Equinox: Equinox means equal share. The vernal equinox means that day and night are equally divided.

The vernal equinox is the midpoint of 90 days in spring. One of the 24 solar terms, around March 20 of the Gregorian calendar every year, when the sun is located at 0 (vernal equinox) of the yellow meridian. On the vernal equinox, the sun shines directly at the equator of the earth, and the seasons in the northern and southern hemispheres are opposite. The northern hemisphere is the vernal equinox and the southern hemisphere is the autumnal equinox.

On the day of the vernal equinox, the sun is almost direct to the equator, and day and night are almost equal in most parts of the world. But near the North Pole (90 north latitude) and South Pole (90 south latitude), we can observe the special phenomenon that "the sun circles on the horizon all day". After the vernal equinox, the position of direct sunlight continued to move northward, and the phenomenon of long days and short nights began to appear in the northern hemisphere.

The vernal equinox is an important solar term, which not only has astronomical significance: the northern and southern hemispheres are equally divided day and night, but also has obvious characteristics in climate. At the spring equinox, China has entered a beautiful spring scenery except the Qinghai-Tibet Plateau, Northeast China, Northwest China and North China. On the vast land, willows are green, yingfei grass is long, wheat is jointing and rape is fragrant.

During the vernal equinox, the daily average temperature in all parts of China rose steadily to above 0℃ except the alpine mountain area and the area north of 45 north latitude. At this time, the cold has passed and the temperature has risen rapidly, especially in North China and Huanghuai Plain. The daily average temperature rises to above 10℃ almost at the same time as the rainy areas along the Yangtze River and the south of the Yangtze River, entering a bright spring.

On the vast land, on the bank of willow green, the grass grows in Fei Ying, the wheat is jointing, the rape blossoms are fragrant, the peach blossoms are red, and Li Bai is coming in spring. South China is a scene of late spring. According to the climate law, the precipitation in Jiangnan increased rapidly at this time, and it entered the period of "peach blossom flooding" in spring; In the northeast, north and northwest regions where "spring rain is as expensive as oil", precipitation is still very little, so resisting the threat of spring drought is the main problem of agricultural production.