The Earth's Climate

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Movements and inclination

The average temperature at a given point on the Earth's surface is not constant all year due to the changing seasons. In the temperate zones the year is divided into four successive seasons.
There are three reasons for this phenomenon: the revolution of the Earth around the Sun, the planet's round shape, and the inclination of its axis of daily rotation, the axis of the poles, in relation to the plane of its orbit around the Sun.
The Earth revolves around the Sun in an elliptical orbit contained within a plane called the ecliptic plane. Because its orbit is only slightly off-center, our planet moves in a nearly circular path. The Earth makes one complete rotation around the Sun in one year.
Because the Earth is round, the Sun's rays are perpendicular to its surface at the Equator, and increasingly oblique as they got closer to the poles.
This means that for a given quantity of solar energy reaching the ground, the heated surface will be smaller at the Equator than at the poles. The Equator therefore receives more energy per surface unit than the poles.
In addition, the closer one gets to the poles, the greater distance the Sun's rays must travel through the atmosphere, losing energy along the way.
As a result, the Equator receives twice as much solar energy than a location at 60 degrees latitude.
However, the orbit around the Sun and the Earth's shape do not explain the phenomenon of the seasons, which is due solely to the fact that the axis of the poles is not perpendicular to the ecliptic plane. In other words, the Earth's equatorial plane is not congruent with the ecliptic plane. There is an angle, or obliquity, of 23.5 degrees between them.
If this angle were zero, at any given latitude, for example in Paris, the quantity of solar energy received would be the same in December as in June, and there would be no difference in temperature between winter and summer.
In fact, the Sun's rays are very oblique at this latitude in December. The quantity of solar energy received is lower resulting in cold winter weather.
On the other hand, in June the Sun's rays reach at the same latitude at much more perpendicular angle. The quantity of energy received is higher. It's summer.
This regular succession of four distinct seasons does not occur outside the temperate zones. For example, between the two Tropics, the Sun is always close enough to perpendicular, so there is no major difference in temperature between summer and winter. In these parts of the world, there are usually only two "seasons" in the climatic sense, a rainy season and a dry season.