Why the Earth is not a sphere and how we measure its true shape
In short: Earth is not a perfect sphere but an oblate spheroid, bulging at the equator due to its rotation. Geodesists use mathematical models called reference ellipsoids to map this shape. These models are essential for GPS, navigation, and defining the coordinates that keep our global systems aligned with reality.
The Earth is technically an oblate spheroid, an ellipsoid of revolution where the polar diameter is shorter than the equatorial diameter by approximately 21 kilometers. This shape arises from the centrifugal force generated by the planet's rotation, a phenomenon first theorized by Isaac Newton in his 1687 work, Principia. Newton proved that a self-gravitating fluid body in equilibrium would naturally flatten at the poles, a concept that shifted scientific focus away from the earlier assumption that Earth was a perfect sphere.
To represent this complex, irregular shape—known as the geoid—geodesists use reference ellipsoids. These are simplified mathematical surfaces that allow for consistent coordinate systems, such as latitude, longitude, and elevation. While a 'mean Earth ellipsoid' describes the global average curvature, regional networks often employ specific reference ellipsoids like the 1841 Bessel or the 1924 Hayford models. These older models persist in local surveys because changing the reference surface would require recalculating the coordinates of millions of established boundary stones.
Modern geodesy has moved far beyond historical methods like meridian arc measurements, which Jean Picard used in 1669 to estimate Earth's radius. Today, satellite geodesy and gravimetry provide highly accurate data, allowing for the precise definition of models like WGS84, which is the foundation for modern GPS. These modern ellipsoids are constantly refined by the International Geoscientific Union to reflect the best available data, ensuring that our digital maps remain anchored to the physical reality of our rotating, slightly flattened home.
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Sources
- Summary by Intellect · source: Earth ellipsoid
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