Not only was he certain it was not flat, Eratosthenes accurately calculated the radius of the Earth by putting sticks in the desert sand. He did this without the ability to look at it from space, or without the complex understanding of the laws of physics we have today. He did it all just by noticing the shadow had different lengths in different cities.
During summer solstice on the tropics, an upright stick would cast no shadow at noon – the Sun is directly above it. But on the exact same day, to the South or to the North, the same upright stick would have a shadow, as if it was no longer upright. Indeed, the stick was now standing at an angle, for it had descended down the circle of the Earth, and now it was tilted to where it stood back on the tropics.
Much surprisingly, Eratosthenes somehow also managed to calculate the distance between the Earth and the Sun. While there's some difficulty in translating his result from Ancient Greek, it either translates to a figure that's insanely off, or to a figure that's within a percent of the real distance. Since getting it with such precision by chance is a practical impossibility, it stands to reason that the latter translation is indeed correct, and he did in fact know how far away the Sun was.
Every sailor has known for ages that the Earth is round. That's how they navigate the seas without getting lost. To tell where they are, they would measure the angle between the horizon and Polaris – the North star when they are North of the equator. Or to the Southern Cross when they are South of the equator. All the time they were at sea, they knew – and saw – how their ship descended up and down the half-circles of longitude. They saw how the North star descended to a lower angle as they went South, until it disappeared, and the Southern Cross appeared from the other opposite side of their ship. If the Earth was not round, no ship would have ever reached its destination.
A man with little tools living in the desert can know, confirm and measure the curvature of the Earth on his own. A man unfairly stereotyped to be drinking, swearing and fighting for a living, can – in the middle of the endless ocean – repeat these same measurements daily to tell you exactly where they are. If a person armed only with the stars, angle-measurer and a clock can tell you exactly where you are, yet all you see is a flat ocean in every direction, they probably know a thing or two about the shape of the Earth that you might not.
I'm also pretty sure that if they can do that, it should inspire you to do better when you're armed with the Internet, all the information in the world, all the photographic evidence that you need, and more technology at your fingertips than anybody in Earth's past.
During summer solstice on the tropics, an upright stick would cast no shadow at noon – the Sun is directly above it. But on the exact same day, to the South or to the North, the same upright stick would have a shadow, as if it was no longer upright. Indeed, the stick was now standing at an angle, for it had descended down the circle of the Earth, and now it was tilted to where it stood back on the tropics.
Much surprisingly, Eratosthenes somehow also managed to calculate the distance between the Earth and the Sun. While there's some difficulty in translating his result from Ancient Greek, it either translates to a figure that's insanely off, or to a figure that's within a percent of the real distance. Since getting it with such precision by chance is a practical impossibility, it stands to reason that the latter translation is indeed correct, and he did in fact know how far away the Sun was.
Every sailor has known for ages that the Earth is round. That's how they navigate the seas without getting lost. To tell where they are, they would measure the angle between the horizon and Polaris – the North star when they are North of the equator. Or to the Southern Cross when they are South of the equator. All the time they were at sea, they knew – and saw – how their ship descended up and down the half-circles of longitude. They saw how the North star descended to a lower angle as they went South, until it disappeared, and the Southern Cross appeared from the other opposite side of their ship. If the Earth was not round, no ship would have ever reached its destination.
A man with little tools living in the desert can know, confirm and measure the curvature of the Earth on his own. A man unfairly stereotyped to be drinking, swearing and fighting for a living, can – in the middle of the endless ocean – repeat these same measurements daily to tell you exactly where they are. If a person armed only with the stars, angle-measurer and a clock can tell you exactly where you are, yet all you see is a flat ocean in every direction, they probably know a thing or two about the shape of the Earth that you might not.
I'm also pretty sure that if they can do that, it should inspire you to do better when you're armed with the Internet, all the information in the world, all the photographic evidence that you need, and more technology at your fingertips than anybody in Earth's past.
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