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Sexagenary cycle
The Chinese sexagenary cycle, also known as the Stems-and-Branches , is a cycle of sixty terms used for recording days or years in China and in neighboring East Asian cultures (notably Japan, Korea, Tibet, and Vietnam).

The sexagenary cycle was used in China from the second millennium BC. It has been found on Shang dynasty oracle bones as a means of recording days. Its use to record years began around the middle of the 3rd century B.C.

This traditional method of numbering days and years no longer has any significant role in modern Chinese time keeping or the official calendar. However, the sexagenary cycle continues to have a role in contemporary Chinese astrology and fortune telling.

Overview


Each term in the sexagenary cycle consists of two Chinese characters, the first representing a term from a cycle of ten known as the Heavenly Stems (天干; tiāngān) and the second from a cycle of twelve known as the Earthly Branches (地支; dìzhī). The first term (jia-zi 甲子) combines the first heavenly stem (jia 甲) with the first earthly branch (zi 子). The second (yi-chou 乙丑) combines the second stem with the second branch. This continues, generating a total of 60 different terms (the least common multiple of ten and twelve), after which the cycle repeats itself. This combination of two sub-cycles to generate a larger cycle and its use to record time have parallels in other calendrical systems, notably the Akan calendar.

The sexagenary cycle is attested as a method of recording days from the earliest written records in China, records of divination on oracle bones, beginning ca. 1250 BC. Almost every oracle bone inscription includes a date in this format. This use of the cycle for days is attested throughout the Zhou dynasty and remained common into the Han period for all documentary purposes that required dates specified to the day.

Almost all the dates in the Spring and Autumn Annals, a chronological list of events from 722 to 481 BC, use this system in combination with reign years and months (lunations) to record dates. Eclipses recorded in the Annals demonstrate that continuity in the sexagenary day-count was unbroken from that period onwards. It is likely that this unbroken continuity went back still further to the first appearance of the sexagenary cycle during the Shang period.

The use of the sexagenary cycle for recording years is more recent. The earliest document showing this usage is a diagram among the silk manuscripts from Mawangdui tomb 3, sealed in 168 BC. An annotation marking the first year of the reign of Qin Shi Huang 秦始皇 (246 BC) is applied to the diagram next to the position of the 60-cycle term (day 52 of 60, yi-mao 乙卯) corresponding that year. Use of the cycle to record years became widespread for administrative time-keeping during the Western Han Dynasty (202 BC- 8 AD). The count of years has continued in demonstrable continuity ever since: the year 1984 began the present cycle (a jia-zi 甲子 year), and 2044 will begin another. Note that the new year, when the sexagenary count increments, is not January 1st, but rather the lunar new year of the traditional Chinese calendar. For example, the yi-chou 己丑 year (coinciding roughly with 2009) began on February 4th.

In Japan, according to Nihon shoki, the calendar was transmitted to Japan in year 553. But it was not until the Suiko era that the calendar was used for politics. The year 604, when the Japanese officially adopted the Chinese calendar, was the first year of the cycle.

The Japanese tradition of celebrating the 60th birthday (還暦) reflects the influence of the sexagenary cycle as a count of years.

The Tibetan calendar also counts years using a 60-year cycle based on 12 animals and 5 elements, but while the first year of the Chinese cycle is always the year of the Wood Rat, the first year of the Tibetan cycle is the year of the Fire Rabbit (year #4 on the Chinese cycle).

Relation to the western calendar
Below is the sexagenary cycle matched up to the Western calendar for the years 1804–2043, or four full 60 year cycles. The sexagenary cycle begins at lichun 'about February 4' according to some astrological sources.