What are solar terms and why are there 24?
Solar terms (節氣) divide the year into 24 segments based on the Sun's apparent motion along the ecliptic. Each solar term corresponds to about 15 degrees of ecliptic longitude, so a year has 24 terms, each lasting roughly 15 days. This system is closely tied to the East Asian agricultural calendar, helping determine planting, harvesting, and festival times.
Origin and use in calendrics
Solar terms are built from observation of the solar cycle, not the lunar cycle. Thus they are a solar element within the lunisolar calendar. Terms such as Lichun, Qingming, Xiazhi, and Dongzhi mark turning points in weather. In Vietnam, many customs are linked to solar terms; for example, Qingming is a time for tomb sweeping and ancestor remembrance, reflecting the seasonal rhythm and agricultural life.
Distinguishing from easily confused concepts
Solar terms differ from "tiết" in "tiết trời" (weather) or "tiết mục" (performance item). They should be distinguished from:
- Can chi: a day and hour counting system using 10 heavenly stems and 12 earthly branches, with a 60-cycle, not a solar marker.
- Pure lunar calendar: a calendar based only on the Moon, without the concept of solar terms.
- Modern climatology: divides seasons by temperature and rainfall, not by 24 fixed markers.
Role in cultural life
Solar terms influence festival calendars, date selection, and some traditional divination systems. However, this is traditional knowledge for reference, not scientifically verified in the field of esotericism. The main value of solar terms lies in astronomical observation and agricultural experience.
Do all places use the same calculation?
There are minor differences between calendrical traditions regarding time zones and starting points. However, the general principle is based on the Sun's position, so solar terms still correspond approximately across cultures.