A Reference ellipsoid is an ellipsoid which is used as a frame of reference for geodetic calculations. It is a Figure of the Earth, which is easier to work with than the Geoid. It is relatively easy to describe the reference ellipsoid using mathematical formulae. Describing the geoid is much more complex, as it results form precise measurements. The fist such models were spheres, used since Ancient Greeks' times. 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. In practice this could only be shown by Pierre Bouguer and Alexis-Claude Clairaut half a century later. The two had done an expedition to Peru and Lapland (1735-1741). It was the comparison of the respective findings that could show this. This measurement of the meridian arc lead to the definition of the meter as the 10.0000 part of the idealized distance between the pole and the equator, 1791. Because of different errors in measurement, it turned out to be 0,022 % too short, and was redefined twice, in 1793, and 1799. The value of 1799 is still the official definition; it is about 0,197 ‰ too short. In 1983, the meter was redefined as the distance, light travels in a certain amount of time, in vacuum.