Sanfu Tian: The sun is small, but the sky is hot

As the saying goes, "Winter Training Three Nine, Summer Training Sanfu", the hottest "three volts" of the year is coming. What is Sanfu Tian? Why is it hot and unbearable during this period?

San Futian in Chinese calendar

In ancient my country, the "dry branch" was used, and 10 Tiangan (A, B, C, D, Wu, Ji, Geng, Xin, Ren, Gui) and 12 earthly branches (Zi, Ugly, Yin, Chen, Chen, 巳 巳 巳The 60 groups of different names of the 60 groups of different names made by noon, Wei, Shen,, 戌, and Hai) are used to remember the year, month, day, and time, and use it.

According to the traditional calendar, from the summer solstice, the third Geng Day is the beginning, the fourth Geng Day is the middle, and the first Geng Day after the beginning of the autumn is the end of the "three volts".

Sanfutian takes Geng Day as the beginning and end, and the generally last time is 30 or 40 days. The astronomical calendar shows that it was in early July 16, 2022, mid -July 26th, at the end of August 15, and lived on August 24th.

Does this mean that it must be hot for 40 days this year? What is the reason for the "San Futian"?

We may think of the scene in the famous article "Two Children" in "Liezi Tangwen":

Confucius traveled to the east, and saw the two children's debate days, and asked him.

Yizer said: "I go to the time when I come from the beginning of the day, and the day is far away."

Yizer said: "I go far from the beginning of the day, and the mid -day is near."

Yizer said: "The beginning of the day is like a car cover, and the middle of the day is like a Panpad.

One child said: "The Cang Cang is cool and cool at the beginning of the day, and it is like the soup in the middle of the day.

Confucius cannot be decided.

The two children laughed and said: "Do you know how to know?"

Is the hotness of Sanfu Tian, ​​really because the sun becomes larger or the earth is closer to the sun?

Tian Tai Hot has nothing to do with the size of the sun

In fact, the sun is a middle -aged star at the stage of the main sequence star. At this stage, the sun burning is stable and the volume changes are very small. Therefore, the change of the sun's own size in one day and even a year is insignificant.

The radiation intensity of the sun is also very stable. Even if there is often a sunspots such as sunspots such as sunspots, the changes in the year are only about 1%. Obviously, this is not the reason for the hot sky.

Is it closer to the sun during this period?

Twenty -four solar terms and earth orbital relationships. Source: The author draws

Not. In fact, during this time, the daily distance is farther than usual. The orbit of the earth running around the sun is not a round, but an ellipse of 0.0167.

The average daily and land distance is about 149.6 billion kilometers, and the daily and ground distance at the distance is 152.1 billion kilometers. At the time of the recent day, the daily and ground distance is 147.1 billion kilometers. The average distance is 1/30.

The long -distance point of the earth is in early July, and the near January is in early January. Therefore, during the period of Sanfu Tian, ​​the daily distance is far away from the average distance.

"The far away is small and near". This year, the time when the earth runs to the distance is 15:11 on July 4th, Beijing time. The relatively decreased solar radiation caused by the change of distance is approximately 6%, which is obviously not the cause of Sanfu.

In addition, the orbit of the ellipse also leads to the unevenness of the solar anniversary. After the earth is the slowest at a distance, it is the fastest after the near future, and the length of the interval between seasons is not equal.

For example, in the autumn equinox of 2021 to the spring equinox of 2022, it passed 178 days and 20 hours and 12 minutes, and the spring equinox of 2022 to 2022 passed 186 days and 9 hours and 31 minutes. In addition, the summer of the northern hemisphere is longer than winter.

The reason for "Sanfu Tian" was found

To answer the hot question of "Sanfu Sky", you need to understand the reasons for the changes in the four seasons.

The root of all this lies in the track characteristics of the earth. While the earth revolves around the sun, it is also revolving around the ground axis. The orbital surface of the earth revolver is called zodiac surface. The plane planes that are vertical with the ground axis are called equator planes. These two planes currently have a 23 ° 26 ′ cross angle, called yellow red clay angle.

It is the existence of this corner that allows the sun to move between 23 ° 26 ′ from the north and south latitude of the earth with the different positions of the earth in the different positions of the orbit.

When the sun was shot to 23 ° 26 ′ north latitude, it was called the summer solstice; it was the winter solstice when the south latitude was 23 ° 26 ′ (south round), and it was spring equinox/autumnal equator when it was illuminated.

Source of the sun during the summer solstice: the author draws

In the summer solstice, the sun in the northern hemisphere is high during the day and long sunshine. Take the Nanjing area as an example. At the summer solstice, the height of the sun at noon is about 82 degrees, and the length of the day is 14 hours and 15 minutes.

Compared with the winter solstice, the height of the sun at noon is about 35 degrees, and the length of the day is only 10 hours and 04 minutes. Therefore, the most solar radiation energy obtained by the surface unit in the summer solstice is about three times the winter solstice day, and the higher the latitude, the greater the ratio. How much to receive solar radiation is the main reason for the changes in the four seasons.

At the same time of the day, the four seasons of the sun location.

However, the hottest time in the northern hemisphere is not in the summer solstice, but a period of time after the summer solstice. Because the changes in temperature are also affected by the balance of the surface heat revenue and expenditure.

In general after the summer solstice, the heat absorbed on the ground is still greater than the heat it released, and the ground temperature continues to rise. This time corresponds to the "three volts" of the traditional calendar, so it created the "three volts" in the northern hemisphere.Hot.After that, the heat released on the ground was greater than the heat absorbed by the ground, and the temperature gradually decreased.

Of course, for local areas, atmospheric circulation, sea and land differences, terrain factors, weather conditions, etc. are all factors that affect temperature, and the hottest time is not corresponding to the date of the 'Sanfu'.

Having said that, how do you come to the heat?

Produced: Popular Science China

Production: Wang Kechao (Zijin Mountain Observatory of the Chinese Academy of Sciences)

Producer: China Science Popularization Expo

Edit: Wang Tingting

The China Science Popularization Expo is the Science Popular Science Platform of the Chinese Academy of Sciences. It is sponsored by the Computer Network Information Center of the Chinese Academy of Sciences. Relying on the high -end scientific resources of the Chinese Academy of Sciences, it is committed to spreading cutting -edge scientific knowledge and providing fun science and education services.

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