Gleaned from the web:<br /><br />The credit is usually given to the Greek philosopher Pythagoras and his school, although several of his contemporaries also believed the Earth was round. <br /><br />Almost 2000 years before Columbus was born, these ancient Greeks argued that because the moon was a shphere and the sun was a sphere, the Earth should be a sphere. <br /><br />Some 300 years after Pythagoras wrote about Earth being round, another Greek, Eratosthenes, calculated the Earth’s radius. He used the distance between two cities and the angle of the sun’s rays made at midsummer at noon. His measurement turned out to be 15 percent too large, but still impressive, considering it was done over 2600 years ago. <br /><br />One way you can see for yourself that Earth is round is by watching a lunar eclipse. A lunar eclipse is when Earth passes between the sun and the moon. Earth then blocks the light of the sun and Earth casts a shadow on the moon. If you watch as the shadow slowly passes over the moon, you will see that the shadow of Earth is as round as the Earth is. <br /><br />----------<br /><br />IMO: Notions that the Earth was a sphere predate written history - the clues are all around us! I would also argue that this was an intuitive guess and there were probably just as many counter-arguments that the Earth was flat!