- What is the Winter Solstice?
The Winter Solstice is one of the important lunar solar terms in China. And it is also an important Traditional Festivals for Chinese people. There are many other names for Winter Solstice, which can also be called as Winter Festival, Long-arriving Festival (“长至节”: because this day is the shortest day in the whole year. After this day, the daytime will become longer and longer in the coming year.) sub-year(亚岁) and so on. More than 2,500 years ago, in the Spring and Autumn Period, the ancestry of Chinese have set the Winter Solstice out by measuring the sun through the tool of TuGui (土圭: a kind of time-measuring instruments in ancient China), so it is the first solar term in the twenty-four solar terms of China. The Winter Solstice comes around annual solar 21st to 23rd December. In this day, the daytime is the shortest and the night is the longest during the whole day for the northern Hemisphere. Also in this day, there are traditional habits for northern Chinese to eat dumplings and for southern Chinese to eat sweet soup balls. There is a saying for Chinese, that is it’s time to eat dumplings, when the Winter Solstice comes.
- How to define the Winter Solstice?
In the eyes of the Lunar solar terms, the Winter Solstice is one of them. And it is situated between the great Snow (大雪) and the Slight Cold (小寒). It is usually from 21st to 23rd December.
About its name, the ancients of Chinese describe it like this:
The extremity of the lunar (阴极之至), the beginning of the solar(阳气之始), the sun starts moving to the south (日南至), the shortest daytime (日短之至), the longest shadow of the moon (日影长之至), so it is be called as the Winter Solstice (故曰“冬至”).
- The measuring method for it.
Usually, the Winter Solstice comes in the day of 22nd December or 23rd December every year, but there is not a stable day for it, so it is the similar to the Tomb-sweeping Day (清明节: anther one lunar solar term of China), because both of them are named as “flexible festivals (活节)”.
So there is a computing method to confirm which day it is. It should take use of the time of UTC time to calculate it out: (Y*D+C) – L.
Expression for the formula:
Y stands for the two numbers of the current year, D is equal to 0.2422, L stands for the number of leap year, the 21 century C is equal to 21.94, 20 century is equal to 22.60.
For example, the date of the Winter Solstice of the year 2088 should be calculated like this: [88×0.2422+21.94]-[88/4]=43-22=21, so the Winter Solstice will come in 21st December.
But except the year of 1918 and 2021, it needs to subtract 1 day for them.
- The geographical meaning of the Winter Solstice.
The extreme partitions for day and night.
A measurement by modern astronomic science found that in the day of the Winter solstice, the sun directly irradiate the Tropic of Capricorn, the sunshine is the most gradient for the Northern Hemisphere, and the daytime is the shortest and the night is the longest at the same time. After this day, the sun starts to move to the Tropic of Cancer.
In the traditional Chinese Yin and Yang and the Five Elements theory system, The winter Solstice is the most important solar term for conversion between Yin and Yang. In the Book of Changes, there is a short description for the Winter Solstice: all of the ancient kings retreated on the day of the Winter Solstice, and suspend business and trade activities (先王以至日闭关，商旅不行).
Because that the high of the sun is the lowest, the shining time is the shortest and the heat of the sun, which is absorbed by earth is more that the one, that is lost by the earth, so the custom of the nine periods(“数九” in Chinese) follows the winter solstice. Every nine days are known as a “nine”. When the third nine comes, the heat, which saved by earth is the least and the weather is the coldest during the while year, so we thought it is the coldest time during these nice days through a year.