What are the dates of summer solstice, winter solstice, vernal equinox and autumn equinox?

The vernal equinox refers to the position of the sun between 0 degrees and 15 degrees of the yellow meridian during the period from March 20th to April 5th. But in fact, it usually refers to the day when the sun is really at 0: March 20 or March 2 1. On June 2 1 or 22 of the summer solstice, the sun reaches 90 degrees of the yellow meridian. During the summer solstice, the direct point of the sun is on the tropic of cancer, and then the direct point of the sun begins to move south. The autumnal equinox is on September 23rd or 24th, and the sun reaches longitude 180 degrees. At the intersection of winter solstice 65438+February 2 1 to 65438+February 22, the sun is located at 270 degrees longitude of the Yellow River, pointing directly to the Tropic of Capricorn. It is recorded in the History of the Later Han Dynasty and the Book of Rites: "Before and after the winter solstice, Jiazi lived quietly, and the officials refused to listen to politics, so they chose an auspicious day and then saved trouble." It can be seen that the winter solstice has been a national festival since the Han Dynasty. This date is different for the following reasons: The time for the Earth to make one revolution around the sun is the period of revolution of the Earth. Generally speaking, the revolutionary period is one year. Because the period of the solar annual apparent motion is the same as that of the earth's revolution, the period of the earth's revolution can be measured by the solar annual apparent motion. Observers on the earth observe that the interval between the continuous passage of the sun through a certain point on the ecliptic is one year. Because the selected reference points are different, the length of "year" is also different. The commonly used cycle units are sidereal year, tropic year and perihelion year.

The sidereal year of the earth's rotation is a sidereal year. This periodic unit is obtained with the star as the reference point. In a sidereal year, starting from the center of the sun, the center of the earth takes a star as the background, starts from a certain point, orbits the sun once, and then returns to the same point in the sky; Seen from the center of the earth, the center of the sun starts from a certain point on the ecliptic, is fixed relative to the stars, runs once, and then returns to the same point on the ecliptic. Therefore, from the perspective of the geocentric celestial sphere, the length of a sidereal year is the time interval when the sun center passes the same star twice in succession on the ecliptic.

The sidereal year is obtained with a star as the reference point, so it is the time when the earth rotates 360 degrees, and it is also the real period of the earth's revolution. In days, its length is 365.2564 days, that is, 365 days, 6 hours and 9 minutes 10 second.

The vernal equinox period of the earth's revolution is the Tropic of Cancer. This periodic unit is obtained with vernal equinox as the reference point. In a tropical year, starting from the sun center, the center of the earth has arrived in vernal equinox twice in a row; Seen from the center of the earth, the sun center has reached the vernal equinox twice in a row. From the point of view of the geocentric celestial sphere, the length of a tropic year is the time interval between two consecutive vernal equinoxes, which depends on the ecliptic.

The vernal equinox is the intersection of the ecliptic and the celestial equator. Its position on the ecliptic is not fixed, and it moves 50 ″ .29 to the west every year. In other words, the vernal equinox is a moving point in a year, and its moving direction is from east to west, which is clockwise. The direction of the sun on the ecliptic is from west to east, that is, counterclockwise. These two directions are opposite. Therefore, depending on the center of the sun, the angle taken by the vernal equinox for two consecutive times is not 360, but 360-50.29, that is, 359 59' 9.71,which is the angle of the earth's revolution in a tropic year. Therefore, the tropic year is not the real cycle of the earth's revolution, but only the time required for the earth to rotate at an angle of 359 59' 9 ".71,that is, 365.2422 days, that is, 365 days, 5 hours, 48 minutes and 46 seconds.