title: [Construction principle of the Taiji diagram is based on the observation of 24 solar terms].
abstract: The Taiji diagram is a common diagram in ancient Chinese works and medical works, but it's not known yet how it was made. Key evidence has been found in the data of astronomical observation in Han Dynasty. It was found that variation characteristics of the curve of the lengths between days and nights are in accordance with the S-curve of the Taiji diagram. The theory contained in the Taiji diagram could also be illustrated well by this curve.
  
  
  CONCLUSIONS
  the Taiji diagram was made according to the diurnal rhythm in a year, which was obtained from observation of 24 solar terms in ancient times. As a great scientific achievement in ancient China, the Taiji diagram has profound influence and should be reevaluated on an academic level.
authors.affiliation: Meteorological Bureau of Taizhou, Zhejiang, 318000, China.
authors.forename: Zhong-Xing
authors.initials: ZX
authors.lastname: Mou