The year is divided into four seasons, mainly due to the Earth’s orbital position around the Sun and the tilt of the Earth itself.

The revolution of the Earth refers to the rotation of the Earth around the Sun, and this rotation orbit is an ellipse that is approximately perfectly circled, and the Sun is located on one of the focal points of this ellipse. Since the speed of the earth’s revolution and the length of its orbit are constant, it takes the earth 365 days, 5 hours, 48 minutes and 46 seconds to complete one revolution, that is, one year.

The Earth’s orbit around the Sun is from west to east, with a speed of 30 kilometers per second. The length of the Earth’s orbit around the Sun is 9.4 billion kilometers.
However, the change of the seasons is not only related to the rotation of the earth, but also to the tilt of the earth. The axis of the Earth is not perpendicular to the plane of the Earth’s orbit, but has an inclination of about 23.5 degrees, which is called the yellow-red angle.

Due to the presence of the yellow-red angle, the direct point of the sun will move between the Tropic of Cancer of the Earth, resulting in changes in the heat distribution of the Earth’s surface in different seasons. When one side of the Earth is tilted towards the Sun, that side receives more sunlight, causing the temperature on that side to rise, forming the summer season; While the other side is away from the sun, there is less sunlight and the temperature is lower, forming a winter season. When the Earth’s axis is perpendicular to the direct point of the sun, sunlight on the Earth’s surface is relatively evenly distributed, forming spring and autumn.

Therefore, the reason for the division of the year into four seasons is the movement of the direct point of the sun and the change in the heat distribution of the earth’s surface caused by the position of the earth’s orbit around the sun and the tilt of the earth itself.

The division of the year into four seasons is a result of the nature of the Earth’s orbit around the Sun and the tilt of the Earth’s own axis. The Earth does not perpendicular to its orbit but maintains a yellow-red angle of about 23.5 degrees (also known as the Earth’s axis inclination), resulting in a periodic north-south direction of the sun (i.e., the point at which the sun’s rays hit the Earth’s surface vertically) throughout the year.

Specifically:

1. During the spring and autumn equinoxes, the direct point of the sun is located at the equator, and most of the world has almost equal days and nights, and the climate is mild, marking the beginning of spring and autumn;
2. During the summer solstice, the direct point of the sun reaches the northernmost point (Tropic of Capricorn), the northern hemisphere receives the most solar radiation, the longest day, and the climate is hot, which is summer; In the southern hemisphere, it is winter, with relatively short sunshine hours and a cold climate;
3. During the winter solstice, the direct point of the sun reaches the southernmost point (Tropic of Capricorn), the northern hemisphere receives the least solar radiation, the longest nights, and the climate is cold, which is winter at this time; In the Southern Hemisphere, on the other hand, summer begins.

Therefore, the north-south movement of the direct sun during the rotation of the earth brings about the alternation of cold and warm in different regions of the earth, and then forms the climate characteristics of four distinct seasons. The type of climate and geographical location of different regions can also lead to different performances of the four seasons, such as in the tropics, where the temperature does not change much throughout the year, and is usually divided into only a rainy and dry season. In the polar regions, long polar days and nights may be experienced due to the more drastic changes in the sun’s altitude angle.