Fortunately, a group of beards from Greece solved this problem long before satellites and rockets were convenient to use.
More than 2300 years ago, there was a great thinker named Aristotle, perhaps most famous for his philosophical thinking about Plato. Aristotle is not only an expert in politics, poetry, drama, music, natural science and philosophy, but also a genius in astronomy. Other ancient Greeks proposed that the earth was spherical through vague poetic language, including Plato and Pythagoras, but Aristotle was the first person to concretize it.
In the book "On Heaven" in 350 BC, he wrote: "Our observation of stars once again shows that not only the earth is round, but also it is a circle without large size. For a relatively small position change to the south or north, it will cause a significant change in the horizon.
"There are indeed some stars near Egypt and Cyprus that are invisible in the northern region; In the north, the observation range of stars has never exceeded the observation range. The rise and concentration of these areas not only shows that the earth is round, but also shows that it is a sphere without large size.
How does Eratosthenes estimate the size of the earth?
Here, we know how this idea came into being, but you can thank Eratos for his theory. Eratosthenes is another bearded man from Greece. He is a librarian, mathematician, poet, historian, astronomer and "father of geography".