Why is it sometimes cold and sometimes hot?

The earth revolves around the sun in an elliptical orbit. In January, the earth is at the perihelion of this elliptical orbit. However, due to the earth's inclination of 23.5 degrees, the area of direct sunlight on the earth will change with the position of the earth in orbit. When the earth is at perihelion, it is winter in the northern hemisphere and the direct point of the sun is located in the southern hemisphere. In the northern hemisphere, the days are short and the nights are long, and the sunshine time during the day is short, so it absorbs less heat. Because of the oblique sunlight, the sun's heat takes the longest time to pass through the earth's atmosphere, and the surface absorbs the least heat per unit area. At this time, the northern hemisphere is in winter. On the contrary, when the earth is at perihelion, the sun shines directly on the northern hemisphere, and the days in the northern hemisphere become longer and the nights become shorter, which makes the temperature in the northern hemisphere higher, resulting in summer, while in the southern hemisphere, due to the oblique sunlight, the days are shorter and the nights are longer, resulting in less energy, so the temperature is lower, resulting in winter. Half a year later, the sun shines directly in the southern hemisphere and obliquely in the northern hemisphere, so the seasons are reversed. It is summer in the southern hemisphere and winter in the northern hemisphere. Repeatedly, the seasons change.