在太空能看见长城吗？

English (for the teacher)

In October 2003, China’s first astronaut, Yang Liwei, had only just come back down to Earth when someone asked him the question so many people wanted to ask: up in space, did you see the Great Wall? Yang gave an honest answer: the Earth looked very beautiful from space, but as for the Great Wall, he hadn’t seen it.

His answer took a lot of people by surprise. For decades, many Chinese people had firmly believed that the Great Wall was just about the only man-made structure you could see from space with the naked eye, and some had even read it in their Chinese textbooks at school.

In fact, the claim is much older than space travel. In 1754, an English scholar wrote in a letter that China’s Great Wall could be made out even from the Moon. In 1932, Ripley’s Believe It or Not!, a newspaper cartoon column devoted to strange and curious facts, put it even more firmly: the Great Wall was the mightiest work of man, and the only one visible to the human eye from the Moon. When those words were written, not a single person had ever left the Earth.

In 1969, humans finally landed on the Moon, and the claim could at last be put to the test. One astronaut who walked on the Moon’s surface later recalled that, standing on the Moon and looking back at the Earth, all he could make out were great stretches of white, which were clouds, a few patches of blue, a little yellow, and now and then a small patch of green. At that distance, he said, nothing made by people could be seen at all.

The reason is not complicated. The Great Wall really is long: a survey published in 2012 put the total length of the walls built across the dynasties at more than 21,000 kilometres. But it is surprisingly narrow: on the Badaling section in Beijing, for example, the wall is only about six metres wide. The Moon is about 380,000 kilometres away, so looking at the Great Wall from there would be like trying to make out a single hair from several kilometres away. What’s more, the wall follows the rise and fall of the mountains, and its colour is almost the same as the rock and soil around it, so even from a bit closer it is hard to pick out from the background.

The year after Yang Liwei’s return, a newspaper reported that the Ministry of Education had told the publisher to stop printing one of the texts in the sixth-grade primary school Chinese textbook. In that text, an astronaut says that from space he could make out two great works on Earth with the naked eye: one a sea dyke in the Netherlands, the other China’s Great Wall. The paper added that printing such a mistake in a primary school textbook was probably the main reason the claim had spread so widely.

So what about from a space station a few hundred kilometres above the ground? Here the answer is less clear-cut. According to NASA, at that height you generally can’t see the Great Wall with the naked eye, though in radar images some sections are actually quite easy to pick out. Cameras do better than eyes, too. In November 2004, an American astronaut on the International Space Station took a photo with a telephoto lens, and many people later came to believe it showed the Great Wall. The funny part is that he said he had not seen the wall with his own eyes when he took it, and at the time he could not say for sure whether it was in the photo at all.

So, can you see the Great Wall from space? From the Moon, certainly not; from a space station, very hard with the naked eye; with a telephoto lens, you might just catch it in a photo. If you really want a good look at the Great Wall, the best way is still to go and climb it yourself. After all, the saying goes “You’re not a real hero until you’ve reached the Great Wall”: it’s the Great Wall you’re meant to reach, not the Moon.