A solar term transition refers to the precise moment when the Sun reaches a specific position along the ecliptic longitude, marking the beginning of one of the 24 solar terms in the traditional Chinese calendar. These solar terms divide the solar year into 24 equal parts, each corresponding to approximately 15° of the Sun’s movement along its apparent path around the Earth (the ecliptic).
Astronomical Basis:
The 24 solar terms are determined by the Sun’s position relative to the Earth, measured in degrees of celestial longitude. For example:
- Spring Equinox (Chūnfēn) occurs when the Sun is at 0°.
- Summer Solstice (Xiàzhì) occurs when the Sun is at 90°. Significance:
It marks the exact beginning of a new solar term.
In traditional Chinese culture, it was used to guide agricultural activities, such as planting and harvesting.
Usage
Solar term transitions are often calculated down to the precise minute or second in modern times. This level of accuracy is essential for maintaining alignment between the traditional lunisolar calendar and the solar year.
Example of a Solar Term Transition:
Let’s consider the solar term Lìchūn (立春), or the beginning of spring:
- The solar term transition happens when the Sun’s ecliptic longitude reaches 315°.
- This is calculated using astronomical methods, and the exact time varies slightly each year due to orbital mechanics.
Solar term transitions are a blend of astronomy and cultural tradition, deeply embedded in East Asian calendars and practices.