Sitian (司天, "governing the heavens") and Zaiquan (在泉, "residing in the earth") are the two governing qi that split each year into an upper and lower half. Sitian governs the first half — from the first qi step at Dahan (大寒) through the third step at Dashu (大暑) — while Zaiquan governs the second half, from the fourth step through the end of the year. Together, they give each year a two-part climatic structure that is finer than the single grand movement but coarser than the six individual qi steps.
How Sitian and Zaiquan split the year
The calculation is deterministic and rooted in the sexagenary cycle. The earthly branch of the year determines the Sitian qi: for example, a Zi (子) year has Taiyang Cold-Water as Sitian, a Chou (丑) year has Jueyin Wind-Wood, and so on through the twelve branches. The Zaiquan is always the qi that sits diametrically opposite the Sitian on the six-qi wheel — so if Sitian is Cold-Water, Zaiquan is Damp-Earth; if Sitian is Wind-Wood, Zaiquan is Ministerial Fire. 《素问·六元正纪大论》 catalogues all twelve Sitian-Zaiquan pairs and their climatic characteristics.
How the pair is calculated from the sexagenary cycle
The relationship between Sitian and Zaiquan is always complementary. They are never the same element, and they always sit opposite each other on the wheel. This means the upper and lower halves of any given year carry contrasting climatic flavours — if the first half is governed by cold, the second half is governed by dampness; if the first half is governed by wind, the second half is governed by fire. The grand movement provides the year's overall character, while Sitian and Zaiquan add a half-year modulation that tells you how the year's climate shifts between its first and second six months.
Complementary half-year climates
TianJi computes the Sitian and Zaiquan pair for any year from its earthly branch and displays them on the half-year view. Each label comes with a citation to the relevant Suwen passage, so you can verify which classical text describes that particular qi pairing. The half-year view also shows how the Sitian qi interacts with the grand movement — whether they reinforce or moderate each other — giving you a clear picture of how the year's two halves differ climatically.
Climatic descriptors, not health predictors
Sitian and Zaiquan are climatic descriptors, not health predictors. Knowing that the first half of a year is governed by Wind-Wood and the second by Ministerial Fire tells you the environmental backdrop is shifting from a windy, expansive quality to a hot, active quality — and classical guidance suggests adjusting lifestyle accordingly. But this is seasonal education, not medical advice. The half-year view helps you plan your year in rhythm with the climate; it does not diagnose, treat, or predict any health condition.