What is Tiết khí?
Solar terms are 24 markers dividing the year by the Sun's position on the ecliptic, used in East Asian calendrics to determine seasons and festivals.
Also called: 24 solar terms · solar term
What are the 24 solar terms?
What are the 24 solar terms and why are there 24 markers?
The 24 solar terms (節氣) are a way of dividing the year into 24 segments based on the apparent movement of the Sun along the ecliptic. Each solar term corresponds to roughly 15 degrees of the ecliptic, so a year has 24 terms, each lasting approximately 15 days. This system is closely tied to the agricultural calendars of East Asia, helping to determine the timing of planting, harvesting and seasonal festivals.
Origins and use in calendar systems
The solar terms were built from observation of the Sun's cycle, not the Moon's cycle. They are therefore a solar element within the lunisolar calendar. Terms such as Lập Xuân (Start of Spring), Thanh Minh (Clear and Bright), Hạ Chí (Summer Solstice) and Đông Chí (Winter Solstice) mark turning points in the weather. In Vietnam, many customs are tied to the solar terms; the Thanh Minh term, for example, is the occasion for visiting and tending graves and remembering ancestors, reflecting the rhythm of the seasons and agricultural life.
How do the solar terms differ from easily confused concepts?
The solar terms are not the same as "tiết" in "tiết trời" (weather) or "tiết mục" (a performance item). They should be distinguished from:
- Can chi: the system of counting days and hours by the 10 heavenly stems and 12 earthly branches, which runs in a 60-unit cycle and is not a solar marker.
- A purely lunar calendar: a calendar that follows the Moon only, with no concept of solar terms.
- Modern climatology: which divides the seasons by temperature and rainfall, and does not use 24 fixed markers.
What role do the solar terms play in cultural life?
The solar terms influence festival calendars, the choosing of auspicious days, and also some traditional divination systems. However, this is traditional knowledge of a reference nature, and it has not been verified by science in the field of metaphysics. The main value of the solar terms lies in astronomical observation and agricultural experience.
Does everywhere use the same way of calculating the solar terms?
There are minor differences between calendar traditions regarding time zones and starting points. However, the general principle is based on the Sun's position, so the solar terms still correspond approximately across cultures.
How do different schools understand the solar terms?
- Classical astronomy: the solar terms are 24 ecliptic markers, each of 15 degrees, used to set the agricultural season and the calendar.
- Traditional agriculture: the solar terms are folk experience guiding the seasons, planting and harvesting.
- Traditional metaphysics: the solar terms are an important factor in choosing days and in some divination systems, of a reference nature.
Related articles
What are the 24 solar terms in the table?
| Order | Name | English | Solar longitude | Term / Qi | What it marks |
|---|---|---|---|---|---|
| 22 | Lập Xuân | Start of Spring | 315° | Term | The beginning of spring, also the marker of the new year in Tứ Trụ (Four Pillars) |
| 23 | Vũ Thủy | Rain Water | 330° | Qi | Spring rain replaces snow, ice melts |
| 24 | Kinh Trập | Awakening of Insects | 345° | Term | Insects wake after winter |
| 1 | Xuân Phân | Vernal Equinox | 0° | Qi | Day and night are equal in length, the sun crosses the equator into the northern hemisphere |
| 2 | Thanh Minh | Clear and Bright | 15° | Term | Clear skies, the grave-tending season for Vietnamese people |
| 3 | Cốc Vũ | Grain Rain | 30° | Qi | Showers nourish newly sown grain |
| 4 | Lập Hạ | Start of Summer | 45° | Term | The beginning of summer |
| 5 | Tiểu Mãn | Grain Buds | 60° | Qi | Grain begins to fill out but is not yet ripe |
| 6 | Mang Chủng | Grain in Ear | 75° | Term | Rice comes into ear, the winter-spring harvest begins |
| 7 | Hạ Chí | Summer Solstice | 90° | Qi | The longest day of the year in the northern hemisphere |
| 8 | Tiểu Thử | Minor Heat | 105° | Term | Heat begins but is not yet at its harshest |
| 9 | Đại Thử | Major Heat | 120° | Qi | The hottest time of the year |
| 10 | Lập Thu | Start of Autumn | 135° | Term | The beginning of autumn |
| 11 | Xử Thử | End of Heat | 150° | Qi | The heat gradually recedes |
| 12 | Bạch Lộ | White Dew | 165° | Term | White dew gathers in the early morning, the air turns cool |
| 13 | Thu Phân | Autumnal Equinox | 180° | Qi | Day and night are equal in length, the sun crosses the equator into the southern hemisphere |
| 14 | Hàn Lộ | Cold Dew | 195° | Term | Cold dew, the chill becomes clear |
| 15 | Sương Giáng | Frost Descent | 210° | Qi | Frost appears in the highlands |
| 16 | Lập Đông | Start of Winter | 225° | Term | The beginning of winter |
| 17 | Tiểu Tuyết | Minor Snow | 240° | Qi | Snow begins in the north |
| 18 | Đại Tuyết | Major Snow | 255° | Term | Heavy snowfall in the north |
| 19 | Đông Chí | Winter Solstice | 270° | Qi | The shortest day of the year in the northern hemisphere |
| 20 | Tiểu Hàn | Minor Cold | 285° | Term | The deep cold begins |
| 21 | Đại Hàn | Major Cold | 300° | Qi | The coldest time of the year |
The solar terms are divided by the Earth's position on its orbit (each term is 15° of solar longitude apart), so they follow the solar calendar rather than the lunar calendar. Only the 12 terms are the markers that shift the month pillar in Bát Tự (BaZi); the 12 qi fall in the middle of the month.
The figures in the table match the results of the lookup tools on Nhật Nguyệt.
