The Earth rotates on itself about an axis which is inclined by about 23 ° 27 ‘with respect to the vertical. Why 23 ° 27 ‘? Because one day a celestial body, a small planet that no longer exists, collided with the earth, which, beneath the shock, bent like a top, but it never re-changed. This inclination constantly changes the duration of day and night at a given point and creates the seasons.

Two days a year however the duration of the day is equal to the duration of the night: 12 hours each. It is the spring equinox on March 20, 21, 22 or 23 and the autumnal equinox on September 21, 22 or 23. These days coincide with the passage of the Sun in one or the other hemisphere.

In summer, the Sun rises very high in the Sun, it is hot and the day lasts longer than the night. In winter it is the reverse, the Sun does not rise very high in the sky, the days are short and it is cold. Why these differences? A diagram will help us understand what is going on.

As seen on the left, in summer (left), the North Pole remains illuminated by the Sun; There is no night for 6 months. Conversely, in winter the north pole (right part) no longer receives the sunlight for 6 months. A little lower in latitude (Norway, Sweden, north of Moscow, etc.), in winter the night lasts barely 2 hours. On the other hand, at the equator, during the dry season as well as during the wet season (there is no summer or winter), the day lasts practically as long as the night; The Sun rises at 6 am and goes to bed at 6 pm.

Why is it cold in winter in the Northern Hemisphere? In December the Earth is a little closer to the Sun than in June. The difference is 1%. So we might think it should be warmer in December. This cooling is not related to the distance that separates us from the Sun, but above all to the position of the Sun with respect to the axis of inclination of the Earth. In fact, in winter in the northern hemisphere, the inhabitants do not long see the Sun above the horizon and its rays are very inclined with respect to the vertical. Conversely, in summer the sun is very high in the sky and its rays, like a very powerful light beam, illuminate a surface smaller than in winter.

This is the inclination of the axis of the Earth on the plane of its orbit which explains why there are seasons and the reason why the duration of the day is not equal to the duration of the night.

As shown in the following drawing, since the same heat is distributed over a wider surface in winter, each illuminated part of the Earth receives less energy and therefore becomes colder. Moreover, since the day is shorter in winter (it is daylight for 8 hours against 12 hours in summer around 50 ° North latitude) the Sun does not have enough time to heat the ground and the cold remains.

In this regard, in the southern hemisphere the seasons are reversed: the inhabitants of Brazil, South Africa or Australia spend Christmas under the Sun of the summer and at home in the mountains it snow in June!

However, distance to the Sun still plays a role in softening the seasons in the Northern Hemisphere. In the winter, since the Earth is slightly closer to the Sun, the northern hemisphere receives a little more heat, while the inhabitants of the Southern Hemisphere have a slightly warmer summers. Conversely, in summer the Earth is a little farther from the Sun which slightly softens the heat in the northern hemisphere, while the southern hemisphere shakes a little more.

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