The Geoid: How Gravity and Space Missions Reveal the Earth’s True Irregular Shape

The accurate representation of the shape of the Earth is known as the Geoid, a model that combines physical and mathematical principles influenced by gravity. This figure is neither perfectly spherical nor geometric, but rather takes on an irregular shape due to mass variations on the planet, detected by international space missions.

The Geoid: Understanding the True Shape of the Earth

According to Michael Watkins from NASA’s Jet Propulsion Laboratory, these irregularities are due to the uneven distribution of materials inside the Earth, which affects its gravitational field creating “hills” and “valleys”.

The Geoid is defined in geodesy as an equipotential surface of the Earth’s gravitational field, a model that integrates gravity, often ignored in simplistic geometric representations.

To accurately map this gravitational field, NASA and other international agencies have launched missions like GRACE (Gravity Recovery and Climate Experiment), highlighting the importance of exact models to understand our planet.

In a popular illustration by NASA, the Earth is presented in an exaggerated “dented potato” shape to highlight the small gravitational variations that, on their real scale, are imperceptible to the human eye.

A report from the Earth Observation Data System explains that the uneven distribution of mass, such as rocks, magma, and water, generates gravitational fluctuations that directly impact the shape of the Geoid.

These differences in mass and density within the Earth’s interior create areas of more intense gravity, forming “hills”, and zones with less attraction resulting in “valleys”.

Geoid vs. Ellipsoid: Physical Reality and Mathematical Models

Contrary to the Geoid, the ellipsoid is an idealized and smooth geometric figure, similar to an American football, frequently used for mathematical calculations due to its simplicity.

While the ellipsoid is mathematically perfect, the Geoid reflects the shape that the ocean water surface would take in a state of rest, considering gravitational forces.

Therefore, the difference between the two models lies in the fact that the Geoid takes into account the actual distribution of mass and density beneath the Earth, affecting how water is distributed on its surface.