Wu

Wu is the seventh of the twelve Earthly Branches (dizhi), distinct from the fifth Heavenly Stem of the same romanisation. Its zodiac animal is the Horse, its element is Fire, its direction is due south, and its double-hour runs from 11 am to 1 pm, the hour of noon, when yang is at its height. 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. Standing opposite the midnight branch Zi, Wu marks the sun's zenith. 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 into which the day is divided.

Wu is the seventh Earthly Branch, corresponding to the Horse, and to the double hour from 11 am to 1 pm, midday, the branch's noon position mirroring Zi's midnight position exactly opposite it on the twelve-branch wheel. Its dominant hidden stem is Ding, giving it the Fire phase, with Ji present as a secondary hidden stem. Wu is a Yang-polarity branch, pairing only with Jia, Bing, Wu, Geng or Ren within the sexagenary cycle, and it governs the fifth lunar month, the height of summer. The branch Wu, the Horse, is an unrelated homophone of Wu the fifth Heavenly Stem (Yang Earth); the two share a romanised spelling but are written with different characters and belong to different halves of the sexagenary system. In BaZi astrology a chart with Wu prominent is read through the Horse's traditional associations of energy, sociability and restlessness, amplified by the Fire phase's own association with rapid, outward-moving activity.

Beyond the animal it carries, a branch enters three fixed sets of relations that the almanac and the fate-calculation tradition use constantly. Wu 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 Zi, the branch six places away, and it forms one of the six harmonies with Wei. 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 Wu names a period of two hours as well as a year and a month; they mark the twelve directions of the compass, Wu standing at south; 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.

Wu is the cardinal branch of the south and holds the noon double hour, standing opposite Zi at midnight. That opposition is the clearest of the six clashes, the pairings of branches six apart that the tradition reads as tension, and the Zi and Wu axis is used in ritual orientation and in siting as well as in the calendar.

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