The beauty of festivals and vernal equinox

According to the astronomical calendar of China, the "vernal equinox" solar term was ushered in at 23: 33 on March 20th. As the "C" solar term in spring, is the vernal equinox a period of time or an instant? How to determine? Astronomical experts from Chinese Academy of Sciences reveal the secret for you.

Zhang Xi, director of the Science Department of Purple Mountain Observatory, China Academy of Sciences, said that the public's understanding of solar terms usually refers to one day or more than ten consecutive days, but in astronomy, the definition of solar terms is an exact moment, which is determined according to the relative position of the sun and the earth.

The earth goes around the sun, and visually, people on the earth also feel that the sun goes around the earth. In astronomy, the track of the sun's annual apparent motion is called the ecliptic, and the angle between the ecliptic plane and the equatorial plane is about 23.5 degrees. The sun runs 360 degrees along the ecliptic, and every time it "passes" 15 degrees, it ushers in a new solar term. The vernal equinox is special because it is the moment when the sun "walks" to 0 degrees of the ecliptic, which is one of the intersections between the ecliptic plane and the equatorial plane.

Because at the vernal equinox, the sun will shine directly at the equator, at this time, the light and heat obtained by the northern and southern hemispheres are the same. On the vernal equinox, people all over the world will see the sun rise in the east and set in the west. The day and night on this day are almost equally divided.

Because the sun does not move at a uniform speed on the ecliptic, the arrival time of the vernal equinox is different every year, and the interval between solar terms is also different. "For example, this year's vernal equinox was at 23: 33 on March 20, and last year was March 20 17: 37." Zhang Xi said.

In meteorology, most parts of China officially entered spring around the vernal equinox. "Since the vernal equinox, except for the Qinghai-Tibet Plateau, Northeast China, Northwest China and northern North China, the average daily temperature in most areas has exceeded 10 degrees Celsius for five consecutive days. The grass turns green, the willows are developed, the swallows fly from the south to the north, and the spring in the meteorological sense falls on the vast land of China. " Jiang Xiaodong, associate professor of Nanjing University of Information Science & Technology Institute of Applied Meteorology, said.

Producer: Hu Qinghai

Coordinators: Liu Xinhui and Liu Zhaoquan

Poster production: Chen Jie

Produced jointly by Jiangsu Branch of Xinhua News Agency.

Ling Jie studio production