Four Pillars (BaZi) 四柱 Glossary

The four two-character “pillars” — year, month, day, hour — that BaZi 八字 reads a moment of birth as. Seven of the eight characters are computable astronomy; the conventions for the remaining boundaries are where tools disagree.

Four Pillars · 四柱 · 八字 · Bāzì · sì zhù · BaZi · birth chart

Source credibility (S/A/B/C/X): S    Rule computability (A/B/C): A

What it is

BaZi （八字） represents a birth moment as four pillars （四柱）: the year, month, day and hour, each written with a heavenly stem （天干） over an earthly branch （地支）. The year pillar follows the Lichun boundary; the month pillar flips at each of the twelve major solar-term instants （節）; the day pillar is a continuous 60-day cycle; the hour pillar is derived from the day stem and the two-hour branch of the birth hour.

School differences

Three conventions must be declared, not assumed: the year boundary (Lichun instant vs Chinese New Year vs January 1), the day boundary (midnight vs the 23:00 late-Zi split), and historical time (whether to apply true solar time for pre-standardisation births). The engine computes all four year-convention / day-boundary combinations and returns them side by side — a difference between them is a convention choice, not an error.

How this engine computes it

• Solar terms come from the arcsecond-level solar-longitude pipeline; the day pillar is cross-checked against the Hong Kong Observatory 1901–2100 and four open-source reference implementations (≈290,000 samples, zero day/hour-pillar divergence where conventions align).
• Every response carries engine_version, ruleset_id and a SHA-256 input_digest (what that is), so any result can be replayed and compared byte-for-byte.
• Hour-unknown charts are computed with the hour pillar explicitly marked absent — the engine does not guess a birth hour.

Related entries

Year Pillar (Lichun boundary) · Solar Terms · Early / Late Zi Hour · True Solar Time · Heavenly Stems & Earthly Branches

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