Jia

Jia is the first of the ten Heavenly Stems (tiangan), the cyclical signs that combine with the twelve Earthly Branches to form the sexagenary cycle of sixty, the backbone of the traditional Chinese calendar for numbering years, months, days, and hours. Its element is Wood and its polarity is yang, so Jia is often glossed as yang Wood, likened in tradition to a great tree or standing timber, upright and pioneering. In the sixty-year cycle each stem pairs in turn with a branch, and because ten and twelve share a common factor the stems and branches recombine only after sixty steps. The stems also carry associations of direction and the five phases used in almanac reckoning and fate calculation. These are the conventions of the Chinese calendrical and cosmological tradition, in which Jia opens the count of stems.

Jia is the first of the ten Heavenly Stems (Tiangan), a cycle first attested on Shang-dynasty oracle bones of the late second millennium BCE, where it was already in use for naming days. Paired with Yi, Jia carries the Wood phase, and as the odd-numbered, first member of that pair it takes the Yang polarity, conventionally likened to standing timber or a tall tree, active and upward-growing, against Yi's Yin Wood, likened to a pliant vine. The ten stems combine with the twelve Earthly Branches (Dizhi) to form the sexagenary cycle of sixty paired terms, the basic reckoning of years, months, days and hours in the traditional Chinese calendar and its continuing use in four pillars (BaZi) astrology; because ten and twelve share a lowest common multiple of sixty rather than pairing stem to branch one-to-one, a stem always recurs with the same Yin or Yang branches, so Jia never falls on an even-numbered branch.

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. Jia pairs with Ji, 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.

In the Four Pillars of Destiny, the fate-calculation system built on the stems and branches, a stem is read as a day master when it heads the day pillar, and Jia as day master is treated as yang Wood in its most upright form, the standing tree rather than the climbing vine that Yi represents. The distinction between the yang and yin members of a phase pair is one of manner and not of substance.

The stem's own attributes here are Wood 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