[Lunar Eclipse in Estonia, October 28, 2004, photographed by Epukas]
[Source: Wikimedia Commons]
The idea spread throughout the Hellenistic world and in the 3rd century CE, Eratosthenes of Cyrene (now Libya) made an attempt to calculate the circumference of the Earth. Depending on whether he used the Egyptian or Attic stade as a unit of measure (he doesn’t say), he was only off by 2%-17% and any error can be attributed to him having the wrong numbers.**
[Ship at horizon, March 2006, photographed by Anton, source: Wikimedia Commons]
The idea of a spherical Earth was so commonly accepted within the Roman Empire that both Cicero and Pliny the Elder refer to it as fact quite offhandedly. By this point no one that we now know of in Roman or Hellenistic lands or the societies that came out of them was arguing that the Earth was flat.
*This doesn’t mean that the Greeks were the first or only people to figure this out. This is not only a problem of what evidence has survived, but also what documents people have studied and circulated.
**I never learned this in history class in school. No, I heard of it in one of my upper level math classes. For shame.
Plato, Phaedo 108-109 - Perseus
Aristotle, De caelo Part 2 - MIT.edu (Search "spherical" and you'll find the reference.)
Aristotle, Meteorology Part 1 - MIT.edu
Aristotle, Meteorology Part 2 - MIT.edu
Eratosthenes - Wikipedia
Philolaus - Wikipedia
Aristarchos of Samos - Wikipedia
Seleukos of Seleukia - Wikipedia
Cicero, Somnium Scipionis - Tertullian.org (The exact reference starts in paragraph 8)
Pliny the Elder, Natural History 2.2 - Perseus
Claudius Ptolemy, Almagest - Central Connecticut State University
Spherical Earth - Wikipedia