Our planet, the Earth, has a spherical shape, flattened towards the poles, called geoid.
This form is determined by the movements experienced by the Earth along its orbit and by the fact that there is, in certain regions of the space that it crosses, enough matter to generate friction.
To get an idea, let's imagine an egg lying down and, although not as pronounced, it does approach the aspect of the planet a bit, since Ecuador is 21.5 km farther from the center of the Earth than both poles. Therefore, if you believed that our planet had a ball-like shape ... you were wrong!
Is called Earth's axis to the imaginary line that would link the poles through the center. He Ecuador divide the planet into two halves or hemispheres. While the equatorial radius is 6,378.1 km, the polar measures 6,356.8 km.
The Earth has an estimated surface area of 510,072,000 km2, being a small star when compared to other planets in the galaxy. Of these, about 29.2% corresponds to emerging land. That is, 3/4 of the Earth's surface is occupied by water in a liquid state, something peculiar among the planets of the Solar System.
The geoid is the reference form for all the measurements to be carried out on Earth since it considers the surface more homogeneous than it really is, measured from sea level, with positive or negative measures depending on whether it is above or below this level
However, the geoid lacks mathematical representation, as it is a equipotential surface of gravity, that is, a surface whose points have the same value of gravity. This is very complicated to measure at practical levels. Therefore, the use of other approaches to the shape of the planet is used.
The ellipsoid of revolution
When an ellipse is rotated 360º, the figure obtained is called the ellipsoid of revolution; That comes to be like a deflated ball. In Geodesy (science that studies the shape of the planet) that form is called reference ellipsoid.
And why do we want to spin an ellipse? Because, as we said, the flattened shape of the Earth reminds of a lying egg that, in turn, looks quite like an ellipse. And since our planet rotates 360º with respect to its axis, while it travels, several scientists have considered that it could be a way of approaching the real form of our planet, by means of reference ellipsoid.
The earth globe
Since the Middle Ages, if not before, the planet was represented by a sphere that had the shape of the continents and seas drawn on its surface. It was called globe and, although it is currently known that it is not accurate, it is also true that the difference between the two semi-axes of the terrestrial ellipsoid is only 22 km, so the mistake made is not so much.
Of course, knowing that this error exists and that the morphology of the Earth is really that of a geoid. For the purposes of calculation, the use of the reference ellipsoid, which does have mathematical representation, is unlike the geoid. In this way, the calculations will be more accurate and real than when using a sphere for terrestrial morphology.
Another curious aspect that became evident once artificial satellites were used to study the Earth is that, because of this flattening, the traditional maps representing the developed countries - especially, of Europe and North America - of great extent, really They were smaller. Similarly, Africa and South America were larger than they were being attributed.
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