A solar term is any of 24 points in traditional East Asian lunisolar calendars that matches a particular astronomical event or signifies some natural phenomenon. The points are spaced 15° apart along the ecliptic and are used by lunisolar calendars to stay synchronized with the seasons....These solar terms have been given names of what is expected to occur during that period (e.g.,"awakening of insects", usually beginning around March 5 or "major heat", usually beginning around July 23). As such, these names serve a social use as well - mainly to provide agricultural cues. Provided that the climate remains stable, these cues - barring annual variations - provide a useful "farmer's almanac." Indeed, given the assumptions of the use over 2000 years in China, the 24 solar terms likely became divorced from the actual position of the sun and became more of a description of the expected climatic conditions at certain periods throughout the year.
Because the Sun's speed along the ecliptic varies depending on the Earth-Sun distance, the number of days that it takes the Sun to travel between each pair of solar terms varies slightly throughout the year. Each solar term is divided into three pentads (候 hou). Each pentad consists of five days (rarely six), so there are 72 pentads in a year.
However, the climate has not remained stable, and in an interesting combination of combining a cultural calendar with climatic measurements, a new paper has been published that shows how - in China - the 24 solar terms have changed in character. First, though, the researchers had to convert solar periods into correlative temperature ranges (in order to make the climate change comparisons). That done, this is what they found:
According to these results, the timings of the climatic Solar Terms during the warming phase (around spring) of the seasonal cycle have significantly advanced (by 6-15 days) from the 1960s to the present.
Across China, timings during the cooling phase (around autumn) have delayed by 5-6 days on average. This is mainly because of a warming shift of the entire seasonal temperature cycle, as illustrated in the figure. Four particular phenology-related climatic Solar Terms, namely the Waking of Insects, Pure Brightness, Grain Full, and Grain in Ear, have advanced almost everywhere in the country (as much as 20 days in North China). This has important implications for agricultural planning. The numbers of extremely cold (Great Cold) days decreased by 56.8% over the last 10 years as compared with the 1960s, whereas those of extremely hot (Great Heat) days increased by 81.4%.
|Graph from Qian et al (2012). Caption from the paper reads: "Climatological mean ALCs (seasonal cycles) of the China mean temperature for the earliest 10 years (blue line) and for the latest 10 years (red line). Dashed lines indicate the temperature thresholds for the 24 Solar Terms"|
One reason why I find this so interesting is that this is the sort of thing that represents an important step in disseminating information about climate change to the public, using cultural vehicles that they know, understand, and are familiar with. Too often, a lot of the cutting-edge science and its critical findings are written in a way that requires a mess of cultural decoding. Often, too, it takes the implicit assumption of the Western world: marking (in this case) the calendar into the 12 months (which - themselves - were rather arbitrarily set way back when) that are commonly used in the West, but may rarely be used so intuitively outside of cities in the rest of the world. Although this article refers to China, the 24 solar terms are used throughout East Asia, and they still mark major celebration points in the Japanese calendar. In other words, this paper's findings speaks in the calendar language that is understood by upwards of 1.6 billion people (i.e., the combined population of China, Taiwan, North and South Korea, Japan, and Vietnam).
The paper, "Climatic changes in the Twenty-four Solar Terms during 1960–2008", can be found here: http://www.springerlink.com/content/g0264r7102x18844/