5. That the Earth is a sphere.
At a time when not everyone believed so, Eratosthenes clearly had no doubt that the Earth is a sphere [3, p.116]. Many early Greek philosophers held that the Earth was a disc or a cylinder. The idea of a spherical Earth was first suggested by Pythagoras of Samos (580-500 BCE), about 300 years before Eratosthenes [20, p.1]. The spherical Earth model did not begin to gain widespread acceptance among scholars until the well reasoned arguments of Aristotle (384-322 BCE), only about 100 years before Eratosthenes [4, p.59]. Around the time of Eratosthenes, the globe was becoming a popular model of the Earth as many Greek scholars came to accept the idea of a spherical Earth [20, pp.2 ]. As it was the prevailing belief in the scientific community at the time, Eratosthenes is justified in making the implicit assumption that the Earth is a sphere.