Is Great Wall visible from the moon?
Popular beliefs ranging from Ripley’s Believe It or Not!’s cartoons from 1930s, which claimed that the Great Wall is “the mightiest work of man, the only one that would be visible to the human eye from the moon,” to Richard Halliburton’s 1938 book Second Book of Marvels which makes a similar claim, have persisted, assuming urban legend status, and sometimes even appearing in school textbooks. The Great Wall is a maximum 9 meter wide and is about the same color as the soil surrounding it. Based on the optics of resolving power (distance versus the width of the iris: a few millimeters for the human eye, meters for large telescopes) only an object of reasonable contrast to its surroundings 70 miles or more in diameter would be visible to the unaided eye from the moon, whose average distance from Earth is 384,393 km. The apparent width of the Great Wall from the moon is the same as that of a human hair viewed from 2 miles away. Unsurprisingly, no lunar astronaut has ever claimed seeing the Great Wall from the moon. A more controversial question is whether the Wall is visible from low earth orbit, i.e., an altitude of as little as 100 miles (160 km). In October 2003, Chinese astronaut Yang Liwei stated that he had not been able to see the Great Wall of China.