The seasons

The alternation of the seasons is caused by the inclination of the Earth’s axis and by the revolution of our planet around the Sun.
The Earth follows a slightly elliptical trajectory with respect to its orbital plane. During its revolution, the Earth’s axis of rotation maintains a constant inclination respect to the ecliptic and the Earth’s two hemispheres do not receive the same amount of radiation, which is dependent on the position of the planet respect to the Sun. The variation of the angle of incidence of the Sun’s rays on the Earth’s surface results in a consequent difference in the amount of heat received. Hence, seasonal variations in temperature are not due to a greater or shorter distance from our star: in fact, the Earth’s orbit is closest to the Sun during the winter solstice and furthest from it during the summer solstice. The inclination of the Earth’s axis respect to the orbital plane also explains the change in the length of day and night during the year.
It must be mentioned that the Moon has had an important role in stabilising the Earth’s rotation axis and therefore favouring the development of life. The more inclined the rotation axis is on the ecliptic plane, the more marked are the differences between the seasons. Had there been no Moon, the gravitational attraction of the Sun and the other planets could have made the tilt of the Earth vary in the course of time. In this case, temperatures would have been more extreme, making the evolution of life on Earth more difficult.

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