The shape of our planet is often taught as a perfect sphere, but the true figure of the Earth is far more complex. Scientists must account for this non-spherical nature in every precise measurement. Accurately defining and modeling this shape is the specialized field of study called geodesy. Understanding the Earth’s true shape requires moving beyond the spherical ideal to two distinct, scientifically defined forms that are fundamental to modern navigation and mapping systems.
The Oblate Spheroid Model
The first scientific refinement of Earth’s shape is the oblate spheroid, a mathematical figure that provides an excellent first approximation. This term describes a sphere that is slightly flattened at the poles and bulges around the equator. This equatorial bulge is a measurable difference in the planet’s diameter. The equatorial diameter of Earth measures approximately 12,756 kilometers, while the polar diameter is roughly 12,714 kilometers. This difference of about 42 kilometers is significant enough to affect satellite orbits and is foundational for many large-scale calculations because it offers a smooth, mathematically defined surface.
Why Earth Isn’t a Perfect Sphere
The primary force driving the Earth’s departure from a perfect sphere is its constant rotation on its axis. As the planet spins, it generates an outward centrifugal force, which is strongest at the equator and diminishes to zero at the poles. This force counteracts gravity to a small degree, causing material to spread outward. Because the Earth is not perfectly rigid and has a fluid interior, the rotational force causes the planet to assume the shape of least resistance, resulting in the characteristic equatorial bulge. This phenomenon was first explored mathematically by Isaac Newton.
Defining the Geoid
While the oblate spheroid is a smooth, mathematical model, the most accurate physical description of Earth’s shape is the geoid, a term that literally means “Earth-shaped.” The geoid is an irregular, undulating surface that represents the shape of global mean sea level, extending through the continents as if hypothetical canals were dug. Every point on the geoid surface has the same gravitational potential. The geoid differs from the smooth oblate spheroid because it accounts for the uneven distribution of mass within the Earth. Variations in crust density and mountain ranges create gravitational anomalies that cause the geoid surface to be lumpy, with highs and lows that can vary by up to 100 meters relative to the smooth spheroid.
Using Earth’s Shape for Global Positioning
The distinction between the smooth oblate spheroid and the irregular geoid is directly relevant to modern navigation and mapping, particularly in the use of the Global Positioning System (GPS). Geodesists use a specific oblate spheroid, known as a Reference Ellipsoid (like WGS 84), as the simple, mathematically defined surface for calculating latitude and longitude coordinates. GPS receivers measure height above this smooth reference ellipsoid, which is called the ellipsoidal height. Traditional altitude, or orthometric height, is defined as the height above mean sea level (the geoid). To convert the GPS-measured ellipsoidal height into practical elevation, mapping systems must use a complex geoid model to determine the vertical offset, or “geoid height,” because users need their height relative to the irregular, gravity-based sea level surface.