Measuring about 13,170 miles long, the Great Wall of China is no doubt the longest man-made project on earth. It was once widely believed that it could be seen from space. Some media even reported with affected seriousness, "The biggest building the astronauts could see from space is the wall, which looks very much like a black worm. " Consequently, this statement has gone deep into people's minds and become an honor that Chinese people have enjoyed for years.

Is the Great Wall truly visible from space? Yang Liwei, China's first astronaut who was lifted into outer space by the spacecraft Shenzhou Ⅴ, gave a definite answer "No. " Yang's negative response may probably put out a fair number of people's passion. But it powerfully corrected the misconception. The wall is indeed majestic, but you won't see it from space!

In fact, Neil Alden Armstrong, the American astronaut who first set foot on the moon in 1969, was asked many times whether or not he had seen the Great Wall from the moon. Recently from a sound recording announced by NASA Johnson Space Center, Armstrong said that he had seen the continents, lakes and blue spots touched with red. But he could not make out any man-made objects on earth from the moon.

The Great Wall is narrow and irregular. In space, something irregular is hard to observe. Measuring about 10 metres wide on average, it easily merges (融入) into the surrounding environment. With solely the naked eye, it is totally invisible at a height of 196,850 feet. To watch the wall on the moon is like seeking for a single hair from about 2,688 metres away. To say an astronaut can see it from space is obviously not true.

However, some people introduced an idea to make the Great Wall visible from space. If we install bright spotlights on the wall, astronauts will see the lights at night. But this suggestion has not been carried out by scientific experiments.