A Moment of Science

A Well Full of Stars

There’s an urban legend that says: If you look at the sky from the bottom of a well, you can see stars, even in daytime. After trying it myself I found this legend to be entirely false.

The theory behind this legend goes like this: Daytime is bright, due to the sun. If you view a sunless piece of sky through a long tunnel, there should be no way for the sunlight to get into your eyes. Therefore, you should see stars. The reason this doesn’t work has to do with how sunlight diffuses through the atmosphere. Sunlight doesn’t just shine directly onto your eyes from the sun. A lot of light is scattered by the atoms of the upper atmosphere like marbles in a child’s game. This scattering causes some of the sun’s light to shine out of every square inch of the daytime sky.

Poke a pinhole through a piece of black crepe paper, then hold it up to the sky. The amount of scattered sunlight shining through the pinhole will be brighter than almost every star. A couple of the very brightest stars are actually a little brighter than a pinhole of daytime sky, but they would be almost impossible to make out. There is almost no contrast between the bright star and the surrounding bright sky.

To all you kids out there, don’t try this experiment yourselves! Wells are very dangerous places.

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