When Does Thu Phân Begin and What Customs Mark It? A View from Sui Shi Guang Ji and Calendrical Science

What Is Thu Phân and When Does It Fall?

Thu Phân is the 16th of the 24 solar terms, after Bạch Lộ and before Hàn Lộ, beginning on 22, 23 or 24 September depending on the year. It is the moment the Sun reaches ecliptic longitude 180 degrees, opposite Xuân Phân, when day and night are nearly equal.

Solar terms are counted not by lunar date but by the Sun's position on the ecliptic, with consecutive terms 15 degrees apart. From 1950 to 2050, Thu Phân begins on 23 September Vietnam time in most years, on 22 September in a few, and on 24 September only rarely.

The Hiệp kỷ biện phương thư, in its section on the Sun passing through the palaces, records: "秋分日躔辰宮初度是為壽星之次", meaning that on Thu Phân the Sun enters the first degree of the Thìn palace, called the Thọ Tinh station^[2]. The book divides the ecliptic into twelve palaces named after the earthly branches, each beginning at a mid-month term, so the Thìn palace runs from 180 to 210 degrees of ecliptic longitude.

Why Does Thu Phân Not Have a Fixed Date?

Thu Phân follows the Sun's actual position, and one circuit of the Sun along the ecliptic takes about 365.24 days rather than the 365 days of an ordinary Gregorian year. The transition time therefore falls nearly 6 hours later each year, then moves back a day in a leap year.

Thu Phân transition times in Vietnam time, 2025 to 2030:

On the lunar month, the Tuế thời quảng ký quotes the Tam thống lịch: "白露為八月節秋分為八月中氣", that is, Bạch Lộ is the term opening the eighth month and Thu Phân is the mid-month term of the eighth month^[4]. The table of solar terms by lunar month in Hoàng Tuấn's Kinh dịch và hệ nhị phân likewise places "Giữa Thu (Thu Phân)" in the eighth month, after Bạch Lộ^[3]. The lunar date of Thu Phân within the eighth month changes from year to year, so it does not coincide with the Mid-Autumn Festival on the fifteenth.

What Does Sui Shi Guang Ji Record About Thu Phân?

The Tuế thời quảng ký (歲時廣記, Sui Shi Guang Ji) is a Southern Song work by Trần Nguyên Tịnh that gathers passages from older books on the observances of the year, arranged by month into four volumes for the four seasons^[4]. Thu Phân appears under the month of Trọng thu, mid-autumn, in volume 3.

That section quotes the Nguyệt lệnh: in the mid-autumn month "日夜分雷乃收聲蟄蟲坏户", day and night are divided equally, thunder ceases, and insects seal the mouths of their burrows. It then quotes the Chu thư Thời huấn, which divides Thu Phân into three pentads: "秋分之日雷乃始收後五日蟄蟲坏户後五日水始涸", on the day of Thu Phân thunder begins to withdraw, five days later insects seal their burrows, five days after that the waters begin to dry; with omens that if thunder does not withdraw there will be lingering rain, if insects do not seal their burrows the people will have nothing to rely on, and if the waters do not dry shelled creatures will do harm^[4]. The entries on Thu Phân in this section are all phenology and calendar; none describes a rite specific to the day.

What Customs Mark Thu Phân in Vietnam?

Thu Phân is not a festival with rites of its own in Vietnam.

In the section Tứ thời tiết lạp of Việt Nam phong tục, Phan Kế Bính lists ten festivals of the year: Nguyên đán, Hàn thực, Thanh minh, Đoan ngọ, Trung nguyên, Trung thu, Trùng cửu, Trùng thập, Táo quân and Trừ tịch; Thu Phân is not among them^[5]. Thanh minh is the only solar term on that list, and he notes "we do not celebrate that festival", only that many people take the occasion to tend graves. The Mid-Autumn Festival (Trung thu) is fixed on the fifteenth of the eighth lunar month, not by solar term.

In China, the rite tied to Thu Phân was a court rite: the Hiệp kỷ biện phương thư records the Qing ritual "秋分酉時祭夜明於夕月壇", a sacrifice to the Moon at the Altar of the Evening Moon in the Dậu hour of Thu Phân^[2]. It was a state rite of the Qing dynasty, not a Vietnamese folk custom.

In folk usage, Thu Phân is mainly mentioned as a seasonal marker for autumn work; no common rite is shared across regions.

Thu Phân in Divination Systems and Feng Shui

Thu Phân is one of the eight nodal terms. The Vietnamese translation of the Hiệp kỷ biện phương thư, in its note on the eight terms and the three wonders, explains that the eight terms are Đông chí, Lập Xuân, Xuân phân, Lập Hạ, Hạ chí, Lập Thu, Thu phân and Lập Đông, and that the three wonders are Ất, Bính and Đinh of Kỳ môn Độn Giáp^[6].

The book Sử dụng la bàn has a table of Thái âm đáo sơn matching the 24 solar terms to the 24 mountains, which reads "Thái âm thu phân đáo dậu sơn"^[7]: Thu Phân corresponds to the Dậu mountain, just as Xuân Phân corresponds to the Mão mountain.

In Tử Bình the birth month changes at the term opening each month, not at the mid-month term. The Kỳ Môn Chỉ Quy gives the astrologers' example: a person born in the fourth lunar month but before the transition to Lập Hạ is still counted as born in the third month^[9]. Thu Phân is a mid-month term, so being born before or after the Thu Phân transition makes no difference: both fall in the Dậu month (the eighth month counted by solar terms), which runs from the Bạch Lộ transition to the Hàn Lộ transition. In 2026 the Dậu month runs from 21:41 on 7 September to 13:29 on 8 October, Vietnam time.

Table Comparing Thu Phân Across Systems

Should One Observe Any Taboos During Thu Phân?

No ritual text sets a taboo for the day of Thu Phân itself.

Day selection avoids the day before Thu Phân, not the day itself. The Hiệp kỷ biện phương thư classes the days before Xuân Phân, Hạ Chí, Thu Phân and Đông Chí as Tứ ly, the four separations, quoting Tào Chấn Khuê: "秋分前一日隂體分而金亦離也", and Lý Đỉnh Tộ: "此日忌出行征伐", on that day one avoids setting out on journeys and military campaigns^[2].

Nhật Nguyệt's suggestion: to use Thu Phân as a marker for your own plans, take that year's exact transition time in Vietnam time from the table above, because in some years it falls close to midnight, as at 00:30 on 23 September 2029.