Up to the middle of the 17th century, the Earth was considered to be perfectly spherical. However, the flattening of the planet Jupiter had been observed in the previous century.
In 1672, during an expedition to Cayenne, the capital of French Guiana, organized by the Paris Academy of Science for the purpose of measuring the Earth's radius, it was observed that in Cayenne the period of oscillation of the same pendulum was different from what it was in Paris.
This unexpected result could only be caused by a decrease in the acceleration of gravity; this decrease, as Christian Huygens (1629-1695) immediately hypothesized, must be due to centrifugal force.
Indeed, while Paris lies at approximately 48 degrees north latitude, Cayenne is only at 5 degrees and therefore closer to the equator. Thus, the same body has a longer circumference near the equator than farther north and undergoes greater centrifugal force in Cayenne than in Paris.
Huygens suspected that the Earth too must be flattened at the poles and he, and Isaac Newton (1643-1727), supposing that the Earth was fluid in origin, calculated the value.
It is understandable that centrifugal force, which is maximum at the equator and null at the poles, could cause a widening of the Earth at the equator and a consequent flattening at the poles in its primitive fluid state.