Eratosthenes (276–194 BC)

Eratosthenes of Cyrene (born in Cyrene 276 BC, died in Alexandria 194 BC) was the first scholar to use the term geography, meaning by that the description of Earth. His studies pertained to geography, astronomy, and mathematics and allowed him to confirm the hypothesis about the round shape of the planet and to measure its size with an extremely good approximation. He was also the first to imagine dividing Earth's surface into portions demarcated by meridians and parallels. With a deep interest in nature, he studied tides and their lunar cycle, and by observing the presence of marine fossils far from the sea, he realized the slow transformation of the coastlines.

The starting point that made Eratosthenes estimate the measurement of Earth's circumference was the observation that at noon, on the summer solstice, the bottom of wells located in the Egyptian city of Syene were vertically illuminated by the sun, while at the same time in the northern city of Alexandria, objects (in particular he was observing an obelisk) were casting their shadows. After noticing that in Syene the sun was at the zenith (he considered it to be crossed by the Tropic of Cancer) and presupposing that the two cities were located on the same meridian, he calculated that the distance between them was around 5,000 stadia, based on the strength of the observed average speed of the caravans of dromedaries. He also correctly asserted that due to the enormous distance that separates the sun from Earth, its rays could be considered parallel when they reach the surface of the planet. Considering that in Alexandria the beams were not perpendicular to the surface but were inclined at an angle of 7.2° relative to the vertical direction, through trigonometry he got the result that the angular distance between the two cities was 1/50 of Earth's circumference, which let him calculate both its dimension and its distance from the sun. He also succeeded in estimating the inclination of the ecliptic.

As a cartographer, Eratosthenes mapped the river Nile from Khartoum to its delta. He also drew a map of the known world, with Persia at its center and its limits described as follows: Thule (the island, described for the first time by Pytheas, that could be equated with Iceland) in the north, Ethiopia in the south, the Pillars of Hercules in the east, and India in the west. The Caspian Sea was imagined to communicate with the Northern Ocean, and as far as the African continent is concerned, its whole northern part was named Libya. Eratosthenes is also supposed to be the inventor of the armillary sphere (also known as spherical astrolabe), which is a model of the celestial sphere.