evidence: Satellite measurements: Satellites that orbit the Earth can make very precise measurements of the planet's shape. By measuring the Earth's gravitational field and variations in its rotation, satellites have shown that the planet is not a perfect sphere but is slightly flattened at the poles and bulging at the equator.
Gravity measurements: Scientists have used gravimeters to measure the Earth's gravitational field. The strength of the gravitational field varies depending on the distance from the Earth's center, and measurements show that the field is stronger at the poles than at the equator, consistent with an oblate spheroid shape.
Earth's rotation: The Earth's rotation causes it to bulge at the equator and flatten at the poles. This is known as the equatorial bulge, and it has been observed since ancient times. The equatorial bulge causes the Earth's diameter at the equator to be about 43 kilometers (27 miles) greater than its diameter from pole to pole.
Geodetic measurements: Geodetic surveys, which measure the shape and size of the Earth, have also shown that it is an oblate spheroid. These measurements have been made using a variety of methods, including laser ranging, satellite altimetry, and ground-based surveys.
Earth's rotation rate: The rate at which the Earth rotates varies slightly depending on the latitude. This is due to the fact that the Earth's oblate shape causes it to be more difficult to rotate at the equator than at the poles. This effect, known as the polar motion, has been measured and is consistent with an oblate spheroid shape.
TL:DR: Rotation, geodetic, gravity measurements, and satellite measurements cause the equatorial bulge
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