The seasons divide the tropical year into four time periods, which are formed by the apparent path of the Sun in the celestial hemisphere. Their sequence is the result of the annual movement of the Earth around the Sun and the inclination of its polar_axis with respect to the plane of terrestrial orbit.
This means that the equatorial plane (which is perpendicular to the axis joining the two poles) is inclined at an angle of 23.5 degrees to the orbital plane of the Earth. This angle changes during the orbit of the Earth around the Sun, and therefore, during the course of a year, the angle of incidence of the Sun's rays falling on the Earth changes as well.
In the winter at a particular place on the Earth, the Sun's rays fall at a flatter angle on the Earth's surface and get distributed over a wider area. In summer, in contrast, the rays fall more directly on a smaller surface and it becomes hotter than in winter. These transitions are not abrupt; they are gradual.
In addition, altitude differences, movements of the air masses as well as ocean currents play a significant role in influencing climatic conditions. The sure signs of the change in the seasons can be seen in the behavior of animals, e.g. the annual migration or processes that take place in the plant world, such as the start of blossoming.
The Sun does indeed rise in the East and set in the West every day, but the location of its rising or setting is not always the same. If the two poles are equidistant from the Sun, that state is called the equinox. It is 21st March, i.e. the start of spring in the Northern Hemisphere. On that day, the Sun rises exactly in the east and sets exactly in the west.
When the North Pole reaches the shortest distance to the Sun, it is 21 June, i.e. the start of the summer in the northern hemisphere. The day of the summer solstice is the longest day as well. On 23 September, at the start of autumn, the day and night are once again of equal length (autumn equinox).
Once again, the Sun rises exactly in the east and sets exactly in the west. The winter starts with the winter solstice on 21 December. Now, it is the shortest day and the longest night. The above applies only to the northern hemisphere. In the southern hemisphere, the situation is the opposite.
