I’m not knowledgeable about it, but if I was guessing I’d say that was probably the main “proof” that the world was not round.
Knowing that the earth was spherical and understanding gravity are two different things.
That’s a powerful thought.
The Aztecs, or at least Nahuatl-speakers, had a trading outpost as far south as Panama. I don’t know how far north they traded.
The Incan Empire extended over 2,500 miles north-south, and there were trading vessels that sailed along the coast. However, it was mostly south of the Equator, and there is no Pole Star at the South Pole.
There are some extremely tenuous indications that they might have had some contact with the Ancestral Puebloans (formerly known as Anasazi) in what is today northern New Mexico and southern Colorado.
Closer to 1500 BC than to 1500 years ago, I think, though at that point it’s the Lapita not yet called Polynesians:
The ancestors of the Polynesians, the Lapita people, set out from Taiwan and settled Remote Oceania between 1100-900 BCE, although there is evidence of Lapita settlements in the Bismarck Archipelago as early as 2000 BCE.
In the mid-2nd millennium BC, a distinctive culture appeared suddenly in north-west Melanesia, in the Bismarck Archipelago, the chain of islands forming a great arch from New Britain to the Admiralty Islands. [ . . . ] Within a mere three or four centuries between about 1300 and 900 BC, the Lapita culture spread 6000 km further to the east from the Bismarck Archipelago, until it reached as far as Tonga and Samoa
Aristotlean theories of gravity incorporate the notion of “downwards” as meaning “towards the center of the Earth.” The idea as to why this happened was totally wacky (objects seek their natural place and for most heavy objects that place is the ground) but they understood that there isn’t a unitary direction “below the Earth” that things fall towards.
The Greeks would have also been well aware of about where the equator was, which is pretty easy to figure out once you know the diameter of the Earth. They didn’t make it to the Southern Hemisphere themselves but they knew people who did (there were lots of Greeks in India and lots of Indian contact with present-day Indonesia, lots of Greeks in Egypt and lots of trade networks extending further south into Africa, etc.) “No one has ever reported that they trade with a nation of upside-down people” would have registered as a relevant fact.
To add to @ZosterSandstorm’s post:
In medieval Europe, there were various different theories and speculations.
A popular one was that the rest of the earth was covered with water, and the only land was in the north. But there was no question that the water was spherical, and ‘downward’ was in different directions around the sphere.
An interesting short text, worth reading, is Sacrobosco’s On the Sphere of the World, written about 1230.
It’s written as an introductory text to astronomy for students, and summarises briefly and very clearly the all basics of astronomical knowledge at the time. It was popular, and widely used as an introductory text in universities across Europe.
SURFACE OF THE SEA SPHERICAL. – That the water has a bulge and is approximately round is shown thus: Let a signal be set up on the seacoast and a ship leave port and sail away so far that the eye of a person standing at the foot of the mast can no longer discern the signal. Yet if the ship is stopped, the eye of the same person, if he has climbed to the top of the mast, will see the signal clearly. Yet the eye of a person at the bottom of the mast ought to see the signal better than he who is at the top, as is shown by drawing straight lines from both to the signal. And there is no other explanation of this thing than the bulge of the water. For all other impediments are excluded, such as clouds and rising vapors.
Also, since water is a homogeneous body, the whole will act the same as its parts. But parts of water, as happens in the case of little drops and dew on herbs, naturally seek a round shape. Therefore, the whole, of which they are parts, will do so.
Sacrobosco says that the area around the equator is probably uninhabitable because of the heat, but that the northern area between the Arctic Circle and the Tropic of Cancer is habitable, similarly the southern area between the Antarctic Circle and the Tropic of Capricorn is ‘habitable’. This implies at least the possibility of people living there.
He gives the size of the earth accurately, but also notes that the size of the earth must be infinitesimal or the size of a point, compared to the size of the whole universe.
Herodotus reported on a Phoenician expedition that supposedly circumnavigated Africa over the course of three years. He was skeptical of the voyage because when they were in southern Africa they observed the sun to their north, although that observation would tend to confirm it.
This falls into the category ‘not even wrong’. The Pre-columbians had no idea that there was anything unusual in the bowl of the Big Dipper, because there isn’t anything unusual at the centre of the bowl. There are many black holes in that region, all of them at the centre of very distant galaxies, and none of them unusual in any significant way. There are also many many other black holes in that region, outside the bowl and throughout the night sky. Only two of these galaxies are visible to the naked eye (Triangulum and Andromeda) and two minor galaxies (the Magellanic Clouds) and none of these are near the bowl of the Big Dipper.
Not even wrong.
That’s a striking idea that’s obvious once said. But it’s easy to overlook when a layman’s just trying to generalize back in time using “Our worldview but with less detailed knowledge” as their framework.
