It is relatively easy to describe the reference ellipsoid using mathematical formulae. Describing the geoid is much more complex, as it results from precise measurements. The first such models were spheres, used since Ancient Greece. In the 17th century, there were doubts that the earth was a sphere. In 1688, Isaac Newton solved a controversy with Giovanni Domenico Cassini by demonstrating mathematically that the rotation of the earth would lead to a flattening around the area of the poles, and not at the equator. 