Why the Earth Is a Squashed Sphere
📐 Measuring the Earth
Question
How do we know the Earth is a slightly squashed sphere - and not a perfect ball, or a flat disc?
Five independent kinds of measurement all agree. (1) Pendulums: a Foucault pendulum (1851, Paris) swings in a fixed plane while the ground turns under it - the rotation rate depends on latitude, zero at the equator, a full turn per day at the pole. (2) Gravity: g is about 0.5% stronger at the poles than the equator, because you are closer to the centre of mass. (3) Arc surveys: the 1735-36 French expeditions to Peru and Lapland showed a degree of latitude is longer near the equator - exactly what a squashed sphere predicts. (4) Rotation: a spinning body bulges at the equator. (5) Satellites: every orbit ever flown is only possible around a ball.
The math
Oblateness f = (a − b)/a = 1/298.257 (WGS-84): equatorial radius a = 6,378.137 km, polar radius b = 6,356.752 km. The equatorial diameter is 42.77 km (~43 km) larger than the polar one. g = 9.780 m/s² at the equator vs 9.832 m/s² at the poles.
Why it matters
A flat-disc model fits none of these measurements. A squashed sphere fits all five - and predicts the exact 43 km difference in diameters that NASA and every surveying agency report.
Linked from
'Ships disappear over the horizon because of distance and angular resolution' Al-Biruni Atmospheric Refraction: Why the Horizon Is Not Where You Think Behind the Curve: The Experiments That Disproved Themselves Coriolis: The Earth's Spin in Your Ballistic Eratosthenes Eratosthenes' Shadow (240 BCE) GRACE: Weighing the Earth with Two Chasing Satellites
+18 more
Great-Circle Navigation: How Ships Actually Cross a Sphere Léon Foucault Measuring the Circumference with Google Maps Polaris Points at Your Latitude Satellite Altimetry: Measuring Sea Level to the Centimetre The Amateur Rockets That Showed the Curve The Coriolis Effect: When It Matters, and When It Doesn't The Final Experiment: The Midnight Sun, Live The High-Altitude Balloon: The Horizon from 30 km Up The horizon is always flat, so the Earth must be flat The Midnight Sun The Obstructed Boat from Pensacola Beach The Salt Flats Are Not Flat - They Are Level The Scale of the Earth VLBI: Measuring the Earth with Radio Waves Watch the Earth Rotate: The Foucault Pendulum We would be flung off by the Earth's rotation Why Planets Are Spheres