Overview
Key data
| Item | Detail |
|---|---|
| Period of use | 463 – 520 CE |
| Compiler | Zu Chongzhi |
| Tropical year | 365.2428 days |
| Synodic month | 29.53059 days |
| Intercalation cycle | 144 in 391 years |
Zu Chongzhi and the dispute over reform
Zu Chongzhi presented his Daming calendar in 462 CE; blocked by a powerful minister, it was never promulgated in his lifetime. Only ten years after his death was it formally adopted, in 510 CE, and it was used until 520.
The dispute turned on the new calendar’s introduction of precession and a new intercalation cycle, against the upholders of the old methods. The victory of the new is a famous case in Chinese calendrical history — the process of that victory was itself an instance of science prevailing over inertia.
Precession brought in
Precession is the slow shifting of the sun’s position among the stars at the solstice, caused by the gradual wobble of the earth’s axis. Zu Chongzhi fixed it at one degree in 45 years and 11 months.
Bringing precession into the calendar was the second great reform. Before him the solstitial point was taken as fixed among the stars; once precession was admitted, epoch, solar terms and stellar coordinates had all to be re-handled, and the calendar entered a dynamic frame.
A better intercalation cycle
Zu Chongzhi proposed 144 intercalations in 391 years, against the old seven in nineteen (about 144.05 in 391), distributing the intercalary months more evenly.
His calendar also fixed the nodal month at 27.21223 days, against the modern value of 27.21222 — a sign of the precision of his observations.
A name to beware
Note that the Jin dynasty had another Daming calendar, by Yang Ji (1127–1179), same name, different calendar.
Compiled and arranged by Chujia Kuang.