Autumn Equinox秋分

Autumn Equinox (秋分, Qiū Fēn) is the sixteenth of the twenty-four solar terms and the point where day and night split into equal halves on the way toward winter. It is an astronomical instant, not a day: the moment the sun reaches 180° of ecliptic longitude and crosses the celestial equator, which falls somewhere between 22 and 24 September depending on the year.

分 here means "to divide", the same character used in Spring Equinox — the two terms are a matched pair, 180° apart on the sun's path, each marking one of the year's two moments of equal day and night. Despite the symmetry of the name, the Autumn Equinox changes nothing in a BaZi chart: it is a mid-month marker, exactly like its spring counterpart.

It falls at the midpoint of the 酉 month that White Dew opened roughly two weeks earlier, and it is followed roughly two weeks later by the next 節, Cold Dew. In the agricultural calendar it marks the point after which nights grow longer than days for the rest of the year's slide into winter, and it sits at the traditional midpoint of autumn itself.

Autumn Equinox is a 氣 — a mid-month term — and a 氣 changes no pillar at all. The month pillar's branch was already set to 酉 when White Dew opened this month, and it stays 酉 straight through to the next 節, Cold Dew. A birth at the Autumn Equinox and a birth a week before or after it can carry the same month pillar with nothing about the equinox itself moving it.

This site's calculator treats every 氣, this one included, as a marker with no effect on the four pillars: it is useful for describing where in the solar year a birth falls, but the year, month, day and hour pillars are computed from the 節 boundaries and the calendar, not from the equinox.