The sun turns in beginning of spring, it rains by the river, crows crow, the spring equinox is dry, the Qingming wheat is busy, and Grain Rain is farming; Long summer goose feathers live, birds come all over the place, awn seeds shovel, cotton is not taken on the summer solstice, the summer is not hot, and the summer is three days; In early autumn, I was busy doing some work, and in summer, I was working with a sickle. White dew smoke is put on the shelves, and the autumn equinox does not produce fields. The cold dew is not cold, and the first frost weather changes. Early winter, October, light snow freezes, heavy snow stubble, no boat from winter to Sunday, slight cold near the twelfth lunar month, and severe cold all year round.
Twenty-four solar terms refer to the special festivals in the trunk and branch calendars that express seasons, phenology, climate change and establish "December construction", which were used to guide farming in the Han Dynasty. Twenty-four solar terms are the product of ancient farming civilization, and are the knowledge system formed by ancient ancestors following the farming season and understanding the changing laws of seasons, climate and phenology in a year by observing the movement of celestial bodies.
succession of the seasons
The twenty-four solar terms in ancient times were based on "variable stars" (the direction of bucket handle), and now they are based on "the annual apparent motion of the sun" (the degree of the longitude of the sun). Although they were established in different ways, in fact, the cause of the struggle is that the earth revolves around the sun, so the time when they intersect is basically the same.
Because the earth revolves around the sun while rotating around the earth's axis, in this process, the earth's axis is not perpendicular to the orbital plane of revolution, but in an inclined state, which leads to the non-parallel equatorial plane and ecliptic plane, forming the ecliptic angle. Due to the existence of the ecliptic angle, the direct point of the sun moves back and forth between the north and south latitudes of the earth, which leads to the change of the height of the sun at noon and the length of day and night, thus resulting in seasonal changes. The direct point of the sun moves back and forth between the Tropic of Cancer and the Tropic of Cancer.
Activity law of direct solar point
At the vernal equinox, the direct point of the sun is 0 on the equator, and then moves north. From summer to the sun, the direct point of the sun is on the tropic of cancer (that is, "the sun arrives in the north and the days are long" as the ancients said), and then it moves south. At the autumnal equinox, the direct point of the sun is 0 on the equator, and then it continues to move south. On the winter solstice, the direct point of the sun is on the tropic of Capricorn (that is, the so-called solar solstice is short), and then it moves northward and repeats.
China is located in the northern hemisphere, and the Tropic of Cancer runs through China, which is a special latitude line of great significance to astronomy, geography, climate and biology. The seasonal laws reflected by the movements of the Big Dipper, the sun and other celestial bodies are closely related to this line, and the natural rhythm of the Tropic of Cancer (the Yellow Cross Point) and its surrounding areas has gradually changed in the sense of seasonal transformation in Japan.