The earth can be divided into two hemispheres, the northern hemisphere and the southern hemisphere. The seasons are completely different. Summer in the northern hemisphere is winter in the southern hemisphere, and winter in the northern hemisphere is summer in the southern hemisphere. The reversal of seasons in the northern and southern hemispheres also leads to historical and cultural differences. For example, in the northern hemisphere, different cultures, arts and important traditional festivals in this area are in winter, because there is nothing to do in winter in agricultural society. As we know, the earth often causes changes in the four seasons, which comes from the rotation of the earth, and the rotation track of the earth closely revolving around the sun is not a standard circle, but an ellipse, so the earth also has perihelion and apohelion.
Interestingly, for everyone in the northern hemisphere, in the process of staying away from the sun, it is neither summer nor winter that is close to the sun. Summer in the northern hemisphere is just near the perihelion of the earth, while winter is near the perihelion of the earth.
Why is it cold when it is close to the sun and hot when it is far away from the sun? This reminds people of another classic problem in ancient China: a child stopped Confucius' carriage and asked him for advice. Confucius promised to learn from the child if he couldn't answer. So the child asked Confucius, "Why is the sun cold in the morning and hot in the afternoon?" ? Confucius replied, "The sun is far away in the morning and near in the afternoon." The child asked, "The sun is far away in the morning. Why does it look big? The sun is near in the afternoon, why does it look small? " Confucius couldn't answer, so he worshipped the child as a teacher. For ancient people, this question is really a difficult coefficient, but for children in 500 AD, it is really not easy to ask such a difficult question.
In ancient times, this question about the morning and evening of the sun was very similar to what people often said about winter in the northern hemisphere. For the earth, the distance from the sun is not the key factor of hot and cold, but the inclination angle of the earth.
Due to the gravitational force of the moon and other unknown factors, there is an included angle between the earth's rotation axis and the ecliptic plane, which is about 23.5 degrees. The summer heat is coming, and the sun shines on the Tropic of Cancer. For people born in the northern hemisphere, the sun will draw the most diagonal lines in the sky on this day, which is the most in broad daylight in a year. After this day, the aspect ratio of the sun in the sky will drop sharply, the angle of view of the sun shining on the ground will gradually skew, and the temperature will become colder and colder. Why does the angle of view of the direct sun become more and more skewed, which will lead to the temperature drop? There are two reasons. One is that the solar radiation source has a certain kinetic energy to the earth.
Whether it is scattering or irradiation, the kinetic energy presented by the sun is certain, but the total area of the earth that needs direct irradiation is much larger than that when it is irradiated. As more kinetic energy is dispersed to a wider area, the total area of each enterprise will of course need to receive less heat, and the temperature will of course be lower than before.
The second is due to the isolation of the earth's atmosphere. The total area of air through which sunlight scatters is larger than that through which sunlight shines. According to the calculation, the thickness of the earth's air penetrated by sunlight in the Tropic of Capricorn is more than 100 km during the winter solstice and the hot summer noon, and a large amount of sunlight is transmitted in the air. When it shines on the ground, the kinetic energy of course decreases. In fact, Confucius encountered the same thing. In the morning, the sun's direct deflection angle is high, so it looks big, but the temperature is low. In the afternoon, the direct angle of view of the sun is small, so it looks small and the temperature is high. So why don't perihelion and apohelion easily do harm to the temperature of the earth?
Of course, the distance from the sun will directly affect the temperature of the earth, but it has little to do with it. Because the perihelion and apohelion of the Earth's orbit are not much different, and because the perihelion and apohelion of the Earth are only more than 4.8 million kilometers apart, their solar irradiance difference is less than 6%. Such a small difference can't save the harm caused by the earth's 23.5-degree inclination.
However, neither the appearance of the earth's guide rail, the inclination of the earth, nor the orientation of the earth's rotation axis are fixed and will not change. It takes about 654.38+ million years for the earth's orbit to change from the most ellipse to the most circle, and the cycle time for the earth's inclination to change from the maximum to the minimum is about 50,000 years, while the cycle time for the inclination direction is 26,000 years. Therefore, the winter in the northern hemisphere is not always near the perihelion, but will change with the law of the earth's fitness movement. One day, the perihelion in the northern hemisphere will become summer, the perihelion in the southern hemisphere will become winter, and so on.