Wu

Wu is the fifth of the ten Heavenly Stems (tiangan), distinct from the Earthly Branch of the same romanisation, the branch Wu of the Horse. Its element is Earth and its polarity is yang, so this stem is glossed as yang Earth, likened in tradition to a mountain or a wall, solid, high, and enduring. As one of the ten stems it combines with the twelve Earthly Branches to form the sexagenary cycle of sixty that numbers years, months, days, and double-hours in the traditional calendar. Standing at the middle of the ten, it carries the yang aspect of the central Earth phase. These are the conventions of the Chinese calendrical and cosmological tradition, used in the almanac and in the four-pillars method of reading a fate from the stems and branches of a birth.

Wu is the fifth Heavenly Stem, the first of the pair carrying the Earth phase, and as an odd-numbered stem it takes the Yang polarity, traditionally likened to a mountain or high plateau, firm and unmoving, against Ji's Yin Earth, likened to level cultivated soil. Wu sits at the structural centre of the ten-stem cycle, a position that echoes Earth's own frequent placement at the centre of the five phases rather than on the points of a single ring. Within the sixty-term sexagenary cycle, Wu combines only with Yang-polarity Earthly Branches: Rat, Tiger, Dragon, Horse, Monkey and Dog. In four pillars (BaZi) astrology a Wu day master is read as dependable and grounding, a stabilising presence for other elements in the chart, though the tradition also warns that too much Yang Earth without relief from Wood can read as stubbornness. Note that Wu is also, confusingly, the name of the seventh Earthly Branch (the Horse); the two are unrelated homophones written with different characters.

Every stem has a partner among the other nine, and the five pairings, called the combinations of the heavenly stems, are fixed: each stem combines with the one five places away from it. Wu pairs with Gui, and the tradition holds that a combined pair takes on a phase of its own distinct from either stem's native phase. The pairing is used in almanac work and in fate calculation, where a combination in a chart is read as a binding between the two positions that carry it.

The ten stems are older than the sexagenary cycle they now form half of. On the Shang oracle bones of the second millennium BCE they name the ten days of the xun, the Shang week, and Shang kings are known to history by posthumous names built on them, so a stem was a unit of the working calendar before it was a term of cosmological art. The five-phase and yin-yang readings attached to the stems are later, systematised under the Han, and the atlas records the two layers as two layers.

Wu is yang Earth, conventionally the mountain or the embankment against the field or garden soil of Ji. Earth occupies the centre in the five-phase spatial scheme rather than one of the four quarters, and the two Earth stems accordingly sit at the middle of the run of ten, fifth and sixth, which is one of the neater correspondences between the stem order and the phase order.

The stem's own attributes here are Earth and yang polarity. Those two facts fix its place in the run of ten, since the stems proceed through the five phases in generation order taking the yang member first and the yin member second, and they fix how it is read wherever it appears, in the year, the month, the day or the double hour of a Four Pillars chart.

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