The beauty of festivals丨Why is Qingming Festival always around April 5th?

Today, April 5th, is the Qingming solar term in the lunar calendar.

Long ago, the ancient Chinese summarized the 24 solar terms to guide agricultural production. There are also many agricultural proverbs related to the solar terms, such as: Plant melons and beans around the Qingming Festival. Do not force planting after the Grain in Ear Festival, etc. This fully demonstrates the superb agricultural wisdom of the ancient Chinese.

Have you noticed that the Gregorian calendar dates of each solar term are almost fixed every year: for example, Rain Water is always around February 19, Qingming is always around April 5, Summer Solstice is always around June 21, Winter Solstice is always around December 21... Not only that, there are exactly two solar terms in each month.

Weren't the 24 solar terms invented by our ancient people? Why do they fit in so well with the Gregorian calendar introduced from the West? This is where we need to explain the relationship between the Gregorian calendar and the 24 solar terms.

1

The meaning of the 24 solar terms

The Gregorian calendar, also known as the solar calendar, is a calendar based on the law of the earth's movement around the sun. People observed the changes in the altitude of the sun and the cycle of the four seasons, and thus came up with the concept of a year. The time it takes for the earth to revolve around the sun is one year. The Gregorian calendar (Gregorian calendar) currently used in the world is a typical solar calendar. With 365 days in a common year, a leap year every four years, no leap year every 100 years, and a leap year every four hundred years, the average length of each solar year is very close to the period of the earth's revolution around the sun.

China entered an agricultural society a long time ago, and the need for a solar calendar became urgent. This is because the sun is the main factor affecting climate change on Earth, and setting dates according to the solar calendar can more conveniently guide agricultural production. Against this background, the ancient Chinese created the 24 solar terms, which is known as the "fifth invention of China."

The plane in which the Earth revolves around the sun is called the ecliptic. While the Earth revolves on the ecliptic, it is also rotating around its axis. Moreover, the Earth's rotation axis is not perpendicular to the ecliptic plane, but makes an angle of 66.5 degrees with the ecliptic plane.

Earth's rotation axis and the ecliptic plane

During the Earth's revolution, the position of sunlight directly hitting the Earth changes. Sometimes the sunlight directly hits the Northern Hemisphere, the temperature in the Northern Hemisphere is higher, and the daytime is longer than the nighttime, which is summer; sometimes the sunlight directly hits the Southern Hemisphere, the temperature in the Northern Hemisphere is lower, and the nighttime is longer than the daytime, which is winter.

Two solstices

Around June 22 every year, the Earth moves to a special position: the point where the sun shines directly on the Earth is the northernmost point on the ground - the Tropic of Cancer. This day is the longest in the Northern Hemisphere, and the ancient Chinese called it the Summer Solstice.

Half a year later, around December 22, the Earth will move to another special position: the southernmost point on the ground where the sun shines directly - the Tropic of Capricorn. This day is the shortest day in the Northern Hemisphere, and the ancient Chinese called it the Winter Solstice.

There are two special days between the winter solstice and the summer solstice when sunlight shines directly on the equator, and the day and night in the northern hemisphere are equal. These are called the vernal equinox (around March 21) and the autumnal equinox (around September 23). These are the equinoxes and solstices.

In order to better solve the problem of agricultural production, people added other solar terms between the equinoxes and solstices, thus forming the 24 solar terms:

There is a song that can easily remember these 24 solar terms:

Spring rains startle the spring and clear the valley, summer is full of grains and summer heat is connected.

In autumn, there are dews and frosts, while in winter, there are snows and slight or severe cold weather.

The names of the 24 solar terms are also very artistic. For example, "Qingzhe" means thunder, which wakes up the insects underground; "Guyu" means abundant rain and growth of grains; "Mangzhong" means the planting of awned crops begins; "Bai Lu" means dew on the grass and trees and the weather officially turns cooler, and so on.

2

Dates of the 24 solar terms

Of course, in ancient China, people did not know that the Earth revolved around the Sun, nor did they know the positional relationship between the Earth and the Sun. However, the clever ancients found a way to determine the winter solstice through an "immediate" method.

In winter, the sun shines directly on the southern hemisphere, and the sun is low at noon, so the shadow cast by the pole is longer. People set the day with the longest shadow as the winter solstice. In summer, the sun shines directly on the northern hemisphere, and the sun is high at noon, so the shadow cast by the pole is shorter. People set the day with the shortest shadow as the summer solstice. According to the different lengths of shadows, people can set the 24 solar terms.

However, the winter solstice refers to the time when the sun shines directly on the Tropic of Capricorn, which is usually not at noon, so the length of the shadow at the winter solstice is not exactly the same every year. Zu Chongzhi, a mathematician and astronomer in the Southern and Northern Dynasties of China, found a way to measure the time of the winter solstice after long-term observation and calculation. Based on the time between two winter solstices, Zu Chongzhi calculated that a year is 365.2428 days, which is less than one minute different from the 365.2422 days accurately measured by modern people.

Statue of Zu Chongzhi

The determination of the dates of the solar terms has also changed in history. In the "Da Ming Calendar" compiled by Zu Chongzhi, the date of the winter solstice was first determined, and then the other solar terms were determined according to the method of one solar term every 15 or 16 days. This method is called Pingjieqi. In the Yuan Dynasty, Guo Shoujing and others presided over the compilation of the more accurate "Shushili" which was used until the end of the Ming Dynasty.

Statue of Guo Shoujing

However, because this calendar did not take into account the precession of the perihelion, the error gradually increased, and the date of the solar eclipse was wrongly predicted. During the Chongzhen period, Xu Guangqi and others established the Calendar Bureau to compile a new calendar.

Matteo Ricci and Xu Guangqi

Influenced by Western missionaries such as Matteo Ricci, Xu Guangqi hired European scientists to translate the works of Copernicus, Tycho Brahe, Kepler and others, and eventually completed the 137-volume work "Chongzhen Calendar", which introduced Western astronomical theories and mathematical calculation methods, and calculated astronomical date tables based on Western methods. Unfortunately, the Ming Dynasty fell before this calendar could be used.

Later, the Qing emperor decided to use this calendar and renamed it "Shixian Calendar". In this calendar, the solar terms were changed to every 15 degrees that the earth rotates on its orbit. This method of defining solar terms is called "fixed solar terms". In winter, the earth rotates faster, so the time interval between two solar terms is short; in summer, the earth rotates slower, so the time interval between two solar terms is long.

In short, the 24 solar terms are determined based on the earth's revolution, and the Gregorian calendar is also determined based on the earth's revolution. Therefore, it is not difficult to understand why the Gregorian calendar date of each solar term is basically determined every year, with a difference of no more than one or two days. The 24 solar terms are a great creation of the ancient Chinese working people. They provide great convenience for guiding agricultural production and also represent the highest achievement of ancient Chinese astronomy.