Geodesy and the Shape of the Earth

Geodesy is the scientific discipline concerned with precisely measuring and representing the Earth's shape, orientation in space, and gravitational field. The earliest recorded scientific attempt to measure the Earth's size is credited to Eratosthenes of Cyrene, who around 240 BCE compared the angle of the sun's shadow at noon on the summer solstice in Alexandria with the sun being directly overhead at Syene (modern Aswan), and combined this angular difference with the known north-south distance between the two cities to estimate the Earth's circumference; depending on the exact length of the ancient "stadion" unit he used, his result corresponds to a figure remarkably close to the modern value of approximately 40,075 kilometers at the equator. Centuries later, Isaac Newton, in the Principia (1687), argued on theoretical grounds that the Earth's rotation should cause it to bulge slightly at the equator and flatten at the poles, producing an oblate spheroid rather than a perfect sphere, a prediction confirmed empirically through 18th-century French geodetic expeditions: Pierre Louis Maupertuis's 1736 expedition to Lapland and Charles Marie de La Condamine's 1735–1744 expedition to Peru (in present-day Ecuador) measured the length of a degree of meridian arc at high and low latitudes respectively, finding degrees longer near the poles as Newton's theory predicted.