Autumn equinox solar terms

Both the vernal equinox and the autumnal equinox point to the equator, and the summer solstice and the winter solstice are the Tropic of Cancer respectively.

The direct point of the sun is the point where the incident angle of sunlight on the earth's surface (that is, the solar altitude angle) is 90 degrees, and it is the intersection of the connecting line between the center of the earth and the sun and the spherical surface of the earth. The meridian where the direct point of the sun is located is 12 noon. The movement of the sun's direct point will cause the change of global heat distribution, and the seasonal movement of the sun's direct point will drive the north-south deviation of the planetary system, and also cause the changes of the length of day and night and the height of the sun at noon.

The vernal equinox and autumnal equinox are called "equal share", which means that the length of day and night is equal. These two solar terms are generally around March 20 and September 23 of the Gregorian calendar every year. After the autumnal equinox, the direct point of the sun continues to move towards the tropic of Capricorn. It reached the Tropic of Cancer on the solstice in summer, and the direct point of the sun has been moving to the southern hemisphere since then. After the autumnal equinox reached the equator, it continued to move south until the winter solstice. When the direct point of the sun reached the southernmost tip of the Tropic of Capricorn, it began to move north. On the day of the autumnal equinox, the day is as long as the night, which is twelve hours. From this day on, the days are getting shorter and shorter. This is the vernal equinox solar term.