Even at a time when we can artificially grow a lamb in a bag and detect gravitational waves rippling through space-time, there are still folks out there grappling with the idea that the Earth is a sphere (albeit a slightly irregularly shaped one).
Fortunately, a bunch of bearded dudes from Greece sorted out this debate long ago, millennia before the convenience of satellites and rockets.
Over 2,300 years ago, there was a great thinker called Aristotle, perhaps best known for his philosophical ponderings with Plato. Along with being a dab hand at politics, poetry, theater, music, natural sciences, and philosophy, Aristotle was also a prodigy of astronomy. Other ancient Greeks had hinted at the idea of a spherical Earth through vague poetic statements, including Plato and Pythagoras, but Aristotle was the first to concretely put it into words.
In the treatise On the Heavens, written way back in 350 BCE, he explained: "Again, our observations of the stars make it evident, not only that the Earth is circular, but also that it is a circle of no great size. For quite a small change of position to south or north causes a manifest alteration of the horizon.
“Indeed there are some stars seen in Egypt and in the neighborhood of Cyprus which are not seen in the northerly regions; and stars, which in the north are never beyond range of observation, in those regions rise and set. All of which goes to show not only that the earth is circular in shape, but also that it is a sphere of no great size.”
Here, we get a sense of how the idea came to be, but you can thank Eratosthenes for really nailing down the theory. Eratosthenes, yet another bearded guy from Greece, was a librarian, mathematician, poet, historian, astronomer, and the “father of geography”.