Sexagenary Cycle (Jiazi)

The sexagenary cycle is a naming wheel rather than a calendar of months and days. Its machinery is two interlocking lists: the ten Heavenly Stems and the twelve Earthly Branches. Each unit of time takes a stem and a branch together, both lists advancing by one each step, and because ten and twelve share a factor of two, the pairing returns to its start after sixty steps, not one hundred and twenty. The first pairing, jiazi, gives the whole cycle its name. Sixty names are enough to label anything that repeats: the cycle has named days continuously since antiquity, names years in the familiar animal cycle, and in the fuller reckonings names months and double hours as well.

The branches carry the twelve animals, which is why 1984, a jiazi year, opened a cycle as the Wood Rat, and the stems carry the five phases in yin and yang pairs, Wood, Fire, Earth, Metal and Water, which is why each animal returns in a different elemental dress every twelve years. A person's birth year, month, day and hour yield four stem and branch pairs, the four pillars read by Ba Zi horoscopy, and the year pairs drive feng shui's cycles and the traditional almanac. In this atlas the stems and branches are entries in their own right; this page is the wheel they turn together, with each year from 1920 to 2020 carrying its sexagenary name.

The most remarkable fact about the sexagenary cycle is not its structure but its continuity. Stem and branch day names appear on the Shang oracle bones around 1200 BCE, already in routine use for dating divinations, and the count of days has run unbroken from that horizon to the present: the cycle recorded on a bone unearthed at Anyang connects by simple counting to the cycle printed in this year's almanac. That makes it among the oldest continuously running human reckonings of time. The application to years came later, entering general use under the Han, and the traditional claim that the cycle began in 2637 BCE, when the Yellow Emperor's minister devised it, is legend rather than history, the kind of founding attribution Chinese sources routinely supply; the atlas records it as such.

From China the cycle travelled with the calendar to Korea, Japan and Vietnam, where it named years, fortunes and generations; in Japan the sixtieth birthday, kanreki, is still marked as the completion of a person's own cycle, the return of their birth pair. Historical events keep sexagenary names as proper nouns, remembered by their year pair rather than a number. The cycle asks nothing of the sky, no observation, no intercalation, which is exactly why it has never needed correcting and never broken.