Solar-Term Month Boundaries in Bazi
Learn which twelve solar terms start Bazi months, the Yin-to-Chou branch sequence, and why lunar month starts and Zhongqi do not change the Month Pillar.
Overview
In OpenFate’s Bazi contract, the Month Branch changes at the exact instant of each of the twelve Jie: Li Chun starts Yin, Jing Zhe starts Mao, Qing Ming starts Chen, Li Xia starts Si, Mang Zhong starts Wu, Xiao Shu starts Wei, Li Qiu starts Shen, Bai Lu starts You, Han Lu starts Xu, Li Dong starts Hai, Da Xue starts Zi, and Xiao Han starts Chou. The alternating Zhongqi do not start a new Bazi month.
At a glance
- Boundary set
- Twelve Jie, not all twenty-four terms
- First month
- Li Chun begins Yin month
- Midyear examples
- Li Xia begins Si; Li Qiu begins Shen
- Winter examples
- Li Dong begins Hai; Da Xue begins Zi
- Last month
- Xiao Han begins Chou month
- Not a boundary
- Lunar month day one or an intervening Zhongqi
Each Jie starts one fixed Month-Branch segment
The sequence is Li Chun–Yin, Jing Zhe–Mao, Qing Ming–Chen, Li Xia–Si, Mang Zhong–Wu and Xiao Shu–Wei.
It continues Li Qiu–Shen, Bai Lu–You, Han Lu–Xu, Li Dong–Hai, Da Xue–Zi and Xiao Han–Chou before returning to Li Chun.
- Keep the sequence canonical.
- Use the exact event instant rather than the printed day alone.
Jie and Zhongqi alternate but have different Bazi roles
The twenty-four solar terms alternate between twelve Jie and twelve Zhongqi. Together they mark the Sun’s progress through the year.
The Month Pillar changes at Jie in this contract. A Zhongqi may be important in other calendrical contexts, but it does not advance the Bazi Month Branch here.
- Classify the term before changing a pillar.
- Do not advance twice within one Bazi month.
A boundary birth is compared to the exact local instant
Published almanacs often list a date, but the event also has a time. A birth earlier and later on that date can belong to different Month Pillars.
Resolve the astronomical instant and birth instant on the same time scale, then display the result in the user’s historical local time.
- No whole-day rounding.
- Preserve time-zone provenance.
A Month-Pillar change is not an immediate life change
The boundary determines a chart symbol. It does not create a guaranteed event or make one side of the term luckier.
If the birth time is uncertain across the boundary, show both candidate Month Pillars and explain which later calculations also change.
- No omen from proximity to a Jie.
- Keep uncertain inputs uncertain.
Sources and editorial basis
- Hong Kong Observatory: The 24 Solar Terms香港天文台列出二十四節氣及精確交節時刻；八字月界採十二個節令而非農曆初一An official astronomical explanation of the 24 solar terms, their 15-degree divisions along the ecliptic, and the alternating major and minor terms.
- Hong Kong Observatory: Heavenly Stems and Earthly Branches香港天文台干支資料提供十二地支次序，月支須按節令序列配屬An official modern reference for the canonical stem and branch orders, the sexagenary cycle, and calendrical use.
- OpenFate Editorial Methodologytests/bazi/test-pillar-invariants.ts 覆蓋驚蟄等節令前後月柱切換，邊界以出生地時區還原後的交節瞬間判定OpenFate separates deterministic chart calculation, traditional interpretation, and modern editorial explanation.