Hánlù

Hanlu, Cold Dew, is the seventeenth solar term, at the sun's longitude of 195 degrees, around the eighth of October. It is a sectional (jie) term. Its name marks dew grown cold and near to freezing as the chill of late autumn deepens. In the farming almanac it is the time to bring in the late crops and to sow winter wheat before the frosts, and it is associated with the last warmth of the season, with autumn outings, and with the appreciation of chrysanthemums. Being a solar marker, it falls near the same Gregorian date each year. These are the observances of the East Asian agricultural tradition, resting on the twenty-four-fold division of the tropical year into equal solar arcs, each term a fifteen-degree step of the sun.

Hanlu, Cold Dew, falls at 195 degrees, around 8 October, and is a jie, or sectional, term, whose name marks a further step in the cooling sequence begun by Bailu's white dew a month earlier, the dew now cold enough that traditional reckoning treats it as foreshadowing frost rather than the mild condensation of early autumn. In traditional Chinese medicine's seasonal framework, Hanlu is treated as the point at which dietary and lifestyle advice shifts more decisively toward warming foods and protecting the body's yang energy against the advancing cold, part of the broader system of correlating diet and daily conduct with the twenty-four terms that also underlies customs recorded at Dashu and Xiaoshu earlier in the year.

The twenty-four solar terms divide the ecliptic into equal arcs of fifteen degrees, and this one falls at 195 degrees of the sun's apparent longitude. It is a sectional term, a jie, one of the twelve that fall in the odd positions of the twenty-four and open the twelve solar months. The sectional terms and the major terms alternate strictly around the year, so a jie is always followed by a zhong 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.

Hanlu, cold dew, is the second of the two dew terms and follows Bailu by thirty degrees of the sun's longitude. The progression from white dew to cold dew to descending frost across three consecutive terms is one of the most finely graded sequences in the list, marking the same phenomenon, condensation on the ground, at three stages of increasing cold.