Our planet is one of 9 that revolves around the Sun. Back in ancient times, there were first ideas about what the shape and size of the Earth.

## How did the idea of the shape of the Earth change?

Ancient thinkers (Aristotle - 3rd century BC Pythagoras - 5th century BC, etc.) many centuries ago expressed the idea that our planet has a spherical shape. Aristotle (pictured below), in particular, taught after Eudox that the center of the Universe is spherical. Proof of this he saw in a character that has lunar eclipses. With them, the shadow cast by our planet on the moon has a rounded shape along the edges, which is possible only under the condition of sphericity.

The astronomical and geodetic studies conducted in the following centuries gave us an opportunity to judge what the form and size of the Earth really are. Today, that it is round, they know from small to large. But there were times in history when it was believed that the planet Earth is flat. Today, thanks to the progress of science, we no longer doubt that it is round, not flat. Indisputable proof of this is cosmic photographs. The sphericity of our planet leads to the fact that the earth's surface is heated unevenly.

But in fact the shape of the Earth is not quite the same as we used to think. This fact is known to scientists, and it is currently used to solve problems in the field of satellite navigation, geodesy, astronautics, astrophysics and other related sciences. For the first time the idea of what the form of the Earth really is in reality was expressed by Newton at the turn of the 17th and 18th centuries. He theoretically substantiated the assumption that our planet, under the influence of gravity, must be compressed in the direction of the axis of rotation. And this means that the shape of the Earth is either a spheroid or an ellipsoid of revolution. The degree of compression depends on the angular velocity of rotation. That is, the body rotates faster, the more it collapses at the poles. This scientist proceeded from the principle of universal gravitation, and also from the assumption of a homogeneous liquid mass. He assumed that the Earth is a compressed ellipsoid, and determined, depending on the speed of rotation, the dimensions of compression. After a while, MacLauren proved that if our planet is compressed at the poles by an ellipsoid, then the equilibrium of the oceans covering the Earth is indeed ensured.

## Is it possible to consider that the Earth is round?

If the planet Earth is viewed from afar, it will seem almost perfectly round. An observer, to whom a great accuracy of measurements is not important, can quite consider it as such. The average radius of the Earth in this case is 6,371.3 km. But if we take the shape of our planet as the ideal ball, we will make precise measurements of various coordinates of points on the surface, we will not succeed. The fact is that our planet is not an ideally round ball.

## Different ways of describing the shape of the Earth

The shape of the planet Earth can be described by two basic, as well as several derived methods. It can be adopted in most cases either for a geoid, or for an ellipsoid. It is interesting that the second variant is mathematically easy to describe, but the first is not described in principle in principle, since practical measurements of gravity at various points of the surface of our planet are being carried out to determine the exact shape of the geoid (and, consequently, the Earth).

## Ellipsoid of rotation

Everything is clear with an ellipsoid of rotation: this figure resembles a sphere that is flattened from below and above. The fact that the Earth's shape is an ellipsoid is quite understandable: centrifugal forces arise because of the rotation of our planet at the equator, while they are not at the poles. As a result of rotation, as well as of centrifugal forces, the Earth "grew stale": the diameter of the planet along the equator is more approximately 50 km than the polar one.

## Features of the figure called "geoid"

An extremely complex figure is a geoid. It exists only theoretically, but in practice it can not be felt or seen. You can imagine a geoid in the form of a surface, the force of gravity at each point of which is directed strictly vertically. If our planet were the right ball filled uniformly with some substance, then the plumb bob at any point would look to the center of the ball. But the situation is complicated by the fact that the density of our planet is inhomogeneous. In some places there are heavy rocks, in others hollows, mountains and hollows are scattered all over the surface, as are unevenly distributed plains and seas. All this changes the gravitational potential at each particular point. The etheric wind that blasts our planet from the north is also to blame for the fact that the shape of the globe is a geoid.

## Who studied the geoid?

Note that the very concept of "geoid" was introduced by Johann Listing (pictured below), a physicist and mathematician, in 1873.

Under it, meaning in translation from Greek the "view of the Earth", meant a figure formed by the surface of the World Ocean, as well as the seas that communicate with it, at an average water level, the absence of perturbations from tides, currents, and atmospheric pressure differences, etc. When it is said that there is such a height above the sea level, it means the height from the surface of the geoid at this point of the globe, despite the fact that there is no sea in this place, and it is several thousand kilometers away from it.

The notion of the geoid was repeatedly refined. So, the Soviet scientist MS Molodensky created his theory of determining the gravitational field and the figure of the Earth from measurements performed on its surface. To do this, he developed a special instrument that measures gravity - a spring gravimeter. He also proposed the use of a quasi-geoid, which is determined from the values taken by the gravitational potential on the surface of the Earth.

## Read more about the geoid

If gravity is measured at 100 km from the mountains, then the plumb line (that is, the weights on the string) will deviate in their direction. This deviation from the vertical to our eye is imperceptible, however it is easily detected by instruments. A similar picture is observed everywhere: deviations of the plumb are somewhere greater, somewhere they are smaller. And we remember that always the perpendicular plumb is the surface of the geoid. Hence it becomes clear that the geoid is a very complex figure. In order to better imagine it, you can do the following: mold the ball from the clay, then from both sides squeeze it to form the flattening, then make the knobs and dents on the resulting ellipsoid. Such a flattened rumpled ball will be quite realistic to show the shape of our planet.

## Why do we need to know the exact shape of the Earth?

Why do we need to know its form so precisely? What does the spherical form of the Earth not satisfy scientists? Should the picture be complicated by the geoid and the ellipsoid of rotation? Yes, there is an urgent need for this: close to the geoid figure helps to create coordinate grids, which are the most accurate. Neither astronomical studies, nor geodetic surveys, nor various systems of satellite navigation (GLONASS, GPS) can not exist and be conducted without determining the fairly accurate shape of our planet.

## Different coordinate systems

In the world currently operates several three-dimensional and two-dimensional coordinate system with the global value, as well as dozens of local. Its the shape of the Earth adopted in each of them. This leads to the fact that the coordinates that were defined by different systems differ. It is interesting that, in order to calculate their points in the territory of one country, will be most convenient to take the shape of the Earth for datum. It's installed now, even at the highest legislative level.

## Krasovsky's Ellipsoid

If we talk about the CIS countries or Russia, then on the territory of these states the shape of our planet is described by the so-called Krasovskii ellipsoid. It was defined back in 1940. Domestic (PZ-90, SK-63, SK-42) and foreign (Afgooye, Hanoi 1972) coordinate systems were created on the basis of this figure. They are still used for practical and scientific purposes. It is interesting that GLONASS relies on the PZ-90 system, which surpasses the accuracy of the WGS84 system adopted in GPS as a basis in GPS.

## Conclusion

To summarize, let's say once again that the shape of our planet is different from the ball. The earth is approaching in its form to an ellipsoid of revolution. As we have already noted, this issue is not idle at all. The precise definition of which Earth has a form gives a powerful tool to scientists to calculate the coordinates of celestial and terrestrial bodies. And this is very important for space and maritime navigation, for construction, geodetic work, and in many other areas of human activity.