nDiscussing winter without first clarifying the time systems can lead to confusion. The existence of the four seasons is due to the Earth’s axial tilt of approximately 23.5 degrees and its orbit around the Sun. When the Northern Hemisphere tilts away from the Sun, the shortest day occurs at the winter solstice, typically around December 21. This marks the start of the astronomical winter. Similarly, the vernal equinox, summer solstice, and autumnal equinox are defined by the Sun’s position on the ecliptic, representing physical facts.n
nFor convenience in statistics, meteorology divides seasons by whole months. In the Northern Hemisphere, winter is defined as DJF, or December (Dec), January (Jan), and February (Feb); spring as MAM; summer as JJA; and autumn as SON. These abbreviations are widely used in climate reports and academic research. When average winter temperatures are announced, they usually refer to data from the DJF period, not starting from the winter solstice. The aim is comparability.n
nThe lunar calendar is a lunisolar system, with many traditional Chinese festivals determined by lunar months and phases, such as the Spring Festival, Lantern Festival, Dragon Boat Festival, and Mid-Autumn Festival. Based on the synodic month, the lunar calendar uses intercalation to stay in sync with the solar year, preventing complete detachment from the seasons. However, the dates of these festivals in the Gregorian calendar can vary by several weeks each year. They do not consistently correspond to a specific solar altitude or daylight condition. Comparing climate change based on the warmth of a particular year’s Spring Festival is of limited significance, as its solar position varies.n
nHowever, the solar terms in the lunar calendar are calculated based on the Sun’s position on the ecliptic, with each term covering 15 degrees. As a result, solar terms like the Beginning of Spring and Beginning of Winter have relatively stable dates in the Gregorian calendar, usually differing by only one day. When discussing changes in daylight and seasonal transitions, solar terms better reflect the actual rhythm of the Sun’s movement, providing more meaningful reference.n
nThere are similar examples in the West. Although most festivals have fixed dates, Easter is set on the first Sunday after the first full moon following the vernal equinox, with dates ranging from late March to late April. While linked to the equinox, it does not occur under the same daylight conditions each year. Long-term temperature comparisons based on Easter can also lead to misconceptions.n
nThe issue is not with the festivals themselves, but with conflating different calendar systems. The solar calendar reflects the geometric relationship between the Earth and the Sun, while the lunar system reflects the Earth’s cycle with the Moon. Although solar terms are incorporated into the lunar calendar system, they are fundamentally calculated based on the Sun’s movement; lunar calendar festivals, however, often vary with the lunar phases. Their purposes differ. Using floating festivals as a climate benchmark makes it difficult to maintain rigor.n
nThe boundaries of the seasons are rooted in celestial mechanics. When discussing whether winters are becoming warmer, we should refer to the Sun’s position and DJF data, not festival memories. Distinguishing between institutional affiliations and astronomical principles is essential for understanding the seasons. Otherwise, we mistake cultural time for natural time.n
