He was also the first person to use the word “Geography”. He also is known to have realized that Earth could be located with a basic grid of lines called Longitude and latitude.
Eratosthenes was the first scientific Geographer who ascertained the length of equator on sound principles. By measuring the lengths of shadows in the Egyptian cities Alexandria and Aswan , together with applying some simple mathematical calculations, Eratosthenes was able as early as 3rd century B.C. to determine the circumference of the earth as 252,000 stadia, which comes to be anywhere between 24,660 and 28,965 miles, roughly equivalent to 39,690 and 46,620 km. These values are based upon some assumptions as the correct length of stadion is not known. By modern calculations, Earth's circumference is 40,075 km, and thus the calculations made 2000 years back, are worth commendable.