Solar terms What is the solar term that divides day and night equally in the Song Dynasty?

Twenty-four solar terms are vernal equinox and autumnal equinox.

1, vernal equinox solar terms.

The vernal equinox begins on March 20th (or 2 1) and ends on April 4th (or 5th) every year. The vernal equinox begins on March 20 or 2 1 day of the solar calendar every year, and the sun reaches 0 degrees of the yellow meridian (vernal equinox). The length of day and night is average, just half of the 90th day of 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. 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; Climatically, there are also obvious characteristics. At the vernal equinox, except the Qinghai-Tibet Plateau, the northeast, the northwest and the northern part of North China, China entered a beautiful spring.

2, the autumn equinox solar terms.

The autumnal equinox is the16th solar term among the 24 solar terms of the lunar calendar, which usually falls on September 22nd or 23rd every year. According to the lunar calendar, "beginning of autumn" is the beginning of autumn and ends with "the first frost", and the "vernal equinox" is just half of the 90-day journey from beginning of autumn to the first frost.

After the autumnal equinox, the direct sunlight moved to the southern hemisphere, and the northern hemisphere got less and less solar radiation, but the ground lost more heat and the temperature dropped significantly.

From the day of the autumnal equinox, the climate has three main characteristics: the position of direct sunlight continues to move from the equator to the southern hemisphere, and the phenomenon of short days and long nights in the northern hemisphere will become more and more obvious. The days will gradually shorten and the nights will gradually lengthen (until the winter solstice reaches the longest night and the shortest day); The temperature difference between day and night will gradually increase, and the amplitude will be higher than 10℃. The temperature drops day by day, getting colder every day, and gradually entering the late autumn season. The situation in the southern hemisphere is just the opposite.

The meaning of "autumnal equinox" has two meanings: first, the sun reaches the longitude of the yellow meridian 180 degrees on this day and directly hits the equator of the earth, so a day is divided into 24 hours, every hour 12 hours; 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. Secondly, according to the method of seasonal division in ancient China, which took beginning of spring, Changxia, beginning of autumn and beginning of winter as four seasons, the autumnal equinox lived in autumn for 90 days and divided the autumn equally.