For centuries, people have visualized the Earth as a perfect sphere. While this spherical model is useful for many purposes, precise measurements confirm that our planet is not perfectly round. The Earth, along with many other celestial bodies, conforms to a specific three-dimensional shape known as an oblate spheroid. This deviation means the Earth is slightly wider around the middle than it is from pole to pole. Understanding this shape is fundamental to astronomy, physics, and mapping.
Defining the Oblate Spheroid
The oblate spheroid is a precise geometric figure defined as the surface created when an ellipse is rotated around its shorter axis. This rotational process results in a shape that is slightly flattened at the top and bottom. The key characteristic of this shape is the difference in length between its two primary axes.
The polar axis, spanning from the North Pole to the South Pole, represents the shorter dimension. Conversely, the equatorial axis, measuring the diameter across the middle, is the longer dimension. For Earth, the polar radius is approximately 6,356.75 kilometers, while the equatorial radius extends to about 6,378.14 kilometers.
This difference results in an equatorial bulge, making the Earth’s diameter at the equator about 43 kilometers greater than its diameter measured through the poles. The Earth’s flattening is very small, representing only about one part in 300 of its diameter. This geometric model is the standard reference for describing the Earth’s shape.
Oblate vs. Prolate: The Two Types of Spheroids
A spheroid is a general term for an ellipsoid of revolution, a three-dimensional shape created by spinning an ellipse around one of its axes. The oblate spheroid is one of two distinct types, differentiated by which axis is used for the rotation.
The oblate form, as seen in the Earth, results from revolving the ellipse around its shorter axis, making the shape “wider than it is tall.” The prolate spheroid is generated by revolving the ellipse around its longer axis. This creates an elongated shape, or one that is “taller than it is wide,” resembling a football. Some moons, such as Saturn’s Mimas, are slightly prolate due to tidal forces.
The Cause of Equatorial Bulge in Rotating Bodies
The primary physical reason celestial bodies like Earth adopt an oblate spheroid shape is continuous spin. As the Earth rotates, the mass is subject to an inertial effect that pushes material outward, most noticeably at the equator. This outward push is commonly referred to as centrifugal force, which opposes the inward pull of gravity.
Gravity attempts to pull the planet’s mass inward toward a common center, forming a perfect sphere. Rotational forces oppose this pull, especially at the equator, where rotational speed is highest. Objects at the equator have a higher tangential velocity because they travel a greater distance in the same 24-hour period compared to objects near the poles.
This greater speed generates a stronger outward push, causing material to accumulate around the middle. The degree of a planet’s flattening is directly related to its rotational speed. Gas giants like Jupiter and Saturn, which spin much faster than Earth, exhibit a far more pronounced equatorial bulge.
How the Earth’s Shape Affects Measurement and Geodesy
The Earth’s oblate shape has practical implications for mapping, navigation, and geodesy—the study of the Earth’s geometric shape, orientation, and gravity field. Since a perfect sphere is inaccurate, geodesists use the oblate spheroid as a reference surface, known as a reference ellipsoid, to define coordinates like latitude and longitude.
The current standard for global positioning systems (GPS) is the World Geodetic System 1984 (WGS84), which is mathematically defined by a specific oblate spheroid model. This reference ellipsoid is necessary because the equatorial bulge affects measurements and gravitational pull. For example, the equatorial bulge makes the summit of Mount Chimborazo in Ecuador the farthest point from the Earth’s center, surpassing Mount Everest.
Accurate mapping also requires a geoid model, which accounts for slight variations in gravity across the surface. The geoid represents the theoretical surface of the Earth if it were entirely covered by water. This surface would be higher where gravity is stronger and lower where it is weaker. This complex model, based on the underlying oblate spheroid, allows GPS and other navigation technologies to provide precise altitude and location data.