Sky Atlas 天·地·人

Learning guide · Four Pillars

Four Pillars, zodiac,
and Day Master.

Four Pillars (四柱, sì zhù) name a four-position calendar structure in this guide. In the Sky Atlas coordinate model, each position pairs one Heavenly Stem with one Earthly Branch. The year, month, day, and hour coordinates are related; the zodiac animal and Day Master each select one part of that structure.

Last reviewed · 2026-10-22 Static guide · no account or tracking Sources listed at the end

Independence and self-links. Sky Atlas is independently published and edited by Zhou Zhe. It operates the portrait tool linked in this guide, so that is a self-link rather than an independent endorsement. This guide has no sponsorship, paid placement, or affiliate relationship with 6tail, overtrue, Wikisource, the Hong Kong Observatory, IANA, or any other cited source. Sky Atlas has no affiliation with any Four Pillars school, lineage, practitioner, calendar provider, certification organization, or regulator. It does not provide personalized readings or guarantee an outcome.

Answer first

Four Pillars (四柱, sì zhù), also called BaZi (八字, bā zì, “eight characters”), label year, month, day, and hour positions. Four stem-branch pairs contain the eight characters counted in the name. A public implementation exposes GanZhi strings for all four positions and derives the year zodiac from the year-branch index. The cited classical passage calls the day stem 日主, which this guide translates as the Day Master.

These are positions in a symbolic cultural model. This guide is general educational context, not a personalized reading. It does not provide medical, legal, financial, safety, fertility, or relationship advice. Do not use a Four Pillars label to make a high-stakes decision; consult a qualified professional or the responsible official source. If someone may be in immediate danger, contact local emergency services.

A practical way to use that distinction is a positional audit: input → four coordinates → selected labels → interpretation. Put each shorthand label back at the stage that produced it. “Horse” then points to a year-branch mapping, not a personality verdict; “Metal Day Master” points to the day stem and its project labels, not the whole chart.

The four columns each answer a different question

Column Chinese What it marks
Year 年柱 · nián zhù The year GanZhi coordinate. This portrait uses the Lichun boundary stated below.
Month 月柱 · yuè zhù The month GanZhi coordinate. In this portrait it is calculated from solar-term timing, not from the lunar month number by itself.
Day 日柱 · rì zhù The day GanZhi coordinate. In the cited classical passage, its stem is 日主, the Day Master.
Hour 时柱 · shí zhù The hour GanZhi coordinate, pairing an hour branch with a stem. It is omitted when birth time is unknown.

“Four Pillars” is a translation of 四柱. “BaZi” counts the same structural table differently: four pairs contain eight characters, with four stems and four branches. Neither name makes one label a complete chart.

Each pillar is a stem above a branch

Heavenly Stem · 天干 庚

The upper character in the project’s pillar display. The cited passage calls 甲 yang and 乙 yin, then relates the other stems to a day stem through five-phase relations, closing with 而十干盡之矣 (“and the ten stems are exhausted”).

Earthly Branch · 地支 午

The lower character in that display. The same passage says 地支十二 (“twelve Earthly Branches”) and treats their generating and overcoming relations as following the same principle despite differences in movement and form.

One pillar 庚午

The public implementation joins one stem (GAN) and one branch (ZHI) into a GanZhi string. The full portrait contains up to four such pairs.

A sample chart and three different labels

The illustrative values below use 4 July 1990 at 12:00 in New York (America/New_York), clock time, a midnight day boundary, and the Lichun year boundary. They are project-engine output, so this is a disclosed method example rather than an independently published chart.

01 / Year 庚午 gēng · wǔ metal · fire
02 / Month 壬午 rén · wǔ water · fire
03 / Day 庚午 gēng · wǔ metal · fire
04 / Hour 壬午 rén · wǔ water · fire
Four Pillars 庚午 · 壬午 · 庚午 · 壬午 The full set of four stem-branch coordinates.
Zodiac animal Horse Selected from the year branch, 午. It is not a sun sign.
Day Master 庚 · gēng yang metal · the day stem used as one reference point.

