we call geoid to the theoretical almost spherical shape that planet Earth assumes, in which the average level of the seas that cross it will be taken as the surface. It is spoken in an almost spherical way because there is a slight flattening at both poles, given by the equipotential surface of the terrestrial gravitational field that coincides with the mean level of the seas. So, if we consider the crust, the earth will not be one hundred percent a geoid, although it will be if it is represented by the mean level of the tides.
The idea of the earth as a geoid was anticipated by the scientist Isaac Newton in his work Principia in the year 1687. Newton would demonstrate it through a homemade exercise: if a viscous body is rotated rapidly in a liquid fluid, the equilibrium form that the mass will present under the design of the law of gravity and rotating around its own axis will be a spheroid squashed at their respective poles.
Meanwhile, Newton’s proposal would be studied and verified in situ later by Domenico and Jacques Cassini; both made an exact measurement of the difference of one degree in the vicinity of the Equator line and compared the differences with European latitudes. Later mathematical and geometric works would also reconfirm the shape originally proposed by Newton.
The shape of the geoid can be determined by means of: gravimetric measurements (measuring the magnitude of the intensity of gravity at various points on the earth’s surface. As a consequence, it is a sphere flattened at its poles, the acceleration of gravity will go in crescendo from the Equator to the Poles), astronomical measurements (they measure the vertical of the place in question and wait for its variations. The variation will be related to the shape) and measurement of the deformations produced in the orbit of the satellites caused because the earth is not homogeneous.
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