Overview
Key data
| Item | Detail |
|---|---|
| Promulgated | 104 BCE (first year of Taichu) |
| Compilers | Deng Ping, Tang Du, Luoxia Hong and others |
| Year head | Jian-yin (first month) |
| Tropical year | 365.2502 days |
| Synodic month | 29 + 43/81 days |
Why the reform was made
The early Han had kept the Qin Zhuanxu calendar, and after long use its day-counts drifted from the heavens. In 104 BCE Emperor Wu ordered Sima Qian, Luoxia Hong, Deng Ping, Tang Du and others to deliberate on reform; the new calendar was promulgated that same year and the reign was renamed Taichu, from which the calendar takes its name.
The synodic month was set at 43/81 of a day — hence the name “the eighty-one-part method.” It differs slightly from the 499/940 of the quarter-remainder calendar, the result of recalibration against observation.
Three innovations
1. The first month restored as year head. From the Qin, when the tenth month began the year, the reckoning returned to jian-yin, and so it remained through the ages.
2. The twenty-four solar terms brought into the calendar. Terms were arranged against the months — the first time the calendar fully carried the system of solar terms.
3. Intercalation by “no zhongqi.” The month containing no zhongqi was made the intercalary month. Simple and workable, this rule was followed by calendrists for more than a thousand years.
Sources
The Han shu’s “Treatise on Pitch Pipes and the Calendar” gives a full account of the Taichu debates and preserves the working methods of the Santong calendar; it is the first source for this calendar. Its phrase “they fixed east and west, set up the gnomon, and let down the clepsydra, to trace the distances of the twenty-eight mansions in the four quarters” records the scale of the observations then made.
Compiled and arranged by Chujia Kuang.