A label such as “I am a Horse” or “I am a Metal Day Master” selects one layer of this table. The full Four Pillars string remains the coordinate set; an interpretation is another layer again.

From input to interpretation

A Four Pillars result is easiest to audit as a chain. Each stage adds a different kind of claim:

Stage In this sample What changes at this stage
Local input 4 July 1990, 12:00, America/New_York; clock time; midnight day boundary; Lichun year boundary Fixes the date, time basis, zone history, and boundary conventions being compared.
Position coordinates 庚午 · 壬午 · 庚午 · 壬午 Pairs four stems with four branches and gives the eight characters.
Selected labels Horse from the year branch 午; 庚 from the day stem Names two parts of the coordinate set.
Interpretation Not supplied by the calculation Requires a named text, school, or editorial method; another reading may differ even for the same coordinates.

Read that chain in order. If another page shows a different label, compare where the chains diverge: a different input or convention may change a coordinate, while the same coordinates may still receive different interpretations. That distinction separates a calculation disagreement from an interpretive one.

Turn a shorthand label into an auditable sentence

The table applies the same receipt to the sample above. The aim is to prevent a short label from borrowing certainty from a different stage of the chain.

Shorthand Position or operation Auditable wording Next check
“I am a Horse.” Year branch 午 → zodiac mapping “Under the stated Lichun boundary, the year branch is 午; the implementation maps it to Horse.” Does another calculator use a different year boundary?
“I am a Metal Day Master.” Day stem 庚 → 日主 plus the project’s yang-metal label “The day stem is 庚, the position this guide calls Day Master; the project display describes it as yang metal.” Is the next claim about the coordinate or about a school’s interpretation?
“My chart is 庚午 · 壬午 · 庚午 · 壬午.” Four coordinate pairs “Under the disclosed input and conventions, these are the four pairs generated by the engine.” Are the time zone, time basis, day boundary, and year boundary stated?
“I need Water.” A recommendation, not a coordinate This guide cannot produce that sentence from the sample. It would require a named strength or need model and a stated goal. Which model, source, and decision is being applied?

Applied this way, “Horse” and “Metal Day Master” are not rival answers. They are compact labels for different stages of the same positional result. A real calculation disagreement is narrower: two results use the same stated input and conventions but still report different coordinates. An interpretive disagreement is also narrower: the coordinates agree, but named texts or methods support different readings.

This gives the reader a stop rule. If a claim cannot be restated with a position and a named source, it is not a coordinate claim. Keep it labeled as interpretation or leave it out rather than presenting it as a fact of the chart.

Terms that are often collapsed together

生肖 Zodiac animalshēng xiào

The implementation derives ShengXiao from the year-branch index. The animal therefore identifies a year-branch association; it does not summarize the day stem, the full pillar set, or a person.

日主 Day Masterrì zhǔ

The classical excerpt calls the day stem 日主 and examines the other stems through their relation to it. The Day Master is a reference position, not a personality score.

纳音 Na Yinnà yīn

The public implementation returns Na Yin through LunarUtil.NAYIN[...] lookups using the year, month, day, or hour GanZhi string. It is not needed to form the four coordinates, and this portrait does not display it.

立春 Lichunlì chūn

“Beginning of Spring,” a solar term used by some Four Pillars implementations as the year boundary. The cited implementation calculates an exact Lichun transition rather than assuming a fixed February 4 date.

The year boundary is a convention, not a holiday fact

Calendars do not have to use the same year boundary. The cited overtrue implementation states that its GanZhi year and animal use Lunar New Year, while the lunar-python implementation contains an exact Lichun-transition year calculation. A dated page near either boundary can therefore show one year label under one convention and another label under the other.

This site states that it uses Lichun. That statement identifies the convention; it does not make Lichun a universal rule for every calendar or school.

