosses the celestial equator, around 20 March, and is a qi, or major, term, one of only four of the twenty-four terms, along with the summer solstice, autumn equinox and winter solstice, that mark an exact astronomical event of day-night equality or solar extremity rather than an intermediate point. Day and night are of approximately equal length at Chunfen everywhere on Earth, not only in China, since the equinox is a global astronomical event defined by the sun's position relative to the equator. Folk custom associates the date with the game of balancing a raw egg on end, said to be easier at the moment of equinoctial balance, though the claim has no astronomical basis, since an egg's balance depends on its shell's surface texture rather than on gravitational alignment.

The twenty-four solar terms divide the ecliptic into equal arcs of fifteen degrees, and this one falls at 0 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 twelve major terms are what make the Chinese calendar work as a lunisolar one. Each ordinary lunar month is expected to contain exactly one major term, and the month takes its number from the term it contains, so the eleventh month is by definition the one holding the winter solstice. When a lunar month falls entirely between two major terms and contains none, it is declared intercalary and repeats the number of the month before it, which is how a thirteenth month is inserted without any fixed rule about when.

Chunfen is the spring equinox, and it is the term at which the sun's apparent longitude is zero, the origin from which all the others are measured. Four of the twenty-four mark the equinoxes and solstices and are the astronomically fundamental members of the set; the solstices in particular are the earliest of the twenty-four to be attested, and the full system of twenty-four was assembled around that older core.