Overview
Key data
| Item | Detail |
|---|---|
| Period of use | 728 – 761 CE |
| Compiler | Yi Xing (the monk Yixing) |
| Tropical year | 365.2444 days |
| Year head | Jian-yin |
Yi Xing and the great survey
The Dayan calendar was compiled under the monk Yi Xing and promulgated in 728 CE, used until 762.
With Nangong Shuo he carried out the first large-scale survey of latitudes in Chinese history, measuring shadow lengths and the height of the pole from Huazhou Baima (in modern Hua county, Henan) to Yuzhou Shangcai, and from these estimating the length of a degree of the meridian — among the earliest such measurements anywhere.
The sun’s inequality
The Dayan calendar recognized that the sun moves fastest at the winter solstice and slowest at the summer solstice, computing its position piecewise by second-order interpolation with greater fineness than before.
The Jiu Tang shu records its rule:
A quotation
That is: true terms for the motions and eclipses, mean terms for the daily calendar. The two were used together, each where it suited — a balance of precision and use.
Influence
The Dayan calendar was the high point of Tang calendrical work and was followed by later calendrists. Japan and Korea also adopted its methods, and its influence reached across East Asia.
Compiled and arranged by Chujia Kuang.