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.