Eötvös Effect: Evidence of Spherical, Rotating Earth

Eötvös effect is the change in perceived gravitational acceleration when moving eastward vs westward. An object will weigh more when moving westward than when it is stationary or moving eastward.

The effect was discovered by Loránd Eötvös in the 1900s after noticing the difference in gravity measurement on moving ships. He noticed that the readings were lower when the boat moved eastwards, higher when it moved westward. He identified this as primarily a consequence of the rotation of the Earth.

The rotation of the Earth causes centrifugal acceleration away from the Earth’s axis of rotation. Along the equator, the centrifugal acceleration is about 0.03 m/s².

When moving eastward, the object is moving in the same direction as the Earth’s rotation, and results in larger effective velocity. This results in higher centrifugal acceleration.

Conversely, moving westward means moving against Earth’s rotation. The velocities cancel each other and result in smaller effective velocity. This results in smaller centrifugal acceleration.

Meanwhile, Earth’s gravitational acceleration stays constant in either case. The difference in centrifugal acceleration when moving eastward and westward results in small but measurable difference in downward acceleration. An object will weigh more when moving westward, and less when moving eastward.

Practical Example

Let’s say we are going to fly in a flight above the equator. We’ll take a 1000 gram weight with us, and before taking off, we calibrate our scale so it shows 1000 gram at sea level. If we are cruising at 925 km/hour at the altitude of 12500m, then our scale will measure 991 gram if the plane is flying eastward. On the other hand, it will measure 999 gram when the plane is flying westward. There’s a difference of 8 gram as the consequence of the Eötvös effect.

Other than the Eötvös effect, the altitude of the aircraft results in smaller gravitational force, as it is farther from the Earth’s center of gravity when it is flying.

Calculation here was done using the online calculator Centrifugal and Gravitational Acceleration in an Aircraft by Walter Bislin.

An Experiment