Our earth revolves around the sun and rotates around its own axis, but the axis is not perpendicular to the orbital plane, but has an inclination of 23 degrees and 27 minutes. It is precisely because of this tilt that the direct point of the sun on the earth's surface moves between the Tropic of Cancer and the Tropic of Capricorn, thus forming four seasons: spring, summer, autumn and winter.

When the sun shines directly on the Tropic of Cancer, the northern hemisphere gets more solar heat, and the day is longer than the night, so the highest temperature in the northern hemisphere is summer. At this time, the sun shines obliquely in the southern hemisphere, getting less solar heat, and the night is longer than the day. Therefore, the southern hemisphere is in the coldest season of the year-winter.

When the earth revolves around the sun for half a week, the direct point of the sun moves from the tropic of cancer to the tropic of Capricorn, and the solar heat gained by the northern hemisphere gradually decreases, from summer to autumn, and then to winter; On the other hand, the southern hemisphere goes from winter to spring, and then to summer.

Climatic characteristics of four seasons and physiological characteristics of human body

When it comes to how people live a long life, Huangdi Neijing clearly points out that "the health of the wise-they must follow the four seasons and go against the cold and heat."

We must abide by the law of the rise and fall of Yin and Yang in spring, summer, autumn and winter, and adapt to the climate change in a year of cold and summer, so that the human body can stay healthy and prolong life, which is an important idea of TCM health preservation. As an important part of diet health, different dietary principles and requirements should be determined according to the climatic characteristics of different seasons and the physiological and pathological characteristics of human body.