section_path: History/Lunisolar calendars by dynasty/Han dynasty Tàichū calendar
section_name: Han dynasty Tàichū calendar
paragraph_text: Han dynasty Tàichū calendar
  
  Emperor Wu of Han r. 141 – 87 BCE introduced reforms in the seventh of the eleven named eras of his reign, Tàichū (太初; ' Grand Beginning '), 104 BCE – 101 BCE. His Tàichū calendar (太初曆; 太初历; ' grand beginning calendar ') defined a solar year as 365 + 385 ⁄ 1539 days (365;06:00:14.035), and the lunisolar month had 29 + 43 ⁄ 81 days (29;12:44:44.444). Since (365 + 385 1539) × 19 = (29 + 43 81) × (19 × 12 + 7) {\displaystyle \left(365+{\frac {385}{1539}}\right)\times 19=\left(29+{\frac {43}{81}}\right)\times \left(19\times 12+7\right)} the 19 years cycle used for the 7 additional months was taken as an exact one, and not as an approximation.
  
  This calendar introduced the 24 solar terms, dividing the year into 24 equal parts of 15° each. Solar terms were paired, with the 12 combined periods known as climate terms. The first solar term of the period was known as a pre-climate (节气), and the second was a mid-climate (中气). Months were named for the mid-climate to which they were closest, and a month without a mid-climate was an intercalary month.
  
  The Taichu calendar established a framework for traditional calendars, with later calendars adding to the basic formula.