The ancient Greeks knew a thing or two. Nowadays we say Copernicus discovered the earth was round, for example, but the Greeks had that figured out almost two thousand years earlier! How did they manage it? They had several different arguments for the roundness of the earth. One of the best comes from Aristotle himself, who wrote a book called On the Heavens back in 340 B.C. The Greeks were a sea-faring culture, and sailed all over the Mediterranean. And they had noticed, Aristotle tells us, that the north star isn’t always in the same spot overhead. Now remember that all the stars seem to move in a big circle over our heads because the earth itself is spinning. The north star is the center of the circle, and stays put.