Observing from a well
Published on Feb 19, 2007 at 2:05 pm.
6 Comments.
Filed under astronomy, sky lore.
Can you see stars in the sky during the daytime from a deep well (or chimney, or similar structure)? The answer is no. Not too long ago, I ran into this misconception again. It’s been around for a very, very long time. It’s wrong, but people still pass it along, so I thought that I’d say a few words about it.
Stars are up all the time, day and night. But, of course, in the daytime, the sky is bright and so you can’t see the stars. So, according to legend, if you are in a deep well, then if you look up you’ll be able to see a dark sky and stars. Of course, it is totally wrong, but this story has been passed down for ages. Obviously, it’s been around for a while. After all, how many deep wells do you run across in modern society?
So, why is this story out there, and why doesn’t it work? First of all, I am guessing that it is dark deep in the well, so perhaps people were thinking that it being dark meant that the sky would be dark, too. Well, it doesn’t work that way! The sky is not bright because the observer is in the light! I guess that misconception must be what is behind this idea.
The sky is bright and blue because it scatters sunlight. This occurs in the sky, not in the observer’s eyes. So, it makes no difference if the observer is in the sunlight, shade, or a dark room looking out a tiny hole. The sunlight is scattered by molecules in the atmosphere in a process known as Rayleigh scattering (named after Lord Rayleigh). The amount of scattering depends upon the size of the molecules and the wavelength of light. The scattering is strongly dependent upon the diameter of the particle doing the scattering. So, an atmosphere composed of different gasses might have different scattering properties. But, the scattering is done high over your head, so when you look at the sky from a deep well, you are still looking through the same part of the sky that was scattering light as someone who is standing next to the well. The sky looks blue because blue light is scattered more than red light. The scattering is inversely proportional to the wavelength of light to the fourth power: λ-4. Since blue light is shorter wavelength than red light, it is scattered more, making the sky appear blue from all that scattered light. Other light is also scattered, though. This is why you can get sunlight shining through a window even if the sun does not directly shine into the window. And, likewise, the scattered light would shine into a well, so you would not see black sky overhead.
So, this myth is false. I am surprised, though, at how persistent it seems to be.
-Astroprof
A Ler…-- Rastos de Luz on February 19, 2007 at 3:20 pm: 1
[…] “Observing from a well“, no Astroprof’s Page. Será que se conseguem observar as estrelas, em pleno dia, quando se observa o céu do fundo de um poço? […]
regehr on February 20, 2007 at 10:51 am: 2
I do not think that your argument is convincing. A quantitative argument seems to be required: the amount of scattered light must overwhelm the brightness of the brightest star, given the built-in constraints of our eyes and visual processing. This begs other questions. How bright would a point source have to be, to be visible during the day? Can stars be imaged during the day using photographic techniques?
Astroprof on February 20, 2007 at 11:15 am: 3
You’re right, a quantitative argument would be more convincing. But, I was writing for the layman here. But the point is that observing from a well doesn’t make the sky black so that you can see stars.
As for seeing stars in the daytime, you really can do that, if you know where to look. Venus and Jupiter can readily be seen during the day if the sky is very dry and clear, they aren’t too close to the Sun, and you know where to look. Sirius can be seen, too. I’ve seen dimmer stars with optical aids in the daylight. I’m not sure what the limit is, though. That is an interesting question, though.
But, the premise of the myth is that observing from a well makes the sky look like night, and that simply is false.
regehr on February 20, 2007 at 11:19 am: 4
Thanks! Couple more questions while I’m at it:
The scattered light should be much less intense above 8000 meters. Can stars can be seen during the day from Everest?
Does the well help even a little bit? Certainly it would permit the iris to dilate, increasing sensitivity.
Astroprof on February 20, 2007 at 11:29 am: 5
Yes! Higher elevations make the whole spotting stars in the daylight far easier. It is not all uncommon to see bright stars in the daytime from very high flying aircraft. And the highest flying aircraft even see nearly black skies, and a lot of stars.
Dilating the iris doesn’t really help, since you are still looking at a sky that is just as bright, and it is a contrast issue. But, the well would help in keeping you from being distracted, so that if Venus just happened to be right over the well, you’d have your attention focussed on it. But, that’s about all the help that you get.
Bev Currie on April 9, 2008 at 1:19 am: 6
All this is theoretical. I note none of these experts actually when down a well to look for themselves. I am a farmer that has helped dig quite a few wells and I have seen something that looked like stars to me and the well wasn’t over forty ft. deep. My father said he could see the stars while he was down the well and so could I.