The great 'invisible' wall of China

London, Feb 24 (UNI) Though it may break many a Chinese heart, but the fact that the Great Wall of China was the only artefact on Earth visible from the moon, is just a myth.

According to astronauts and remote-sensing specialists, even as the Great Wall runs some 7,200 kilometers long, it is constructed from material that makes it difficult to discern from space. The bitter truth that experts believe is that it was only visible from low orbit under a specific set of weather and lighting conditions such as when the sun was low on the horizon.

It caused the wall to cast extended shadows that made it possible to discern its silhouette.

The 'fable' about the barrier's visibility dates back to a 1932 Ripley's Believe It or Not! cartoon that claimed ''the mightiest work of man'' was visible to the human eye from the moon.

When Neil Armstrong returned from the moon in 1969, he admitted that he could not make out any man-made structures from the lunar surface.

''The problem is that the human eye is most sensitive to contrast, and the color of the wall is not that different from the ground on either side of it,'' Journal Scientific American quoted former NASA astronaut Jeffrey Hoffman, who could not spot the Wall during his five space shuttle missions, as saying.

He, however, was able to identify roads, airport runways and irrigation ditches simply because they stood out in their environments.

''Moderate-resolution satellites could pick up the structure under specific weather conditions such as when snow covers the fields near the wall and snow has been cleared on the wall, and that allows us to see the wall,'' Ronald Beck of the USGS's Land Remote Sensing Programme said, adding, ''The key is contrast.'' Since China's first man in space, Yang Liwei, returned in 2003 and admitted not seeing the Great Wall, scholars have asked for more research and improve astronaut training.

''Some astronauts have said that they didn't see it, but that doesn't mean it isn't there. A shuttle passes by so quickly,'' Wei Chengjie of the Sciences Institute of Remote Sensing Application said.

Meanwhile, industrialisation and pall of pollution is further forcing the Wall to fade from view.

UNI XC SYU SZ RAI0901