The question of Earth’s true form extends far beyond the simplified model of a perfect sphere, which is a convenient, yet inaccurate, representation of our planet. While a spherical shape served as a sufficient approximation for centuries of navigation and early cartography, modern science reveals a more nuanced and complex geometry. The shape of Earth is not a simple geometric figure, but a composite form understood through two distinct scientific models that account for the planet’s rotation and its uneven distribution of mass.
Understanding the Oblate Spheroid
The first, more precise approximation of Earth’s figure is the oblate spheroid, a sphere slightly flattened along its axis of rotation and bulging around the equator. This measurable deformation is a direct consequence of the planet’s rotation. As Earth spins, centrifugal force pushes mass outward, with this force being greatest at the equator where rotational velocity is highest.
This outward push creates the equatorial bulge and corresponding flattening at the poles. The difference is small relative to the planet’s overall size, but significant for geodesy and satellite navigation. The equatorial diameter measures approximately 12,756 kilometers, while the polar diameter is about 12,714 kilometers, a difference of roughly 42 kilometers. This means a person at sea level on the equator is physically farther from the center of the Earth than a person at the poles. The oblate spheroid model provides a smooth, mathematically defined surface used for global positioning systems and large-scale mapping efforts.
The Irregularity of the Geoid
While the oblate spheroid is a mathematical model, the most accurate representation is the geoid, which accounts for the planet’s uneven gravitational field. The geoid is defined as the shape the surface of the oceans would take if only gravity and Earth’s rotation were acting upon them, representing the global mean sea level extended continuously beneath the continents. Because water seeks a constant gravitational potential, the geoid is an equipotential surface, meaning the force of gravity acts perpendicular to it everywhere.
The geoid is an irregular, “lumpy” surface because the distribution of mass within Earth is not uniform. Variations in density within the crust and mantle—such as thicker crust under mountain ranges—cause the gravitational pull to vary slightly by location. Where there is a greater concentration of mass, gravity is stronger, causing the sea level to rise and creating a “bulge” relative to the smoother oblate spheroid. Conversely, areas with a mass deficit cause a corresponding dip in the geoid surface. The geoid undulates above and below the idealized oblate spheroid, but these variations are small, with the difference between the highest and lowest points being less than 200 meters. This gravity-based model is used as the zero-reference surface for measuring true elevation, or orthometric height.
How We Confirmed the Shape
The realization of Earth’s non-spherical shape was a progression of centuries of observation and measurement. Early thinkers established the spherical nature of the planet through observations, such as ships disappearing hull-first over the horizon. The Greek scholar Eratosthenes provided the first accurate calculation of the Earth’s circumference in the 3rd century BCE.
He accomplished this by measuring the difference in the angle of the sun’s shadow at noon on the summer solstice between two Egyptian cities, Syene and Alexandria. Using the distance between the cities and the angular difference, he calculated the circumference with remarkable accuracy.
This confirmed the spherical model, but the subtle deformation remained a subject of theoretical debate until the 18th century. The debate centered on theories proposed by Isaac Newton, who predicted polar flattening, and others who suggested polar elongation. To resolve this, the French Academy of Sciences launched two major geodesic expeditions in the 1730s: one to Lapland near the Arctic Circle and another to Peru near the equator. By measuring the length of a degree of latitude at both locations, the expeditions confirmed that a degree was shorter near the equator than near the pole. This proved the Earth was wider at the equator and flattened at the poles, validating the oblate spheroid model.