Is the Great Wall of China visible from the moon?

 By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

10 February 2013

After reading last week’s article, one reader commented that “It’s like saying a microorganism lands on you and you start wobbling because of its weight!” If you missed the piece, it was investigating whether the mass displaced during the construction of large structures can affect the rotational motion of the Earth.

The reader’s comment made me wonder whether the two situations are comparable. The mass of a typical microorganism, say, an amoeba is about 10 billionths of a gram and an adult human being weighs about 75kg. Therefore the weight ratio is 1-to-75-billion. Thus, in fact, the effect of the water in the Three Gorges Dam on the movement of the earth is much less than that of an amoeba “sitting” on a man!

Another reader was not convinced that the Great Wall of China cannot be seen from the moon. Now, there are two interesting facts about this myth. First of all, it was started somewhere in the year 1754 – over 200 years before the Neil Armstrong landed on the moon! Secondly, no lunar astronaut has ever claimed to have seen the wall from the moon. Nevertheless, the legend persists; so let’s interrogate it using some figures.

Two numbers are important for this interrogation: the distance to the moon from Earth (384,000km) and the resolving power of the human eye (one minute of an arc). While the first one is self-explanatory, the second figure one needs some clarification.

We know that circle is divided into 360 degrees. Now each degree is subdivided into 60 minutes of arc. Therefore, a minute of arc is an angle of one 60th of a degree, or 0.01667 degree…very small indeed.

The human eye can only distinguish an object if the lines from the extremities make an angle greater than 0.01667 degree. Anything small than that appears like a point. Now the farther away an object is, the small this angle becomes. This explains why distant stars look like bright spots in the sky even though they are millions of kilometres in width.

At over 6,000km, the Great Wall of China is quite long, but it is not very wide: it measure about 9m. Thus when viewed from the moon (384,000km away), the width makes and angle of 0.0000013 degree. This is much smaller than the resolution limit of the human eye. Therefore, the Great Wall cannot be seen from the moon.

A comparison can be made with a stand of human hair. Its average width is about 0.1mm. Viewing a 9m-wide wall from 384,000km is similar to looking at piece of hair from 4.5m away. Do you think you would be able to see it? Well, there is only one way to find out: try it!

It turns out that the only distinguishable features on Earth from the moon are those measuring at least 50km in width; but even those are just barely visible. The Great Wall disappears when one reaches about 70km above the ground. That’s not very far; is it?