Sanfu, the traditional Chinese calendar’s period of peak summer heat, spans 30 to 40 days each year, determined by specific astronomical and calendar calculations according to the Purple Mountain Observatory of the Chinese Academy of Sciences. Understanding the exact length of this period requires looking at how traditional almanacs calculate the intervals between seasonal markers.
How the Timing of Sanfu is Calculated
Sanfu is divided into three distinct segments: Chufu (the initial period), Zhongfu (the middle period), and Mofu (the final period), according to Xinhua News Agency. While Chufu and Mofu consistently last for 10 days each, the duration of Zhongfu varies from year to year. Depending on the alignment of the Ganzhi calendar system—a sexagenary cycle combining heavenly stems and earthly branches—Zhongfu can last for either 10 days or 20 days. Consequently, years with a 10-day Zhongfu feature a 30-day Sanfu period, while years with a 20-day Zhongfu extend the heat season to 40 days.
The Role of the Xia Zhi and Geng Days
The timing of Sanfu is anchored to the Xia Zhi (Summer Solstice) solar term. According to the traditional calendar rules described by astronomical authorities, Chufu begins on the third Geng day following the Xia Zhi. The Geng days refer to days marked by the ‘Geng’ character in the heavenly stems of the 60-day cycle. Mofu begins on the first Geng day after Liqiu (the beginning of autumn). Because the number of days between the third Geng day after the summer solstice and the first Geng day after the beginning of autumn fluctuates, the length of Zhongfu adjusts accordingly to bridge the gap.
Cultural Significance and Health Practices
During the Sanfu period, high temperatures and humidity drive traditional health practices across China, such as Sanfu Moxibustion, a form of traditional Chinese medicine therapy. Clinics and hospitals often see an influx of patients seeking seasonal preventative treatments, which practitioners time specifically to the onset of each stage of the fu days. Meteorological departments routinely issue high-temperature alerts throughout this window, as historical climate data shows these weeks typically record the highest sustained temperatures of the year.