Eratosthene measured the earth diameter somewhere around 200BC
https://en.wikipedia.org/wiki/Eratosthenes#Measurement_of_the_Earth's_circumference

A better question to ask is when circular earth became dominant belief. That, I am not aware of good documentation. That said, Columbus knew the earth to be round so it wasn't super obscure knowledge.

NASA has a short history here:  https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question54.html

On 26/01/2018 11:33 AM, Steffen Nurpmeso wrote:
John Haxby <john.haxby@oracle.com> wrote:
 |> On 26 Jan 2018, at 07:35, Paul Eggert <eggert@cs.ucla.edu> wrote:
 |> On 01/24/2018 10:33 PM, Clive D.W. Feather wrote:
 |>> Robert Elz said:
 |>> 
 |>>> Once upon a time, the world was always flat, everyone knew that,
 |>>> the pope even proclaimed it...
 |>>> 
 |>> Cite?
 |
 |A spherical earth pre-dates a Pope by quite a few centuries.  One of \
 |the better known Greeks (whose name escapes me) even estimated the \
 |diameter of the earth.  A flat earth is a relatively new fad.

Now i am anything but a christian, yet i would support the
philosophical reasoning.

 |> I doubt whether any Roman Catholic pope proclaimed the Earth flat \
 |> as a matter of doctrine. That being said, flat-Earthers are still \



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