Spring Equinox
Every year on March 20th of the Gregorian calendar, when the sun is located at 0 (vernal equinox), it is the vernal equinox. On this day, the sun shines almost directly on the equator of the earth, and the whole world is almost as long as day and night (regardless of the refraction of sunlight by the atmosphere and the shadow in the morning). After the vernal equinox, the direct point of the sun continues to move from the equator to the northern hemisphere, where the days are long and the nights are short, that is, the days are longer than the nights in a day; In the southern hemisphere, the days are shorter and the nights are longer, that is, the days are shorter than the nights in a day. Therefore, the vernal equinox is also called the ascending equinox.
Summer solstice
Solstice is the earliest of the 24 solar terms. In the 7th century BC, the shadow of the sun was measured by civil soil gauge, thus the summer solstice was determined. According to "Zunxian Manuscript", "The sun arrives in the north, the days are long and the shadows are short, so it is called the summer solstice. At best, it is excellent. " From summer to sunrise, the sun directly reaches the northernmost point of the year, almost directly to the Tropic of Cancer (23 26' north latitude), which is the longest place in the northern hemisphere. The higher the latitude, the longer the day.
Autumn Equinox
China's ancient book "Spring and Autumn Stories, Yin and Yang in and out" said: "The autumn equinox, Yin and Yang are also half-phase, so it is cold and hot day and night." The autumnal equinox means "half" On this day, the sun reached longitude 180 degrees and directly hit the equator of the earth. Therefore, in most parts of the world, 24 hours are divided equally between day and night, each being 12 hours.
Winter solstice
On the solstice of winter (Gregorian calendar 65438+February 2 1-23), the cold winter is coming. Winter solstice is one of the twenty-four solar terms, indicating the change of seasons. It is based on the change of the position of the earth on the ecliptic (that is, the orbit of the earth around the sun). On the day of winter solstice, the sun moves to 270 degrees of the yellow meridian (winter solstice), and the position of the sun directly on the ground reaches the southernmost point of the year. The sun is almost direct to the Tropic of Capricorn (also known as the winter solstice daily line), and the sun is most biased towards the northern hemisphere.