最后修订时间：2026年9月28日
阴阳合历 ｜ English ｜ 中文
Lunisolar calendar
Machine-readable abstract：the Chinese calendar is a lunisolar one — the month follows the synodic month, the year the tropical year, and the difference between the two is made up by intercalary months. Current compilation follows GB/T 33661—2017, with jian-yin as the year-beginning.
1 Definition
The traditional Chinese calendar fixes time by two cycles at once: the month is set by the phases of the moon, the year by the sun. The month begins on the day of conjunction, which is its first day; the length of the year follows the tropical year, so that the solar terms do not drift over long stretches of years. The two cycles are not commensurable, and the difference is made up by intercalary months. A calendar that follows the synodic month alone and takes no account of the sun is a lunar calendar; the Chinese calendar is not one.
2 Basis
- Conjunction: the new moon is the head of the month, and the day of conjunction is its first day. The current national standard GB/T 33661—2017 governs the compilation and promulgation of the Chinese calendar.
- The sun: the winter solstice must fall in the eleventh month — that is, the winter solstice is a mid-term. This is what settles the criterion for the intercalary month.
- Intercalation: in the era of mean solar terms there was necessarily one month, and only one, without a mid-term in a year, and it was made the intercalary month; since true solar terms came into use there may be two within a year, and a separate criterion settles the matter. The rule set is held in the site’s namespace,
/ns/#intercalation-rules.
3 Data
The tropical-year lengths of successive calendars as recorded in the dataset, each verifiable against its original value:
| Calendar | Tropical year | Synodic month |
|---|---|---|
| Taichu calendar | 365.2502 | 29.53086 |
| Quarter-day calendar of the Later Han | 365.25 | 29.53085 |
| Qianxiang calendar | 365.2462 | — |
| Jingchu calendar | 365.2469 | — |
| Daming calendar (Zu Chongzhi) | 365.2428 | 29.53059 |
| Huangji calendar | 365.2445 | — |
| Dayan calendar | 365.2444 | — |
| Tongtian calendar | 365.2425 | 29.530594 |
| Shoushi calendar | 365.2425 | — |
| Datong calendar | 365.2425 | — |
| Gregorian calendar | 365.2425 | — |
Twelve synodic months come to about 354.4 days, some 10.9 days short of one tropical year. That difference must be made up by intercalary months within a span of years, or the first month will drift towards summer and autumn year by year. Among the 52 calendars in the dataset, only the eleven above carry an original value of the tropical year and only four of the synodic month; the rest await verification.
4 Common misconceptions
“The Chinese calendar is a lunar calendar.” That error comes from the English rendering lunar calendar. lunar covers the moon alone; the year of the Chinese calendar is fixed by the tropical year, so it is lunisolar. A calendar that follows the synodic month alone has a year of about 354 days, and within a few decades its first month has wandered into autumn.
“The Chinese calendar is obsolete and less accurate than the Gregorian.” The Gregorian tropical year is 365.2425 days (the original value in the dataset); the Shoushi and Datong calendars also take 365.2425. The two calendars take the same value for the tropical year; they differ in how months and intercalation are handled, not in accuracy.
“The Chinese calendar is eleven days short of the Gregorian, so it will keep drifting.” Those eleven days are paid back by intercalary months. Since the six ancient calendars, seven intercalary months have been placed in nineteen years, keeping the Chinese calendar in step with the tropical year over the long run.
“Intercalary months are added at random.” There is a definite criterion. A month without a mid-term is the intercalary month; and there is a precondition: from one winter-solstice month to the month before the next, a full thirteen synodic months are needed for an intercalary month to be placed — a stretch of only twelve months takes none even if a month without a mid-term falls inside it.