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By dint of exploring with the imagination and curiosity that distinguishes us, the other planets of the Solar system, it could happen that we find ourselves a little ignorant about what are the characteristics of our Planet Earth. May it ever be! So let's try to observe it together, trying not to be biased but to look at it as we did with the planet Saturn, for example, or with the planet Mars.
Planet Earth is in fact the third planet of the Solar system but it has a primacy that we must recognize, an objective primacy: it is the only one that hosts forms of life. At least for now, we can say, one day, we may find ourselves denying it, after discovering we have neighbors in space. Because we are the only inhabitants of the Solar system?
All thanks to the conditions on our planet, a winning combination of temperature and pressure, but that's not all. Also the distance of a planet from the Sun, the composition of its atmosphere and, obviously, the eventual play an important role presence of water than on Earth there is plenty. In addition to being the only inhabited planet, ours is also the one with the greatest geological variety.
Planet Earth: motion
Since we live there, we are not always aware of its movements, we take them for granted as we take for granted perhaps that a day lasts twenty-four hours and a year three hundred and sixty-five days. Here, let's analyze how the Earth moves and what are the consequences especially on us who live on it. As with the other planets, even theorbit of the Earth it identifies in space a plane called ecliptic, it moves following this orbit with a defined velocity that has been measured, it is equal to 30 Km / s.
Going to better see the characteristics of our planet's orbit, to better describe it and distinguish it from that of other planets, we go to indicate eccentricity: 0.0167. It is quite small compared to that of other planets. Another parameter that we can give to give an idea of the Earth's orbit is the radius which in this case is equal to 149.598.000 Km. We complete the sequence of numbers with the distance from the Sun to perihelion, equal to 147.10 million of Km, and the distance from the sun to the aphelion, equal to 152.10 million km.
Let's do a little digression on the stars, even if we are talking about the Earth, because they are what we who live on its surface see. The fact that at different times of the year different constellations can be seen is nothing more than a consequence of the Earth's revolution motion around the Sun with the characteristics we have indicated.
In fact, the orbit intersects the celestial sphere in a circumference that is a sort of ribbon in which the various appear constellations of the Zodiac. Let's now move on to rotation, a movement that is carried out around an actually inclined axis, to be exact 23 ° 27 'on the ecliptic plane. If the axis were "straight" it would be quite another thing but since it is not, what happens is that the two terrestrial hemispheres, the southern and the northern ones, are alternately exposed to periods of maximum and minimum insolation. We thus have the seasons and the Earth is not the only planet to have them because the others also rotate slightly inclined on the orbital plane.
Planet Earth: characteristics
We have seen how it moves, now let's get to know how it is made. Let's start with the weight and dimensions: the planet Earth has a mass of about 6 billion billion tons (5.976 1027 g) with an average density of 5.517: no other planet exceeds this number, this makes the Earth the densest planet in the Solar system. Seen from where we are it is impossible to understand it but those who have been able to observe it from elsewhere tell us that the Earth has the approximate shape of an ellipsoid, its flattening is however very slight, just compare the terrestrial equatorial radius and the polar one which are respectively 6,378 Km and 6,357 Km.
On closer inspection we are not faced with an ellipsoid that orbits the Sun and also rotates on itself but more than anything else to a planet with a completely irregular shape, with many bumps and depressions that make it unique. This is why the most experts define it as a geoid, a term to indicate a shape similar to that of a pear which has the “tip” at the North pole and the most flattened part at the South pole.
Before moving on to describe the Earth's atmosphere, let's not forget the faithful Moon, its only and precious satellite that gives rise to various phenomena, among the best known that of the tides.
Planet Earth: atmosphere
The Earth's atmosphere is composed of approximately 78% nitrogen, 21% oxygen and 1% argon, for the rest there are other gases that represent a minimum percentage, 0.04%. Its pressure at sea level is 1013 millibars. Among the "ingredients" of the earth's atmosphere, I focus onoxygen, because it is very important for the development and maintenance of life, and oncarbon dioxide, protagonist of the greenhouse effect without which the average temperature would be -21 ° C instead of + 14 ° C, the water would be frozen and there would be no life.
To better study our atmosphere, we have divided it into five main layers, through which the temperature varies considerably, increasing or decreasing with height: the troposphere, the stratosphere, the mesosphere, the thermosphere, the exosphere.
The Earth, on closer inspection, was also divided into layers, each of which with different chemical and physical properties. From the inside to the surface, we distinguish: the core, the mantle and the crust.
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