I will point out that this knowledge was common among the educated populace. The unlettered serfs likely still thought it was flat.
Are you Randall Munroe?? 
Some unlettered serfs today still think it’s flat. 
Not per my understanding (limited as it is 
). The idea of Heliocentricity in India predates the Greeks:
“ It has also been suggested that the idea of a heliocentric solar system first found its voice in Vedic literature, with the Sun often referred to as the “centre of spheres.” The Aitareya Brahmana (9th century BCE) also states:
“The Sun never sets nor rises. When people think the sun is setting, it is not so; they are mistaken. It only changes about after reaching the end of the day and makes night below and day to what is on the other side.””
As I understood it, the big question in the 1480’s was whether it was possible to sail around Africa, or whether that western coastline extended all the way to the south pole. Portugal spend a lot of time and money looking for the route.
The concept that heaven and earth were different was very obvious - there were no on-earth examples of the apparent perpetual motion seen in the heavens, or of the perpetual bright lights from there.
Newton showed that the orbits could be explained by gravity, mathematically. However, he had to do the math to explain how a gravity from a big ball of matter was equivalent to the same force from the center of that ball - for which he invented calculus. Then showed the same force (acceleration) was equivalent both on the surface of the earth and responsible for the moon’s orbit using his explanation of gravity. His numbers for the moon’s orbit apparently were too good, modern physicists think he fudged the numbers a bit. But basically he showed that a gravity 9.81m/s^2 on the earth’s surface also was the force that allowed the moon to orbit in the path it does. By implication, the gravity of the sun also accounted for the planetary orbits, etc.
I’m sure the issue with flat earth is that the no-deep-thinkers of any era never really thought beyond the fact that, with geographic variations, the world was apparently flat. The question was what the intelligentsia were thinking about. As I understand, various cultures, particularly the Greeks, had an obsession with numbers and geometry, and did try to use that to describe the world around them. The question is not whether some Aztec or Incas travelled longer distances, but to what extent the intellectuals of their society did, and whether they cared to measure the sky at each location. The Mayans, for example, were obsessive (from what I gather) about the cycles of the stars and planets in the sky, but did they obsess about why the sky changed as they moved north.
For that matter, how did the Greek philosophers pay their way? If someone has to go from Athens to Alexandria to measure the sun’s angle, who pays for the cost of the trip, in an age when simple food and lodging was the most expensive part of the cost of living? Similarly, how important that a Mayan educated class person go 600 miles north?
I’m re-“reading” the audiobook of James Loewen’s Lies my Teacher Told Me, and one item he just stated was that “people on both sides of the Atlantic knew the earth wasn’t flat”. He therefore says that at least one American civilization knew that the Earth was round, but (at least in the audio edition) he doesn’t say who, or substantiate this with a reference (Although one may exist in the print edition. It’s been too long since I read that, and don’t recall if there was one.)
This page says that Mayas, Aztecs, and Incas appeared to think the Earth was flat:
As cool as that would be if it were true, I can’t in conscience corroborate your understanding. 
After all, there are all sorts of “suggestions” about all sorts of ancient sacred literature worldwide supposedly containing all sorts of metaphorical references to cosmological ideas that happen to agree with modern scientific theories. That doesn’t mean that actual scientific texts in those traditions actually used or endorsed those ideas.
In particular, in ancient and medieval Sanskrit astronomy, there are hundreds if not thousands of surviving technical treatises explicitly discussing the physical form of the universe, and absolutely all of them use the standard geocentric model of pre-modern spherical astronomy. All of them except Āryabhata and maybe a few of his followers also concur with the geostationary nature of this model.
AFAIK the first time any form of heliocentricity shows up explicitly in actual Sanskrit treatises, as opposed to just being speculatively inferred from mystical statements in Vedic texts, is in the school of Nilakantha Somayajin in 15th-c. Kerala (and possibly also through Nilakantha’s teacher Paramesvara). Their work proposed what we might think of as proto-“Tychonic” orbital models where the visible planets, or at least the inferior planets, are orbiting the sun while the sun is orbiting the stationary earth.
But the Kerala school’s innovations were never widely adopted in pre-modern Indian astronomy. The heliocentric hypothesis as we understand it today became established in Indian astronomy after 1700 through the influence of European Copernican/Keplerian/Newtonian theories.
If Greek philosophers did make such an observation, it was most likely done as a side project on a trip made for a different purpose. Generally, philosophy was more of a hobby rather than something people made a living at.
Anyway, Eratosthenes, the guy who computed the diameter of the Earth, used data compiled by bematists, people who measured Egypt for the purposed of taxes and agriculture. And then he assumed that Syene was on the Tropic of Cancer, since there was a factoid around that the sun shown on the bottom of a well in Syene on only one day per year. He didn’t actually go to Syene and do any measurements. He did some at Alexandria where he lived.
Kimstu - thanks for the nice post. I read through more on this and I believe you are absolutely correct. Thank you again
I can see that the intelligentsia of the middle ages were court functionaries or church officials. i assume the same for the intelligentsia, the educated class of the Mayans, Aztecs, and Incas. I don’t recall ever noting anything about a day job for Plato, Aristotle, Pythagoras, Euclid, Socrates, etc. - except IIRC it was Aristotle who was Alexander the Great’s tutor for a while?