Today, scientists measure Earth by improving on a method developed by another Greek polymath, Eratosthenes, around 250 B.C. Eratosthenes used trigonometry to determine the distance between Aswan and Alexandria by measuring the position of the sun at the same time in both locations. He used this calculation and basic geometry, building on Aristotle's earlier ideas, to calculate the circumference of Earth to within 1% accuracy — staggering for over 2,000 years ago!
This practice formed the basis of geodesy, the science of measuring Earth's size and fundamental properties, such as its geometric shape, its orientation in space, and its gravity field.