Not so much a theory of the universe as a simple picture of the planet we call home, the flat-earth model proposed that Earth’s surface was level. Although everyday experience makes this seem a reasonable assumption, direct observation of nature shows the real world isn’t that simple. For instance, when a sailing ship heads into port, the first part that becomes visible is the crow’s-nest, followed by the sails, and then the bow of the ship. If the Earth were flat, the entire ship would come into view at once as soon as it came close enough to shore.
The Greek philosopher Aristotle provided two more reasons why the Earth was round. First, he noted that Earth’s shadow always took a circular bite out of the moon during a lunar eclipse, which would only be possible with a spherical Earth. (If the Earth were a disk, its shadow would appear as an elongated ellipse at least during part of the eclipse.) Second, Aristotle knew that people who journeyed north saw the North Star ascend higher in the sky, while those heading south saw the North Star sink. On a flat Earth, the positions of the stars wouldn’t vary with a person’s location.
Despite these arguments, which won over most of the world’s educated citizens, belief in a flat Earth persisted among many others. Not until explorers first circumnavigated the globe in the 16th century did those beliefs begin to die out. Yet a diehard core of flat-earth believers persisted even past the days of the Apollo Moon missions and those glorious images of a spherical Earth suspended against the blackness of space.