关键词: 节气, 干支, 回归年, 日序, 轨道角动量守恒

Abstract: We present a general method to determine the period of Zhongfu ( second of the three ten-day periods of the hot season，sometimes lasting twenty days instead of ten) by accounting the daily order after the summer solstice. Therefore，in the long term，the 20-days of Zhongfu accounts for 71.37%. And the 10-days Zhongfu accounts for 28.63%.

Key words: solar term, heavenly stems, tropical year, daily order, conservation of orbital angular momentum