Solar Terms (Jie Qi) 节气 Glossary

The 24 astronomical markers of the sun’s longitude — the instants that flip month pillars, define the Lichun year boundary, and key every date-selection officer to the solar-term month.

Solar terms · 節氣 · 节气 · jiéqì · jieqi · 二十四节气 · Lichun

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

What it is

The sun's annual path is divided into 24 equal longitude steps of 15°, each named and fixed to an instant, not a date: 立春 at 315°, 驚蟄 at 345°, and so on. Twelve are "sectional" terms （節） starting months of the solar-term year; the other twelve are "mid" terms （氣）. The whole edifice of BaZi month pillars and traditional date selection stands on these instants — the month in 月建 is the solar-term month, not the lunar month (they differ by ~7.4 days on average, up to 16).

School differences

No school disputes the astronomy; the traps are computational — approximating term dates to the day, or mixing the lunar-month “month” with the solar-term month. Both are fatal to a whole month's officers, which is why the engine keys everything to instants and exposes the birth-side LMT caveats separately.

How this engine computes it

• Term instants are solved from the DE441-based solar-longitude engine to sub-arcsecond precision, then cross-checked against the Hong Kong Observatory for 1901–2100 (machine table: 4795/4800 within tolerance, every divergence attributed in the accuracy report).
• Each term page lists the exact instant in Beijing time; period/chart/year-pillar boundaries all read the same pipeline, so the period, the year pillar and the month pillar never disagree about February 4.
• Range: 1901–2199, with historical time caveats (pre-1929 local mean time) surfaced per response.

Related entries

Year Pillar (Lichun boundary) · Four Pillars (BaZi) · Yun Period · Daily almanac

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