Yin

Yin is the third of the twelve Earthly Branches (dizhi), distinct from the yin of the yin-yang polarity, which is a different word. Its zodiac animal is the Tiger, its element is Wood, its direction is east-northeast, and its double-hour runs from 3 to 5 am, the hour before daybreak. As one of the twelve branches it combines with the ten Heavenly Stems to form the sexagenary cycle of sixty that numbers years, months, days, and double-hours. In the traditional calendar the branch Yin is associated with the first month of the lunar year in the common reckoning. These are the conventions of the Chinese calendrical and cosmological tradition, in which the twelve branches order the twelve-year animal cycle, the months, and the twelve two-hour periods of the day.

Yin is the third Earthly Branch, corresponding to the Tiger in the twelve-animal cycle, and to the double hour from 3 am to 5 am, the pre-dawn hours. Its dominant hidden stem is Jia, giving it the Wood phase, with Bing and Wu present as secondary hidden stems. Because the traditional Chinese calendar's civil and agricultural month count begins with Yin rather than with Zi, the first lunar month of the year is called the month of Yin, a convention that has held since the Han dynasty's adoption of the Xia calendar's month ordering, even though Zi retains its place as the first branch of the twelve and the first double hour of the day. Note that the branch Yin, the Tiger, is an unrelated homophone of yin the polarity opposite yang; the two are written with entirely different characters.

Beyond the animal it carries, a branch enters three fixed sets of relations that the almanac and the fate-calculation tradition use constantly. Yin belongs to the trio Yin, Wu and Xu, one of the four groups of three branches spaced four apart which are read as combining into Fire. It clashes with Shen, the branch six places away, and it forms one of the six harmonies with Hai. These relations are properties of position in the twelve, not of the animal.

The branches also do more work than the year cycle they are best known for. They divide the day into twelve double hours, so Yin names a period of two hours as well as a year and a month; they mark the twelve directions of the compass, Yin standing at east-northeast; and they carry the twelve stations against which the movement of Jupiter was tracked, which is one proposed origin for the twelve-year cycle itself.

The stem-and-branch day count is the longest continuously running day count known. Numbering days by ganzhi begins in the Shang inscriptions and has run without acknowledged interruption since, through every dynastic change and every calendar reform, because the cycle of sixty is independent of any astronomical quantity and therefore needs no correction. Chinese historical dates from the classical period onward can be checked against it.

Yin has a standing the other branches do not, because in the calendar in use since the Han the year begins with the month of Yin. The Chinese New Year falls in that month, the solar term Lichun opens it, and a year's animal changes with it, so the branch marks the practical beginning of the annual round even though it stands third in the list of twelve.

Sources Chinese Mathematical Astrology: Reaching Out to the StarsHo Peng Yoke with The Chinese Sexagenary Cycle and the Ritual Origins of the CalendarAdam Smith