Worked boundary check

The project engine gives the following method example on either side of the stated Lichun transition. These are engine outputs, not an independently published reading.

Clock time · Asia/Shanghai Four Pillars Year animal Day Master
2024-02-04 15:00 癸卯 · 乙丑 · 戊戌 · 庚申 Rabbit 戊
2024-02-04 17:00 甲辰 · 丙寅 · 戊戌 · 辛酉 Dragon 戊

The animal follows the year branch, so it changes from Rabbit to Dragon across this boundary. The Day Master still selects the day stem and remains 戊 at both times. Neither named label equals the complete coordinate string.

How to diagnose two different results

Start with the four coordinate pairs, not with the final reading. Find the earliest column that differs, then check the input or convention that can move that column. The worked example above is one case: the year animal changes while the day pillar and Day Master stay fixed, so the year boundary is the first thing to inspect.

First difference Check first What the check can explain
Year pair or animal differs; day pair and Day Master match Whether each result uses Lichun, Lunar New Year, or another stated boundary, and how close the local time is to that transition A year-boundary choice can change the year column and animal without moving the day column.
Month pair differs; day pair matches Whether both results use the same solar-term cutoff and historical local-time offset Under the portrait method described here, the month column follows solar terms; a cutoff or local-time difference can change the first affected month.
Day pair or Day Master differs The exact civil date and time, UTC offset or daylight-saving state, and whether the method uses midnight or a late-Zi day boundary The Day Master is the day stem, so a changed day column moves that label with it.
All four pairs differ That date, time, place, time-zone identifier, and implementation version were normalized the same way If upstream inputs differ, comparing the labels is premature.
Coordinate pairs match but readings differ Which label mapping, named text, school, or editorial method produced each reading Matching coordinates remove the calculation layer from the disagreement.

This table is a triage aid, not proof that one result is defective. A difference that remains after the same input, conventions, and implementation are recorded is the one worth investigating. For calendar cross-checks, the Hong Kong Observatory’s conversion page offers selectable Gregorian-lunar tables for 1901–2100, while the IANA Time Zone Database describes code and data representing local-time history. The calendar and time-zone guide explains that layer. Stop at the model’s scope: use the comparison to understand cultural structure, not to decide a health, financial, legal, safety, fertility, or relationship question.

What it does not mean

  • The zodiac animal does not summarize a whole personality.
  • The Day Master does not replace the rest of the chart.
  • The four pillars are not four predictions.
  • Na Yin is not a missing scientific measurement.
  • A boundary convention does not make one result morally better or more authoritative.

The model becomes more intelligible when its terms remain distinct: a calendar coordinate, a reference stem, a symbolic association, and a school-specific reading are four different kinds of claim.

Sources

  • 6tail, lunar-python source code, accessed 2026-10-21, for the public implementation’s four GanZhi coordinates, zodiac derivation, Lichun calculation, and Na Yin lookups.
  • Wikisource, 三命通會 (四庫全書本)/卷05, accessed 2026-10-21, for the classical use of 日主, Ten Stems, Twelve Branches, and 四柱.
  • overtrue, chinese-calendar conventions, accessed 2026-10-21, for a documented Lunar New Year boundary on its GanZhi year and animal.
  • Hong Kong Observatory, Gregorian-Lunar Calendar Conversion Table and The 24 Solar Terms, accessed 2026-10-21, for selectable conversion tables covering 1901–2100 and the definition of solar terms through solar longitude.
  • IANA, Time Zone Database, accessed 2026-10-21, for its description of code and data representing local-time history.
  • Four Pillars names and sample values are calculated by the project engine. Interpretive explanations are the site’s editorial layer and do not imply a single authoritative school.

Next step

Next step

Inspect the terms in the portrait, then compare the stated conventions with the calculator-disagreement guide. The portrait shows characters, pinyin, short definitions, and common misunderstandings without turning one label into a personal verdict.

See the terms in context