Qiufen falls at celestial longitude 180°, around September 22-24. Day and night equal again, but unlike spring equinox, yin energy now starts to dominate.
Tradition: this used to be the Mid-Autumn moon-worship date in pre-Tang times; now Mid-Autumn Festival follows the lunar calendar. Climate: cooling accelerates; rice harvest in full swing.
The equinoxes and solstices are the oldest stratum of the system, because they can be found directly from the shadow of a gnomon and the star culminating at dusk. The Canon of Yao chapter of the Book of Documents fixes the four mid-season months by four such stars, and the one assigned to mid-autumn is described as the time when day and night are balanced — an observational tradition far older than the complete set of twenty-four terms. The Explanation of the Trigrams assigns Dui, the trigram of the west, to true autumn. The trigram-and-qi system is a separate development, built by the Han scholars Meng Xi and Jing Fang, and it later drew this stretch of the year into its own framework.
Qiufen (秋分 Qiū Fēn) is a mid-term, sitting at the centre of the You month, and it opens no new pillar. Its significance in chart work is positional: metal takes command at Liqiu, enters the unmixed You month at Bailu, and at Qiufen stands squarely in the middle of its season. On the traditional five-step scale this is where metal prospers, water is supported, earth rests, fire is confined and wood is dormant — the background against which a chart's elemental balance is usually judged first.
When autumn begins has no single answer, and the traditions are candid about it. Calendrical and Bazi practice draws the line at Liqiu, as it has since pre-Qin times. If equal day and night is the criterion, Qiufen is the more natural boundary, or else the season's midpoint. Modern meteorology uses another rule again, requiring a run of daily mean temperatures to settle below a threshold, which can place the start of autumn a month or two later in the south. Each standard serves its own purpose; only mixing them produces contradictions.
For births around Qiufen, classical practice often begins with climatic adjustment. Autumn metal is at full strength while cold is gathering, and the transmitted view is that metal too hard wants fire to temper it while metal too turbid wants water to wash it; the Qiongtong Baojian tradition is usually cited for pairing Ding fire with Geng and Ren water with Xin, a book whose substance is the Ming-dynasty Lanjiangwang that Yu Chuntai edited and printed under its present title only in the Qianlong reign. This is a starting thread rather than a formula. The day master's own strength, the arrangement of wealth, officer, resource and output, and whether a recognised structure forms all still have to be weighed.
Qiufen and Chunfen share a day length and reverse a direction. After the spring equinox daylight lengthens and yang advances; after the autumn equinox the nights lengthen and yin gains. On the branches, Mao and You stand in direct clash — the sharpest of the cardinal oppositions, wood facing metal. Classical phrasing pairs the two as spring generating and autumn completing, and when a Mao–You clash turns up in an annual reading it is usually read for those two senses, movement and settlement, and not given a flat verdict of good or bad.
Day and night are not exactly equal on the equinox: refraction lifts the sun into view early and holds it visible late, and the sun is a disc and not a point, so daylight runs a few minutes long and true equality falls some days afterwards, at a date that depends on latitude. Qiufen is also not the Mid-Autumn Festival. Mid-Autumn is fixed to the fifteenth of the eighth lunar month while Qiufen is a solar marker, so the gap between them changes every year; they share a season, not a date.
What separates them is the direction that follows. After Chunfen daylight lengthens and yang advances; after Qiufen the nights grow and yin gains ground. Chunfen sits in the middle of the Mao month and Qiufen in the middle of You, and Mao and You clash directly, wood against metal. Classical shorthand pairs them as generation in spring and completion in autumn.
Close, but not exactly. The sun is a disc rather than a point, and atmospheric refraction makes it appear to rise early and set late, so daylight on the equinox usually runs several minutes longer than night; exact equality arrives a few days later and depends on latitude. The term itself is defined by the sun reaching 180° of celestial longitude, not by a measured day length.
No. Mid-Autumn falls on the fifteenth day of the eighth lunar month, a luni-solar date, while Qiufen is the moment the sun reaches 180°. Their spacing changes from year to year, sometimes by days and sometimes by more than a fortnight. There was an old rite of offering to the moon at the autumn equinox, but the familiar moon-viewing customs attached themselves to the lunar festival instead.
As a mid-term it opens no pillar — the month remains You on both sides of it. What it marks is the high point of metal: metal prospers, water is supported, earth rests, fire is confined, wood is dormant. A metal day master holds the month command here, while fire and wood day masters sit in restrained positions. Holding the month command is only the first step, though; the rest of the chart still decides the reading.
It depends on the standard. Traditional calendars and Bazi use Liqiu. If you define seasons by equal day and night, Qiufen is the better candidate. Modern meteorology requires a sustained fall in daily mean temperature, which can put the start of autumn one to two months later depending on the region. All three are in use, and there is no need to force them to agree.