Si

Si is the sixth of the twelve Earthly Branches (dizhi). Its zodiac animal is the Snake, its element is Fire, its direction is south-southeast, and its double-hour runs from 9 to 11 am, the late morning. As one of the twelve branches it combines with the ten Heavenly Stems to build the sexagenary cycle of sixty used to number years, months, days, and double-hours in the traditional calendar. The branch names the sixth animal of the twelve-year cycle and the corresponding month and double-hour, carrying the Snake's associations of wisdom and subtlety in the almanac. These are the conventions of the Chinese calendrical and cosmological tradition, in which the twelve branches, paired with the ten stems, recombine only after sixty steps to give the great cycle its length.

Si is the sixth Earthly Branch, corresponding to the Snake, and to the double hour from 9 am to 11 am, the later morning. Its dominant hidden stem is Bing, giving it the Fire phase, with Wu and Geng present as secondary hidden stems. Si is a Yin-polarity branch, pairing only with Yi, Ding, Ji, Xin or Gui within the sexagenary cycle, and it governs the fourth lunar month. Note that Si the branch and Si the given cardinal direction south (written with a different character in some regional systems) are sometimes confused by learners; the branch Si is not itself a compass direction, though its Fire phase associates it with the general southern quadrant of the branch-direction wheel. In BaZi astrology a chart with Si prominent is read through the Snake's traditional associations of intelligence, subtlety and a private, watchful temperament, paired with the Fire phase's quickness and capacity to illuminate what the Snake's caution would otherwise keep hidden.

Beyond the animal it carries, a branch enters three fixed sets of relations that the almanac and the fate-calculation tradition use constantly. Si belongs to the trio Si, You and Chou, one of the four groups of three branches spaced four apart which are read as combining into Metal. It clashes with Hai, the branch six places away, and it forms one of the six harmonies with Shen. 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 Si names a period of two hours as well as a year and a month; they mark the twelve directions of the compass, Si standing at south-southeast; 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.

Si opens the summer in the month assignment. Its trio membership shows how the branch system layers relations on top of the simple sequence: Si belongs to the Metal trio while its own phase is Fire, and Fire overcomes Metal in the five-phase conquest cycle, so a branch's trio and its own phase can pull in different directions and the tradition reads both.

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