Yǔshuǐ

Yushui, Rain Water, is the second solar term, at the sun's longitude of 330 degrees, around the nineteenth of February. It is a principal (zhongqi) term, one of the twelve major markers that anchor the lunar months, a month being required to contain its principal term. Its name announces the shift from snow to rain as the air warms, the moisture the early crops need. In the farming almanac it is a time for tending winter wheat and preparing for spring sowing, and in some regions it carries family customs of honouring parents and elders. Being a solar marker, it falls on nearly the same Gregorian date each year. These are the observances of the East Asian agricultural tradition, resting on the division of the tropical year into twenty-four solar terms of fifteen degrees each.

Yushui, the Rain Water term, falls at the sun's ecliptic longitude of 330 degrees, around 19 February, and is a qi, or major, term, the second of the twenty-four and the counterpart major term to Lichun's sectional opening. Its name signals the point in the traditional agricultural calendar when precipitation is expected to shift from snow toward rain across much of the North China Plain, where the solar term system originated, though the same fixed calendar is applied across China's much wider range of climates, with correspondingly uneven fit to local conditions in the south or in the far north. Traditional pentad lore associated with this period describes otters beginning to catch fish and lay them out, geese flying north, and plants and trees beginning to bud, signs read together as marking winter's genuine retreat.

The twenty-four solar terms divide the ecliptic into equal arcs of fifteen degrees, and this one falls at 330 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 terms have not always been spaced the same way. Until the seventeenth century they were reckoned by equal intervals of time, dividing the year into twenty-four equal parts, a method called pingqi. The Shixian calendar, prepared with Jesuit assistance and promulgated in 1645, redefined them by equal intervals of the sun's apparent longitude instead, fifteen degrees apart, which is the definition still used. Because the Earth's orbit is elliptical the sun does not move at a uniform apparent rate, so the intervals between terms now vary by about a day between the shortest and the longest.

Yushui, rain water, is the major term of the first month in the ordinary numbering, and its name belongs to the group of terms named directly for weather rather than for a season or a crop. That group, which includes the two dews, the frost, the two snows and the four terms of heat and cold, is the largest of the naming families among the twenty-four and reflects the climate of the North China plain where the system was fixed.