Why are there seasons?


There are seasons on Earth: spring, summer, autumn, winter. But, this cannot be just explained by the distance of our planet from the Sun. Where do the earth’s seasons come from?

Spring, summer, autumn, winter: the Earth is punctuated by the seasons. It is not the only one concerned by this astronomical phenomenon: all the planets of the solar system have it. But, where do the earthly seasons come from? One could spontaneously attribute these stable climatic periods of the year to the distance of the Earth from the Sun. We could say that it is naturally colder when the planet moves away from the Sun, and hotter when it approaches it.

As NASA aptly puts it, ” while this idea makes sense, it is incorrect “. Indeed, the orbit of the Earth around the star does not draw a perfect circle, but rather an ellipse. However, when the Earth is at its minimum distance from the star (the perihelion), it is winter in the northern hemisphere. And when it is at its maximum distance from the Sun (aphelion), it is summer there. Moreover, the difference in distance between the moment of perihelion (about 147 million kilometers between the Earth and the Sun) and that of aphelion (152 million kilometers) is not so great, compared to the total distance .

Earth’s axis of rotation is tilted

The explanation is actually based on the axis of the earth, that is, the imaginary line that connects the North Pole to the South Pole of the planet. It is around this axis that the Earth is in rotation (that it turns on itself). Every day, the planet makes a complete revolution on this axis. However, this axis of rotation is inclined by 23.4° and it is this phenomenon which is at the origin of the seasons.

Inclined to what? In the plane of the ecliptic, that is to say the plane of the Earth’s orbit around the Sun (there is therefore a link with the revolution of the Earth around the Sun, but not the one mentioned above ). This axis is always directed in the same direction. Thus, during part of the year (with a total duration of 365.25 days) the northern hemisphere is tilted towards the Sun, then it is the southern hemisphere during the other six months of the year. The Sun, higher in the sky for the hemisphere which is tilted towards the star, heats up more: it is summer. It heats less directly on the other hemisphere: it is winter. At the time of early spring and fall, both hemispheres of the planet receive comparable amounts of solar heat.

The equinoxes and solstices. We see that the axis of rotation of the Earth is tilted. // Source: Wikimedia/CC/Duduf

Why is this axis of rotation tilted? This characteristic would be linked to an event that occurred in the past of the Earth: the hypothesis of an impact with a body baptized Theia. This event, in addition to being the supposed origin of the Moon, would also have slightly “tilted” the Earth.

What do the seasons look like on Mars?

Concerning the other planets, the phenomenon can be explained either by an inclined axis of rotation (like the Earth), or by an elliptical orbit (the initial intuition was therefore relevant, but not in the case of the Earth), or the two.

Mars is in the latter case: its orbit is more elliptical than that of the Earth, which means that it receives more or less heat at certain times of the year, and its axis is tilted. During the summer, in its southern hemisphere, Mars is close to the Sun. Therefore, summer in the Martian southern hemisphere is shorter and hotter than summer in its northern hemisphere. Mars seasons are also longer, as the year is 687 days long.

Hubble observations have also made it possible to observe potential seasonal changes on Saturn or Neptune.


If you liked this article, you will like the following ones: do not miss them by subscribing to Numerama on Google News.

All about the star of the solar system, the Sun



Source link -100