This article appeared in the
in The Textbook Letter, Volume 12, Number 2.
The Great Wall of China is the only man-made object on Earth that is visible from the Moon.
The Great Wall of China is the only man-made object on Earth that is visible from space.
"The Great Wall of China is so huge that it is one of the few human-made features on Earth visible from space."
I have placed the third statement in quotation marks because I have copied it from McDougal Littell's high-school book World History: Patterns of Interaction (1999).
We can reasonably assume that, in all of those statements, the unexplained term "visible" means: visible to the naked eye. If we use that interpretation of "visible," we find that the first of the listed statements is flatly wrong: Earth has no man-made object that can be perceived, with the unaided eye, by an observer on the Moon. The second and third statements are worse than wrong. They are nonsensical, because the claim that a feature of our planet is "visible from space" has no meaning. Where is "space"?
In an astronomical or astronautical context, the term space denotes all of nature that lies beyond Earth's atmosphere. That denotation suffices in many cases, but it doesn't suffice if we want to examine the claim that a person in "space" can see some particular object on Earth. To evaluate such a claim, we have to decide where -- i.e., at what altitude above Earth's surface -- space starts. This decision must be arbitrary, for the atmosphere has no tangible boundary. As altitude increases, the atmosphere becomes thinner and thinner until it just fades away, so to say.
Over the years, various altitudes -- such as 50 kilometers, 100 kilometers, and 50 miles -- have been used for formally specifying where space begins. The first of those altitudes, 50 kilometers (about 31 miles), reflects a fact of physics: When a person has ascended to 50 kilometers, 99% of the atmosphere lies below him. The second figure, 100 kilometers (about 62 miles), was promulgated by a group of aeronautical experts, in the 1950s, as an estimate of the altitude at which the atmosphere would cease to exert certain effects on the behavior of a space vehicle. The third figure, 50 miles (about 81 kilometers), was adopted by the United States Air Force for the purpose of defining an astronaut. The Air Force holds that any officer who has flown a vehicle to an altitude of 50 miles has flown in space and may wear astronauts' insignia.
If we use any of those three values to establish where space begins, then the Great Wall of China is indeed visible from space. This, however, doesn't make the Wall unusual. There are countless other man-made things show the same characteristic.
The Wall's height varies from 15 feet to 30 feet, and its width varies from 15 feet to 25 feet. Even if the Wall is well illuminated by the Sun and is casting a shadow whose width is equal to the Wall's height, the width of the Wall and the width of its shadow, combined, cannot exceed 55 feet or so. This is considerably less than the width of a six-lane highway. (In the United States, the width of each lane of a typical state highway or Interstate highway is about 12 feet.) Many highways are visible from space, and so are many railroads, airports, race tracks, stadiums, dikes, dams and canals. So are the major pyramids on the Giza Plateau.
To conclude: In making the statement that "The Great Wall of China is so huge that it is one of the few human-made features on Earth visible from space," the writers of McDougal Littell's World History: Patterns of Interaction have just repeated one variant of a vulgar rumor. We needn't be surprised to find that they have done this, for we already know that the "history" in this book includes Hollywood hokum about Robin Hood [see note 1, below], pop-culture fantasies about Mother Teresa of Calcutta [note 2], and popular delusions about the impostor Rigoberta Menchú -- delusions that were promoted by Menchú herself as she created her fraudulent public persona [note 3]. McDougal Littell's writers should consider trying to gather, understand and present legitimate information, instead of parroting stuff that they have overheard at cafeterias and bus stops.
William J. Bennetta is a professional editor, a fellow of the California Academy of Sciences, the president of The Textbook League, and the editor of The Textbook Letter. He writes often about the propagation of quackery, false "science" and false "history" in schoolbooks.
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