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Why the Bazi month cannot simply follow the lunar month
This article uses the common convention that changes month pillars at the solar-term transition instant. A lunar month’s first day cannot replace that boundary; near it, time references and record precision also need checking.
The month pillar follows jie, not the lunar month’s first day
A Bazi month pillar cannot simply copy the lunar month. This article uses the seasonal-month convention common in Ziping chart construction: a month begins at the transition instant of the relevant jie solar term. Neither the first day of a lunar month nor the first day of a Gregorian month replaces that boundary. A birth on the transition date requires comparison of the specific times.
The twelve jie are Beginning of Spring, Awakening of Insects, Clear and Bright, Beginning of Summer, Grain in Ear, Slight Heat, Beginning of Autumn, White Dew, Cold Dew, Beginning of Winter, Great Snow, and Slight Cold. Under this article’s convention, adjacent jie define a seasonal month. The other twelve, the principal terms or zhongqi, do not serve as month-pillar boundaries.
The month pillar is the stem-and-branch combination representing the birth month in Bazi; a solar-term transition is the instant a particular term begins. A calendar marking Beginning of Spring on a date gives only the date. If the birth is that day too, find the transition’s time and confirm that the two time readings can be compared directly.
Distinguish three kinds of month
A lunar month begins with the new moon, a Gregorian month runs from its first day to its end, and a seasonal month extends from one jie transition to the next. All are months, but each has a different beginning. Looking up a lunar birthday and assigning a month pillar use different boundaries; a month number from one cannot simply be copied to the other.
Their boundaries operate independently: a lunar month’s first day often falls in the middle of a seasonal month, as does a Gregorian month’s first day. Being born in the first lunar month therefore does not necessarily mean the Yin month. First locate the birth instant within its solar-term interval, then assign the pillar.
When a lunar year has a leap month, its calendar contains an ordinary month and a leap month of the same name. Seasonal-month boundaries still follow the twelve jie in sequence. An additional lunar month does not repeat a solar-term boundary, and two lunar months sharing a name cannot automatically be assigned the same month pillar.
Jie and principal terms are different
The twenty-four solar terms alternate between twelve jie and twelve principal terms, or zhongqi. Slight Heat is a jie, while Great Heat is a principal term. Everyday use often calls all twenty-four solar terms, but month-pillar discussion must distinguish the two categories or a non-boundary principal term may mistakenly be treated as a month change.
Volume two of Sanming Tonghui calls Slight Heat the sixth-month jie and Great Heat the sixth-month principal term, showing this distinction. Its discussion of deriving months and hours also says that the month is derived from the year: calculating the month stem depends on the year stem, and cannot be done independently from a lunar month number alone.
Classical texts help explain seasonal months and traditional derivation, but cannot replace modern transition-time tables. When checking software today, separate its month-changing rule from the astronomical data used to locate the boundary instant. The classical chapters cited here do not supply a modern formula for calculating minutes and seconds.
Teaching examples around a transition
Take a teaching assumption: a transition occurs at exactly 14:00 local time, without reference to a real year or particular term. A birth securely recorded at 13:59 belongs to the preceding seasonal month; one securely recorded at 14:01 belongs to the following month. They lie on opposite sides of the same boundary.
This does not mean minute-level records are always sufficient. If the record is merely “around 14:00,” or birth and transition lie within the same minute, check the original record and the precision of both times. A software display of seconds cannot turn a family’s approximate memory into a birth time accurate to the second.
The time references must also agree. The Hong Kong Observatory page uses Hong Kong time, UTC+8. An overseas birth recorded in local time must first be converted to a comparable reference, with the historical time-zone rule checked. Two clocks from different regions cannot be compared directly to decide which event came first.
Check software boundaries and record precision
First read the software’s month-change explanation. The 6tail calendar-library documentation provides two kinds of interface within one library: ordinary stem-and-branch month calculation changes at midnight at the start of the transition date, while the Exact convention changes at the precise transition instant. They can produce different results on that day, so establish which interface the software actually calls.
Then check the data’s precision. Compare the birth record, transition instant, and their time references. If the birth time is known only within a range, check whether that range crosses the boundary. If it does not, its interval can be established. If it does, retain both possibilities and seek the original record.
Finally, save the basis for the check: the month-change convention, transition source, time-zone conversion, and certainty of the birth time. The month-pillar interval depends on those specific conditions. Resolve them before deriving the corresponding month stem, and do not let the software’s definite format conceal gaps in the data.
Sources and further reading
- Sanming Tonghui, volume two: seasonal terms and deriving months and hours ↗
- lunar library author documentation: stems and branches for year, month, day, and hour ↗
- Hong Kong Observatory: solar-term dates and times ↗
Published after NineHound editorial verification; the illustrative scenarios explain the method.