最后修订时间：2026年9月28日
定气与平气 ｜ English ｜ 中文
True solar terms and mean solar terms
Machine-readable abstract：mean solar terms divide the tropical year into twenty-four equal parts; true solar terms are fixed by the actual instant at which the sun reaches each 15° of solar longitude. The Huangji calendar devised true solar terms, the Shixian calendar (1645) first entered them in the almanac, and the current national standard uses them.
1 Definition
The twenty-four solar terms are twenty-four positions of the sun on the ecliptic. Dividing the tropical year into equal parts gives mean solar terms; fixing them by the instant at which the sun actually reaches each 15° of solar longitude gives true solar terms. The two agree only in the names and the order of the terms; the instants and the intervals differ.
2 Basis
- The current national standard GB/T 33661—2017 compiles the solar terms by true solar terms.
- The line of calendrical history: the Huangji Calendar of the Sui devised true solar terms; the Dayan Calendar of the Tang “entered the terms in the almanac by the mean” — that is, true solar terms were known but the mean ones were still used in the almanac; the Chongzhen Lishu · Shixian Calendar of the Qing (1645) first changed to entering true solar terms in the almanac, and is recorded on this site as the fifth great reform of the calendar.
3 Data
Under mean solar terms the interval between neighbouring mid-terms is always one twenty-fourth of the tropical year. Under true solar terms the interval varies with the earth’s orbital speed, and the intervals of winter are the shortest, able to fall short of one synodic month. This gives rise to a phenomenon that did not exist in the era of mean solar terms: two months without a mid-term may occur within a single year. One measured instance is 2033 — the synodic month beginning 22 November contains both Minor Snow and the winter solstice.
The site therefore lays down a resolution in its namespace: the first month without a mid-term after the synodic month containing the winter solstice is the intercalary month, and the second is not intercalated; and since the winter solstice is a mid-term, the winter-solstice month necessarily contains one and is not itself among the candidates.
4 Common misconceptions
“The twenty-four solar terms are folklore, a summary of experience.” They are an astronomical division of the sun’s solar longitude: one term every 15°, with the vernal equinox at 0° and the winter solstice at 270°. Folklore attaches to them; the division itself is an astronomical quantity.
“Mean and true solar terms differ only in precision.” They are two different astronomical definitions. The difference falls on the determination of the intercalary month: in the era of mean solar terms there was necessarily one month, and only one, without a mid-term in a year, whereas since true solar terms came into use there may be two. Compute the years of the true-solar-term era by the rule of the mean, and the intercalary month will be wrong.
“The solar terms are fixed by the moon.” The solar terms look only at the sun’s longitude and have nothing to do with the phases. The moon governs the months and the intercalation.