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What would we see during the day when we look into the sky (other than clouds), if it weren't for Rayleigh scattering making the sky blue? Would the sky be dark, like at night?

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Slightly off-topic, but the sky is still blue at night. It's just that there's much less light being scattered by it, so we can't really see it. Check out this site for some pretty cool photos. The reason it looks blue in the photos is because the exposure is long, letting in enough light to see the 'blueness' –  Brian Funt May 18 at 22:53

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Consider this picture of earth from the moon: earth from the moon

Most other photographs taken from the moon would do. From how earth is illuminated, or the shadows of objects on other photographs, it is clear that we are in broad daylight. The sky it pitch black however.

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It's worth noting that there are no stars in the image because the camera's exposure is set low in order to capture the bright Moon and Earth in the foreground. However, the human eye has a much greater dynamic range than the film with which this image was taken, and I believe that some stars are easily visible during daylight on the Moon. –  Nathaniel May 19 at 0:46
    
Good remark! Even in broad daylight you should be able to see the stars much more clearly than anywhere on earth (except possibly for adaptation of the human eye to the brightness of the sun (and earth), so you should block out reflected light), because the light would reach us unobstructed, and also the sky should be darker where there are no stars. Moreover the stars wouldn't twinkle, which is an atmospheric effect. –  doetoe May 19 at 8:12
    
@Nathaniel Do you have a citation for some stars being visible during daylight? Based on the personal difficulty of spotting Venus during the day and an informal survey done my one of the astronomy mags a few years back that put spotting Mercury in daylight at the threshold of human ability in near perfect conditions I'm skeptical about more than a few stars being visible. –  Dan Neely May 19 at 13:27
    
@Dan Neely: that is near perfect conditions in the presence of an atmosphere –  doetoe May 19 at 13:38
    
@DanNeely if I had a citation I would have given it instead of prefacing the statement with "I believe that...". But Venus is easily visible during daylight as long as it's in your foveal vision. The difficulty in spotting it in your peripheral vision arises from the brightness of the sky surrounding it; on the Moon the surrounding sky is black, so that wouldn't be an issue. Even if you can't see many stars on the Moon when the sun is in your eyes, simply blocking it with your hand would cause them to become visible. (I have no citation for that but would be very surprised if it isn't true.) –  Nathaniel May 19 at 13:45

Yes. And the sun would be brighter and bluer because no light on a direct path from the sun would be scattered away.

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