What are solar terms?
Solar terms (tiết khí) divide the solar year into 24 roughly equal parts (about 15 days), each corresponding to the Sun advancing 15° along the ecliptic (360° ÷ 24). Unlike lunar months, which follow the Moon, solar terms follow the Sun, so they track the seasons closely: when spring starts, when to harvest, when the cold peaks. That is why they still matter to farmers even though the Gregorian calendar is everywhere.
Each term begins at a precise moment when the Sun reaches its longitude; calendars list the day containing that moment, so the date can shift by a day between years or time zones.
The 24 terms
| Term | Usually on | Meaning |
|---|---|---|
| Lập xuân | 3–5 Feb | Start of spring; opens the solar-term year. |
| Vũ thủy | 18–20 Feb | Rain and damp begin; ice melts. |
| Kinh trập | 5–7 Mar | Insects awaken; first thunder. |
| Xuân phân | 20–22 Mar | Spring equinox: day and night about equal. |
| Thanh minh | 4–6 Apr | Clear and bright; Vietnamese families often visit graves. |
| Cốc vũ | 19–21 Apr | Grain rain: showers that feed the crops. |
| Lập hạ | 5–7 May | Start of summer. |
| Tiểu mãn | 20–22 May | Grain begins to fill but is not yet full. |
| Mang chủng | 5–7 Jun | Bearded grains ripen; busy harvest and sowing. |
| Hạ chí | 21–22 Jun | Summer solstice: longest day in the Northern Hemisphere. |
| Tiểu thử | 6–8 Jul | Minor heat: the heat turns fierce. |
| Đại thử | 22–24 Jul | Major heat: the hottest period. |
| Lập thu | 7–9 Aug | Start of autumn. |
| Xử thử | 22–24 Aug | End of heat. |
| Bạch lộ | 7–9 Sep | White dew appears; cooler. |
| Thu phân | 22–24 Sep | Autumn equinox: day and night about equal. |
| Hàn lộ | 8–9 Oct | Cold dew. |
| Sương giáng | 23–24 Oct | Frost descends; colder. |
| Lập đông | 7–8 Nov | Start of winter. |
| Tiểu tuyết | 22–23 Nov | Light snow in cold regions. |
| Đại tuyết | 6–8 Dec | Heavy snow in cold regions. |
| Đông chí | 21–23 Dec | Winter solstice: shortest day in the Northern Hemisphere. |
| Tiểu hàn | 5–7 Jan | Minor cold. |
| Đại hàn | 20–21 Jan | Major cold: the coldest period. |
Dates by year
The table below is calculated directly from the Sun's longitude in Vietnam time (UTC+7); change the year to see it.
Nodes and mid-terms
The 24 terms alternate between 12 "nodes" (jié) and 12 "mid-terms" (zhōngqì). Nodes mark the start of a month (Lập xuân, Kinh trập, Thanh minh, Lập hạ, Mang chủng, Tiểu thử, Lập thu, Bạch lộ, Hàn lộ, Lập đông, Đại tuyết, Tiểu hàn); mid-terms mark the middle (Vũ thủy, Xuân phân, Cốc vũ, Tiểu mãn, Hạ chí, Đại thử, Xử thử, Thu phân, Sương giáng, Tiểu tuyết, Đông chí, Đại hàn).
Solar terms and the leap month
A lunar year is about 11 days shorter than a solar year, so every few years a leap month is inserted. The Vietnamese rule uses mid-terms:
- The lunar month containing the winter solstice is always month 11.
- In a year with 13 months, the leap month is the first month containing no mid-term; it takes the number of the month before it.
That is why this site computes the Sun's longitude alongside new-moon times. Try the calendar to see a leap month.
What they are used for
- Farming: sowing and harvesting by season.
- The lunar calendar: fixing month 11 and the leap month.
- Day selection: the solar-term month (starting at Lập xuân as the Dần month) sets the day officers and the Hoàng đạo star of each day; see Auspicious days.