What are vernal equinox and autumnal equinox?

Vernal equinox: one of the 24 solar terms, which begins on March 20th or 2 1 day of Gregorian calendar, and the sun reaches 0 degrees of the yellow meridian (vernal equinox). The length of day and night is average, which is exactly half of 90 days in spring, so it is called "vernal equinox". On the vernal equinox, the sun shines directly at the equator, and day and night are almost equal. Then the direct sunlight position gradually moved northward, and the days were long and the nights were short. During the spring equinox, China has entered a bright spring except the Qinghai-Tibet Plateau, Northeast China, Northwest China and North China.

Autumnal equinox: one of the 24 solar terms, usually on September 22nd or 23rd every year. On this day, the sun shines directly on the equator of the earth, and it is divided into 24 hours day and night. There is no extreme day and night phenomenon in the world. After the autumnal equinox, the polar night range near the North Pole and the polar sun range near the South Pole gradually expanded.