Green Power surveys 24 Solar Terms and Butterflies:
From Solar Terms to Butterfly Watching
Date of Release: 31 July, 2004
The origin of the solar terms calendar
The 24 solar terms calendar was devised by the ancient Chinese to match natural phenomena, and combined observations from astronomy, meteorology, biometeorology (which covers relationships between weather and living things) and agricultural experiences. Farmers in old times well understood the solar terms and aspects of biometeorology that were closely related to agriculture.
Solar terms are spaced along the ecliptic, according to the earth’s revolution around the sun; hence, they synchronize with the solar calendar. The earth takes a year to make a 360-degree circuit of the sun. Dividing 360˚ into 24 equal parts (each part being 15˚), means a solar term is arrived at when the earth travels 15˚.
There is interval of about 15 days between solar terms, the 24 solar terms together forming the 365 days of the year. The 24 solar terms are: Spring Commences, Spring Showers, Insects Waken, Vernal Equinox, Bright & Clear, Corn Rain, Summer Commences, Corn Forms, Corn on Ear, Summer Solstice, Moderate Heat, Great Heat, Autumn Commences, End of Heat, White Dew, Autumn Equinox, Cold Dew, Frost, Winter Commences, Light Snow, Heavy Snow, Winter Solstice, Moderate Cold and Severe Cold. In this order, the odd terms are called “Jie Qi”, or “Jie” in brief; the even ones are called “Zhong Qi”, or “Qi” in brief. Together they are called “Jie Qi”.
Butterfly populations and species increase after Spring Commences
The solar terms are important references for agriculture and daily events. In fact, some ecological studies also rely on the solar terms. To establish a more solid study on butterfly ecology, on 4 February –Spring Commences –2005, Green Power began recording local butterfly populations and keeping track of their life cycles and behaviour. The research was conducted at two butterfly hotspots: Shing Mun and Tai Po Kau. The changes of butterfly species and populations in both places were obvious and similar, indicating the effect of the solar terms. Take Shing Mun as an example: we recorded four species, and a total of four individuals, on Spring Commences. Yet in the following days the species number and populations gradually increased. By Moderate Heat (5 July*), there were 45 butterfly species at the same site. By Great Heat (20 July*), 50 species were recorded, accounting for 40% of the known species at the site.
Analyzing the data collected in the half-year study, we found that in the period from Spring Commences to Great Heat, temperatures increased as the solar terms changed. Butterfly populations and species number also rose. This reflected the close link between the butterflies and temperature. The pattern showed up in both Shing Mun and Tai Po Kau. The higher the temperature on the solar terms, the more species and individual butterflies we could observe. The reason might be that butterflies are cold-blooded animals that depend on high temperatures in the environment to enhance their activity.
* Due to weather conditions, the actual days of surveys were slightly different - by a maximum of three days - from the solar terms.
Different adult butterflies first appear during different solar terms
Furthermore, we discovered that different adult butterflies first appeared on days marking the start of different solar terms. Some appeared earlier and some later. Metalmarks, Browns, and Whites or Yellows are more active. They appeared in Shing Mun and Tai Po Kau on the eve of Spring Commences. Duffers or Fauns, by comparison, appeared later. We spotted Large Faun at both sites as late as Bright & Clear on 5 April.
Metalmarks are more active. The adult Punchinello appeared as early as Spring Commences. (Photo taken by Ho Kin Yip/ Provided by Green Power)
Large Faun appeared later. The adults emerged after Bright & Clear. (Provided by Green Power)
Fig. 1 Butterfly species recorded in Shing Mun on days during particular solar terms. (Average temperature on survey days is on top of the bars. Survey on Great Heat was cancelled due to poor weather conditions.) (Jie Qi and butterfly families in Chinese)
Fig. 2 Butterfly species recorded in Tai Po Kau on days during particular solar terms. (Average temperature on survey days is on top of the bars. Survey on Great Heat was cancelled due to poor weather conditions.) (Jie Qi and butterfly families in Chinese)
Solar terms can help with butterfly watching
In butterfly watching, we usually use the conventional Gregorian calendar that divides the year into the four seasons of Spring, Summer, Autumn and Winter and the twelve months. The 24 solar terms are more detailed than the Gregorian calendar. In addition, the names of the solar terms give hints on regarding the meteorology and/or biometeorology. For instance, Spring Showers and Frost indicate weather characteristics; Corn Forms and Corn on Ear have agricultural meaning; Insects Waken signifies biometeorology. These names suggest linkage between butterflies and season change, and can help butterfly watching novices to learn about the timing and behaviour of butterflies. Further, encouraging butterfly watching in accordance to solar terms may help nurture butterfly watching in Hong Kong. In the long term it could benefit local butterfly conservation work.
During the survey, we attempted to discover the relationship between solar terms and butterfly behaviour. We learned that:
1) On Spring Commences the lengths of day and night are equal, signalling the end of the winter days with days and longer nights. Most butterflies are active in the daytime. Therefore, more butterflies appear after Spring Commences;
2) Spring Showers is the second solar terms day, signifying the start of precipitation which will also gradually increase. Rainfall is good for the growth of vegetation that butterflies rely on, facilitating the reproduction of butterflies. The survey also found that during Spring Showers, there was an obvious rise in number of butterfly species at Shing Mun and Tai Po Kau;
3) Bright & Clear brings clear days with thriving vegetation. There were more individual butterflies at both sites, indicating these clear and warm days are ideal for butterfly watching;
4) Moderate Heat and Great Heat are the hottest solar term days. As butterflies are most active in hot weather, we observed the greatest number of adult butterflies on the two days.
The adult swallowtails first appeared on Spring Showers, and busily fed among bushes with flowers. (Photo taken by Lai Hoi Fai/ Provided by Green Power)
Bright & Clear signals the start of clear and warm days. Many Whites or Yellows seek their partners around this solar term. (Provided by Green Power)
Green Power will continue to collect data on the forthcoming solar terms. We hope the information can help people to establish timetables for butterfly watching by making use of the solar terms. Through this, we hope to arouse public interest in and awareness of butterfly watching.
References
Gregorian-Lunar Calendar Conversion Table (including solar terms):
Hong Kong Observatory website: www.weather.gov.hk/gts/time/conversion.htm