Bazi and Its Calendar: Four Pillars, Sixty Terms and the Solar Year

Bazi (八字) means “eight characters”. It is the Chinese art of reading a birth moment, and before anything can be read the moment has to be written down in the right way. That writing is the whole foundation of the method: a calendar of stems and branches, a year that turns on the Sun rather than the Moon, and an hour read from the sky rather than from a clock. This article covers that foundation. The interpretation of the chart, the five phases, the day master and the ten gods, is the subject of a separate article.

Eight characters, four pillars

A birth moment is written as four pairs: year, month, day and hour. Each pair is one of the ten heavenly stems (天干) standing over one of the twelve earthly branches (地支). Four pairs make eight characters, and the whole reading is what those eight are to each other.

The four pairs are called the four pillars, 四柱, and they are not four separate topics. They are one structure read from four distances. The year is the outermost frame. The month is the season, and it is the strongest single influence in the chart. The day is the person. The hour is the closest and the latest. Nothing in Bazi is read in isolation from the rest.

One character carries the reading: the stem of the day pillar, called the day master (日主). Everything else in the chart is described by what it is to that one stem: what feeds it, what it produces, what it can hold and what rules it.

A consequence follows that is worth stating plainly. Two people born in the same two-hour window in the same place have the same eight characters and therefore the same chart. A Bazi chart is not a private possession. It is a description of a moment, shared by everyone who arrived in it.

Ten stems, twelve branches

Two series run alongside each other. The ten heavenly stems, 甲 乙 丙 丁 戊 己 庚 辛 壬 癸, are the five phases taken twice, once in a yang form and once in a yin: 甲 and 乙 are both Wood, 丙 and 丁 both Fire, 戊 and 己 Earth, 庚 and 辛 Metal, 壬 and 癸 Water. The twelve earthly branches, 子 丑 寅 卯 辰 巳 午 未 申 酉 戌 亥, are the twelve double-hours of the day, the twelve months of the year, and the twelve animals by which most people know them: Rat, Ox, Tiger, Rabbit, Dragon, Snake, Horse, Goat, Monkey, Rooster, Dog and Pig.

Pair the first of each series and you get 甲子. Step both forward and you get 乙丑, then 丙寅, and so on. Ten and twelve would suggest a hundred and twenty possible pairings, yet there are only sixty. The reason is that both series alternate. Every odd-numbered stem and branch is yang, every even-numbered one is yin, and the two counters step together. A yang stem can only ever land on a yang branch. 甲丑 is not a term that was left out of the table; it is a term that cannot be formed.

E. J. Eitel, writing about Chinese practice for European readers, put the mechanism in a single clause: each character signifies at once an element and “either an even (female) or uneven (male) number” (Feng-Shui, 1873). That parity is the whole of it. Sixty is not a chosen round number. It is the least common multiple of ten and twelve, and the cycle closes there because it has nowhere else to close. Eitel is worth reading for the structure and worth reading around for the tone: he describes Chinese cosmology as a European sceptic of his century, and the framing is his rather than the tradition’s.

Where the sixty-term cycle came from

The sexagenary cycle is older than any of the divination built on it. It was already used to name days on the oracle bones of the Shang dynasty, in the second millennium BC. Chinese tradition attributes its invention to Nao the Great (大撓), a minister of the Yellow Emperor, which is a way of saying that it was already immemorial when the first surviving accounts of it were written.

For Bazi the decisive detail is easy to read past: the cycle names days as well as years. The same sixty terms run over the years and, independently, over the days, and the two counts have nothing to do with each other beyond sharing a vocabulary. The year pillar comes from a solar boundary. The day pillar comes from a counter that has been running since antiquity.

That day count is the strangest and most reliable thing in the system. It has never been reset. No leap day interrupts it, because it does not care about months, and no calendar reform interrupts it, because it does not care about years. When much of Europe skipped ten days in October 1582 to adopt the Gregorian calendar, the sexagenary day count skipped nothing. It simply went on, one term per day, across a discontinuity that broke the other calendars of the world.

Comparisons with the Babylonian base of sixty are often made and are probably a coincidence of arithmetic. Sixty divides neatly in many ways, which is why more than one civilisation reached for it.

Why the year turns at Lichun

Chinese New Year falls on a new moon, somewhere between 21 January and 20 February. 立春, Lichun, “the beginning of spring”, falls when the Sun reaches 315° of ecliptic longitude, which is always on 3, 4 or 5 February. They are different dates decided by different bodies, and in Bazi it is Lichun that starts the year.

This is not a fine point. Between the two dates the calendar year and the Bazi year disagree, and the window can be up to a fortnight wide or more. On the order of one birth in thirty falls inside it. Anyone born there has one animal sign in the newspaper and a different year pillar in their chart, and both are correct about different systems: the newspaper reports the lunisolar calendar, and the chart reports the solar year that the four pillars are built on. Getting this wrong is the single most common error in popular treatments of the subject.

The reason is that Bazi is solar throughout. Its months turn at solar terms, its year turns at a solar term, and its logic is seasonal. The month pillar is the strongest statement in a chart precisely because it says where in the Sun’s year the person arrived. A lunar boundary would put a person in a year that does not match their own month pillar, which is incoherent rather than merely inaccurate.

