How do we know Earth is round?

We know Earth is round because many ordinary observations all point to a curved planet. Faraway ships vanish from the waterline upward, shadows differ between distant places at the same time, and Earth's shadow on the Moon during eclipses is always rounded. Satellites and astronauts confirm the same shape.

One beautiful clue came from Eratosthenes, who compared sunlight in two Egyptian cities about 2,400 years ago. A well in one place showed overhead sun, while a pole far away made a slanting shadow. From that angle, he estimated Earth's size with astonishingly good math.