The perpetual calendar is a chronological reference book that correlates the Gregorian calendar, the lunar calendar, and the sexagenary cycle (Heavenly Stems and Earthly Branches). Its primary function is to provide the cyclical timekeeping system of the Heavenly Stems and Earthly Branches, a unique and ancient Chinese calendrical method. Based on celestial phenomena and aligned with seasonal changes, it reflects the waxing and waning of the Five Elements (Wu Xing) and holds irreplaceable significance in various disciplines. The sexagenary cycle is particularly valuable in traditional Chinese medicine, qigong, Yijing numerology, agriculture, and calendrical studies.
However, when one needs to determine the cyclical dates but lacks a perpetual calendar, it often becomes a challenge. This article introduces a method—requiring only the memorization of a 14-character mnemonic—that enables rapid mental calculation of the sexagenary dates for any given year. This method, known as the “Silver Plate” , was historically a secret oral tradition among folk diviners, passed down strictly between masters and disciples. Today, we unveil this method to the public for further exploration and study.
The “Silver Plate” is based on the lunar calendar for calculating the sexagenary cycle. The lunar calendar consists of “long months” (30 days) and “short months” (29 days). Ignoring short months, two lunar months total 60 days, matching the 60-unit sexagenary cycle. Thus, every two months repeats one full cycle (Jiazi). Knowing the first day of the first lunar month allows one to deduce the first days of the third, fifth, seventh, ninth, and eleventh months (with slight adjustments for leap months).
Since short months exist, knowing their number and distribution in a given year enables simple adjustments—delaying the cyclical count by the number of short months passed. This makes mental calculation of the sexagenary dates straightforward.
To mentally compute the perpetual calendar, four key elements must be known:
- The year’s cyclical designation .
- The sexagenary date of the first day of the first lunar month.
- The date and time of the “Start of Spring”.
- The number and distribution of short months in the year.
The “Silver Plate” ingeniously encodes all four elements into a 14-character mnemonic. To maintain secrecy, its creators employed “encryption,” assigning multiple names to the same Earthly Branch. This served both poetic rhythm and obfuscation.
The method also abbreviates months with shared short-month patterns:
- Water Formation : Months 3, 7, 11 (Chen, Shen, Zi)
- Fire Formation: Months 1, 5, 9 (Yin, Wu, Xu)
- Wood Formation : Months 2, 6, 10 (Hai, Mao, Wei)
- Metal Formation : Months 4, 8, 12 (Si, You, Chou)
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