What are the vernal equinox, autumnal equinox, summer solstice and winter solstice?

At the vernal equinox of March 2 1, the sun shines directly at the equator;

Summer solstice, that is, June 22, the sun shines directly on the Tropic of Cancer (23 degrees 26 minutes north latitude);

The autumnal equinox, that is, on September 23, the sun shines directly at the equator;

On the winter solstice (65438+February 22nd), the sun shines directly on the Tropic of Capricorn (23 degrees and 26 minutes south latitude).

The vernal equinox is one of the 24 solar terms, and the vernal equinox is the midpoint of 90 days in spring, which is generally around 2 1 every year. The direct point of the sun is on the equator, and the world is divided equally day and night. In the time period, it refers to the position where the sun is located between 0 and 15, that is, from March 20th to April 5th. But in fact, it usually refers to the day when the sun is really at 0 degrees of the yellow meridian: March 20 or March 2 1. On the vernal equinox, the sunrise and sunset time is 6 o'clock all over the world.

The solstice in summer is the longest day and the shortest night in the northern hemisphere. On the day of the summer solstice, except the south pole of the Antarctic circle and the polar night area, the sunrise direction in all parts of the earth starts from the northeast and sets in the northwest.

Take Beijing as an example. In summer solstice, dawn begins at 2:42 am and gets up at 4:45 am. At night 19:46, 2 1:49 it was completely dark. If we continue to go north, the night will be further shortened. In Nordic cities such as Stockholm, Helsinki and Oslo, the midnight sky is still bright, so you can read books under the night sky. When we reach the area north of 66.4 north latitude, we can see the midnight sun, which we think is a miracle. People there can enjoy sunbathing 24 hours a day.

Around the summer solstice, the earth is located near the perihelion, and its running speed is slightly slower, resulting in the sun spending 8 days more in the northern hemisphere than in the southern hemisphere in a year, so the summer in the northern hemisphere is slightly longer.

Every year around September 23rd, when the sun reaches longitude 180 degrees, it enters the "autumnal equinox" solar term. The autumnal equinox, like the vernal equinox, is the earliest solar term established by the ancients. According to "Spring and Autumn Stories Yin and Yang Incoming", "The autumnal equinox is accompanied by Yin and Yang, so it is cold and hot day and night."

The meaning of "autumnal equinox" has two meanings: First, in ancient China, four seasons were divided according to beginning of spring, Changxia, beginning of autumn and beginning of winter, and the autumnal equinox fell within 90 days of autumn, which was equally divided; 2. At this time, 24 hours a day are divided equally, each 12 hours. This day is the same as the "spring equinox" day. On the day of the "autumnal equinox", the sun is almost directly at the equator. After that, the position of direct sunlight moved south, and the days in the northern hemisphere became shorter and the nights became longer.

When the sun reaches the longitude of the Yellow River at 270 degrees, it is about 1 day within 30 days before and after the Gregorian calendar 65438+February 22nd or the lunar calendar1month15th (full moon). On this day, the sun shines almost directly on the tropic of Capricorn, with the shortest day in the northern hemisphere, the extreme night in the northern hemisphere and the extreme sun in the southern hemisphere. Later, the position of direct sunlight moved northward and the days became longer.

65438+February 265438+ 0, 22 or 23 every year. The direct point of the sun is on the tropic of Capricorn (that is, 23 26' south latitude), and the days are the longest and the nights are the shortest in all parts of the southern hemisphere. The higher the latitude in the southern hemisphere, the longer the day and the shorter the night, and the phenomenon of extreme day appears in the Antarctic circle. The northern hemisphere is the opposite.

The farther away from the tropic of Capricorn on the earth, the more inclined the sun's rays are, and the less solar heat is obtained. In contrast, the southern hemisphere gets more heat than the northern hemisphere at this time. Because the energy radiated by the sun to the ground is still less than that radiated by the ground to the air, the temperature continues to drop in a short time. Generally, the months of 12, 1 2 are winter in the northern hemisphere and summer in the southern hemisphere.

References:

Equinox-Baidu Encyclopedia

Summer solstice-Baidu encyclopedia

Equinox-Baidu Encyclopedia

Solstice in Winter-Baidu Encyclopedia