Da Yan Calendar
Da Yan Calendar (a calendar and book) was drafted by Zhang Sui (also named Seng Yixing) in 727, and was later sorted out by Zhang Shuo and Chen Xuanjing. Enforced in 729, the calendar remained in use until 751. In 733, it was introduced to Japan, where it was used for over a hundred years.
The calendar consists of seven parts that elaborate on the specific computation. There are another 12 articles dealing with theoretical issues, mainly the articles Zhang Sui wrote in the calendar.
The calendar was made on the basis of the astronomic data gathered through practical observation with the instruments that were made at the beginning of calendar making, providing an insight into the asymmetrical movement of the sun and the moon. In terms of the computational method, a new formula was created, marking a progress from the Huang Ji Calendar made in the Sui Dynasty (581-618).
An innovative calendar in Chinese history, Da Yan Calendar inherited the strong points of calendars of the previous dynasties, while making improvement on the shortcomings and defects, symbolizing a significant breakthrough in the calendar-making field. The most prominent point about it lies in the calculation mastery of the speed of the sun's movement along the ecliptic.
Well organized and logically deducted, Da Yan Calendar was more precise than its predecessors, and was used for reference during the following dynasties.