The diagram @Colibri posted in post #31 clearly shows the heavens and the Earth being assembled very differently. And no sign that anyone thought that difference was at all queer or unexpected.
Having a pole star makes it easier to point out the difference in a moment, but as long as you have any stars at all (which both hemispheres do), it’s fairly easy for anyone with any familiarity with the stars to see changes of latitude. When I moved from Cleveland to Philadelphia for college, even that fairly small difference of latitude was noticeable to me, and not from Polaris’ altitude.
[Emphasis mine; Citation needed]
Yes, that issue is a really good example of being careful not to project our own assumptions into the past.
I wonder how many people can really, accurately explain how Newtonian mechanics implies Keplerian orbits - I’m guessing if you sample the American adult population around two thirds won’t be able to say anything accurate about it at all, maybe 20% will regurgitate something approximating the basic formulas for projectile or satellite motion but be unable to give a three-sentence summary of how they are related, and a little over 10% will truly get it. This is one of the key notions that I think most societies would never have gotten to on their own prior to the environment of the Enlightenment.
Kepler’s Second Law is just the conservation of angular momentum, and Kepler’s Third Law is easy to show mathematically, at least for the case of circular orbits, from Newton’s gravitational law and centripetal acceleration. But even if you gave me unlimited scratch paper and a full day to work on it, I’m not sure I could deliver a derivation of the First Law, that orbits are ellipses with the Sun at one focus, from Newtonian principles. I mean, given a derivation someone else had done, I could follow along and even explain it step-by-step, but I couldn’t easily generate it from scratch.
I was referring more to the concept of an orbit as a never-ending freefall with the planet receding behind it, but the mathematics are also very modern - Kepler had to anticipate certain developments in calculus to prove his empirical laws and you need Newton and Leibniz to really do it rigorously. It’s a clear dividing line between what I think a devoted Mayan physicist could have worked out on his own and what wasn’t possible without the intervening centuries of scientific advancement.
“If I have seen further it is by standing on the shoulders of giants.”
– Isaac Newton
From the Indian context
Aryabhatta, 476–550 CE, was the first to observe the spherical nature of earth :
“ Aryabhata explicitly mentioned that the Earth rotates about its axis, thereby causing what appears to be an apparent westward motion of the stars.[15] In his book, Aryabhata, he suggested that the Earth was sphere, containing a circumference of 24,835 miles (39,967 km).[21]Aryabhata also mentioned that reflected sunlight is the cause behind the shining of the Moon.[15]”
Very unlikely that it wasn’t known to Indian scientists well before his date, actually. We have later fragments of Sanskrit treatises using spherical cosmological models and trigonometry that are almost certainly no later than second or third century CE.
That came about because Hellenistic spherical astronomy (along with its main selling point, horoscopic astrology) entered South Asia at some point during the Roman Empire, via the Indo-Greek states in the northwest of the subcontinent, and was combined with existing Indian calendric astronomy. All of these developments had already reached a highly mature stage by the birth of Āryabhata in 476 CE.
What does seem to be an innovation of Āryabhata’s, or at least a contemporary notion of which his work provides the earliest surviving record, is the inclusion of the hypothesis of the earth’s rotation in the standard geocentric spherical model. This had been suggested earlier (along with the bizarre notion of heliocentricity!) by Aristarchus of Samos.
But neither Aristarchus nor Āryabhata AFAIK ever made any significant converts to the georotatory hypothesis among ancient and medieval astronomers. Astronomers would readily adopt the sphericity hypothesis because it was necessary to make the whole system of quantitative astronomical prediction concerning local appearances of celestial phenomena work (especially for events like eclipses). And with a bit of outside-the-box thinking it was not that difficult for a trained professional to conceive the spherical model as a coherent and self-consistent whole, once you skated over the intuitively awkward concept of objects being all over the surface of a spherical earth without falling off it.
But the idea that the whole big lump of mud was actually spinning around at a rate of multiple stades per minute, much less flying through space even faster, without any of us inhabitants ever feeling the breeze from it, or seeing birds or projectiles in the air being whirled away westward? Bish please. 
Georotation and heliocentricity were the crazy crackpot speculations of pre-modern astronomy. A few nutters visionaries like Āryabhata or Aristarchus might consider them plausible and even elegant, but they simply could not prevail over the vast amounts of what seemed at the time to be irrefutable common-sense physical evidence contradicting them.
In one text I’ve read, there are some stories from the Norse that suggest Columbus came north and talked to them about their high-northern trips. It’s highly speculative, though.
Columbus says that he visited Iceland (Thule), as part of a trading voyage from Bristol. However, if he heard of visits to Greenland or beyond he never mentioned them otherwise.