The shape of the earth Seen from space and devoid of atmosphere, it is practically spherical, like most celestial bodies. However, due to the rotational movement around its axis and the fact that it is not a rigid or homogeneous body, a better approximation to the real shape of the Earth is that of the flattened spheroid at the poles.
The other planets in the solar system are also flattened at the poles to a greater or lesser extent, due to rotational motion. However, the difference between the equatorial and polar radii of the Earth is small, just 0.3% of the mean radius of the Earth, estimated at 6371 km.
Even though the eccentricity is small, in many cases it must be taken into account to determine a location accurately. Saying that a point is within a radius of 500 meters may be sufficient in many cases, but not when we are trying to locate a buried fossil..
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The ancient Greeks already knew that the Earth was round. Pythagoras (569-475 BC) was among the first to suggest it, but Eratosthenes, around 250 BC, devised a way to test it and put it into practice.
Eratosthenes observed that during the summer solstice in the city of Siena (near the first cataract of the Nile) the Sun shone vertically and the objects did not cast a shadow.
At the same time, much further north, in Alexandria, 800 km from Siena, a pole driven into the ground cast a 7.2º shadow..
Once Eratosthenes had this data, he calculated the radius R of the Earth as:
R = L / θ
Where L = 800 km and θ = 7.2 º = 0.13 radians. With these values he obtained R = 6200 km, a fairly good approximation of the true terrestrial radius..
There is a science in charge of correctly establishing the measurements and the shape of the Earth: geodesy. With the arrival of artificial satellites, the precision of the measurements increased, since before the first half of the 20th century, they were all made from the surface.
It soon became clear that the spheroid flattened by the poles also does not accurately reflect the shape of the Earth, because it does not take into account all the landforms on its surface..
So the scientists defined the geoid, an imaginary surface on which the gravitational potential of the Earth is constant.
Certainly the geoid is also an approximation, so it is concluded that the true shape of the Earth is ... that of the Earth itself, unique and different from any other object in the universe. That is, the shape of the Earth cannot be compared to another object, since it is unique.
The shape of the Earth determines how light and heat from the Sun are distributed, determining factors for life.
In addition, the spheroidal shape, linked to the movements that the Earth makes -as a planet and also its internal movements-, plus the inclination of its axis, is responsible for its physical characteristics..
On the other hand, the shape of the Earth makes it difficult to represent the surface on paper, as we will explain later..
Let's now see some details about these consequences of the shape of the Earth.
The Sun and the Earth are about 150 million kilometers apart, so it can be considered that the solar rays that strike our planet arrive parallel.
However, due to the rounded shape of the Earth, the angle they form with the vertical is not the same for everyone and therefore they do not illuminate or heat it uniformly..
At the equator, the solar rays arrive perpendicular to the surface, then they can be concentrated in a smaller area and produce greater heating (see upper image).
As we move towards the poles, the sun's rays fall lower and spread over a larger area, give rise to temperate regions and those that are definitely the coldest: the poles..
Geographers divide the surface of the terrestrial globe into three zones:
-Intertropical zone, located on both sides of the equator, between two terrestrial circles called tropics. North of the equator is the Tropic of Cancer and south of Capricorn.
-Temperate zone, to the north and south respectively of the Tropics of Cancer and Capricorn, up to the Arctic polar circles to the north and Antarctic to the south.
-Cold zone, from the polar circles to the respective pole.
The fact that the sun's rays are distributed unevenly, together with the inclination of the Earth's axis of rotation, gives rise to many climatic scenarios, such as the seasons.
That is why life has adapted to the conditions of light and heat in innumerable ways, giving rise to a great variety of living beings, both animals and plants..
The value of the acceleration of gravity g is not the same at the poles as it is at the equator, because the radii are slightly different. According to the law of universal gravitation, the intensity of the Earth's gravitational field, which coincides with g, is given by:
g = GM / rtwo
Where G is the universal gravitational constant, M is the mass of the Earth and r is the radius of the same.
Its average value is 9.81 m / stwo at sea level, however at the equator its value is minimal, since there the bulge is greater: 9.78 m / stwo, while it has its maximum at the poles, with 9.83 m / stwo.
Since weight is the force with which the Earth pulls objects towards its center, it turns out that weight differs slightly depending on the latitude we are at. It is the reason why space rockets are launched from places closer to the equator.
Since the Earth is not a perfect sphere, it turns out that the top of Everest in the Himalayas, although it is the highest peak in the world, is not the furthest place from the center of the planet. This honor goes to Chimborazo, a majestic volcano in the Ecuadorian Andes.
From its origins, humanity has developed maps to know its environment, locate other people and locate resources. In such a way that taking into account the shape of the Earth is important to locate the points with precision, a task carried out by cartographers..
When you want to represent a curved surface on a plane, the problem of distortion arises, which brings with it inaccuracies.
Representing small areas in two dimensions is easier. But to create maps of a country, a continent or the whole world, you have to move each point of the curved surface and place it on the paper with the minimum of distortion..
To solve the problem, cartographers have created numerous projections. Examples of them are the cylindrical, conical and azimuthal projections.
In conclusion, the shape of the Earth and the life on it are deeply interconnected, the former conditioning the latter permanently..
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