Choose a legend: The Great Wall of China is the one of the few man-made structures visible from orbit. Or, more remarkably, it's the only human artifact on Earth visible from the moon. Both are false, say astronauts and remote-sensing specialists. Although the Great Wall spans some 4,500 miles (7,200 kilometers), it's constructed from materials that make it difficult to discern from space.

The unglamorous truth is that the wall is only visible from low orbit under a specific set of weather and lighting conditions. And many other structures that are less spectacular from an earthly vantage point—desert roads, for example—appear more prominent from an orbital perspective.