The tradition has long been clear about this. The Huai Nan Tzu, compiled in the second century BC, already sets out the year as a circuit of solar stations rather than a count of moons, and the almanac tradition Eitel described in the nineteenth century was still organising the agricultural year around the same twenty-four terms.

The twenty-four solar terms, and the twelve that matter

Cut the ecliptic into twenty-four arcs of fifteen degrees and mark the instant the Sun enters each. Those are the 二十四節氣, the twenty-four solar terms: 立春 Start of Spring, 雨水 Rain Water, 惊蛰 Awakening of Insects, and so on round to 大寒 Major Cold. They are a solar calendar embedded in a lunisolar one, and they are what Chinese agriculture actually ran on.

The twenty-four alternate between two kinds. The twelve that fall on multiples of thirty degrees are the 中氣, the major or middle terms. The lunisolar calendar uses them to decide which lunar month is which and where a leap month goes. The twelve in between, at 315°, 345°, 15°, 45° and so on, are the 節, the “joints”. Bazi uses the joints and does not use the middle terms at all.

So a Bazi month runs from one joint to the next. 寅月, the Tiger month, opens at 立春 and closes at 惊蛰; 卯月, the Rabbit month, opens at 惊蛰; and so round the twelve. A Bazi month has no fixed length in days, does not begin on the first of anything, and has no relationship to the Moon. Somebody born on 5 or 6 March may belong to one month pillar or the next depending on the minute at which 惊蛰 fell that year. A careful calculation therefore takes the solar-term instants from an astronomical ephemeris for the year in question, not from a table of approximate dates.

The Huai Nan Tzu devotes a chapter, 天文訓 (“Treatise on the Patterns of Heaven”), to this apparatus: the stations of the year, the correspondence of season to phase, and the state’s business of keeping them aligned. Evan Morgan’s partial English translation, Tao, the Great Luminant (1933), is the readiest way into it for an English reader.

The clock on the wall is not the Sun

The hour pillar comes from the double-hour in which a birth fell. 子 runs from 23:00 to 01:00, 丑 from 01:00 to 03:00, and so on. Each is two hours wide, and the boundary is a hard edge: a minute either side of it changes two of the eight characters.

The tradition read that hour off the Sun, because until recently there was nothing else to read it off. A modern birth certificate records something quite different: the reading of a clock synchronised to a zone meridian that may be hundreds or even thousands of kilometres from the birthplace.

Two corrections separate the clock from the Sun. The first is longitude. The Earth turns fifteen degrees an hour, so every degree east or west of the zone’s meridian is four minutes of solar time. China is the extreme case, running a single clock across roughly sixty degrees of longitude; a birth in Kashgar, in the far west, is recorded on Beijing time while its own Sun is nearly three hours behind. The second is the equation of time, the Sun’s own irregularity, which comes from the combined effect of the Earth’s elliptical orbit and its axial tilt and reaches about a quarter of an hour either way over the year.

Together the two routinely reach half an hour, and half an hour is a quarter of a double-hour. This is not a refinement. It is the difference between the right hour pillar and the wrong one for a large share of births. Practitioners differ on whether to read the hour on true solar time or on standard zone time, and many free calculators use the latter without saying so; whichever is chosen, the choice should be made knowingly.

Summer time is a different matter. A clock running an hour ahead of its own zone is simply wrong for this purpose, and no school reads a birth from it. Daylight saving must be removed from the recorded time before anything else is done, using historical records of what the country in question was actually doing in the year of birth, since those rules have changed often.

Whoever fixes the calendar

The Shu King (Book of Documents) opens its account of the sage-king Yao with an act of government that a modern reader can easily pass over. He commissions astronomers, sends them to the four quarters, and charges them with fixing the seasons and delivering the year to the people. It is the first substantive thing he does, and it is placed before anything about law or war.

That order is deliberate. In the Chinese conception the ruler’s legitimacy was bound to the correct regulation of time. An accurate calendar meant the seasons were properly aligned, the agricultural year worked and the mandate held. A calendar that drifted was evidence that something at the centre had gone wrong. Astronomical offices were state institutions, their records were state property, and for long stretches private astronomy was forbidden.

The twenty-four solar terms are a product of that apparatus. They are not folklore. They were an administrative instrument for telling a very large agricultural population when to plant and when to harvest: computed by an office, promulgated by an authority, and printed in an almanac that reached villages which had little else in print.

Bazi inherits that machinery wholesale. Its months are the state’s solar terms, its years turn on one of them, and its day count is the count the astronomical office kept. Computing Lichun to the minute from an ephemeris is the modern form of the work an imperial bureau once existed to do.

The mythological layer sits on top of the same idea: Fu Xi receiving the trigrams, the Yellow Emperor’s minister devising the cycle. E. T. C. Werner’s Myths and Legends of China (1922) is a readable English gathering of that material. The through-line is consistent. In this tradition the calendar is not neutral infrastructure. Someone owns it, and the four pillars are written in its terms.