It is amazing how persistent urban legends can be, especially when they maintain at least a degree of plausibility. For example, the moon or the sun really seem bigger when we see them at the horizon. We may think it results from a refractive effect through the atmosphere, but it is only an optical illusion. Wherever the moon is located in the sky, it has exactly the same apparent diameter. Take photographs, and use a ruler, but don't trust your brain.

From this moon, I have been told that the only man-made visible structure was the Great Wall of China. I used to spread the rumour myself, because it made some kind of sense. However, the truth is that such a lengthy structure happens to be way too thin for the limited sharpness of the human eye. When one can see the famous wall from space, one can also see many other human structures.

Finally, it seems obvious that the tail of a comet is oriented in the opposite direction of its trajectory. But the reality is that there is no friction in space, so there is no such force to push the tail behind the comet. The only force strong enough to produce the tail is the solar wind. And that is why we can observe that the comet actually always points towards the sun.

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