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As stars always propagate light, I was thinking that we should see them even during the day. I searched a lot and I find three questions related to the current one.

  1. Why do two beams of light pass through one another without interacting?

  2. Can photons pass through each other?

  3. Star visibility in outer space even during the day?

I couldn't get the answers given to these questions, but I could find some clues!

I guess beams of light pass through one another without interacting (and so we should see stars during the day) because of

  1. This sentence of Anna's answer to the first question: "Thus two light beams have no measurable interactions when crossing"

  2. The "Because" in the beginning of John Duffield's answer to the first question.

  3. This sentence of Anna's answer to the second question: "Thus we can say that for all intents and purposes photons scatter on each other without interacting"

In other hand, I guess we cannot see stars during the day because of this sentence of udiboy1209's answer to the third question: "if you can sustain the heat and the blinding radiation from the sun, you should be able to see stars when you are facing the sun"

May someone please clarify me by (as much as possible) simple explanations?

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  • $\begingroup$ One can see Venus during the daytime, if one knows where to look and the conditions are right (it shouldn't be too close to the sun) and I have done this several times in my life. Observers have reported that one can also see Jupiter and the brightest stars: "On the visibility of Sirius in daylight" by Colin Henshaw, Journal of the British Astronomical Association, vol.94, no.5, p.221-222. $\endgroup$ – CuriousOne Jul 5 '16 at 3:41
  • $\begingroup$ @CuriousOne Thank you because of your attention! But we cannot see most stars during the day while they propagate light. $\endgroup$ – lucas Jul 5 '16 at 3:45
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    $\begingroup$ It's simply a matter of signal to noise ratio. The background brightness of the daylight sky is simply overpowering the brightness of the stars. I am pretty sure that a committed person with a digital camera can take hundreds of pictures of the daylight sky and process them to reject the background enough to discover dozens, if not hundreds of stars in the residual images. I would almost assume that someone has done that already. If not, it would be worth trying. $\endgroup$ – CuriousOne Jul 5 '16 at 3:49
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    $\begingroup$ Of course the light from the stars reach your eyes, they are just not able to separate it from the background unless they are exceptionally bright, like Sirius or, if you accept a planet, Venus and Jupiter. A digital camera doesn't have that problem. If the photon statistics allows the separation, the starlight can be separated from the background. $\endgroup$ – CuriousOne Jul 5 '16 at 5:35
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    $\begingroup$ This is a very common question. Google your exact title : there are 3+ pages of answers. $\endgroup$ – sammy gerbil Jul 5 '16 at 6:31
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The daylight sky has a brightness of about magnitude 3 per square arcsecond. The brightest stars have an integrated intensity of about zeroth magnitude.

If your eyes had an angular resolution approaching 1 arcsecond then you would easily be able to see bright stars in the daylight sky - they would be about 10 times as bright as the sky. Unfortunately, the resolution of the eye is more like 1 arcminute. That means when comparing the starlight to the sky, the star is blurred over an area such that the contrast ratio with the sky is no longer large enough to discern it. However, even with this, if you knew exactly where to look, you could make out the very brightest stars, if your eyesight were good and this is obviously the case of bright objects like Venus, which are visible in the daytime sky.

If you look through a telescope (which increases collection of both starlight and daylight equally) then you can easily see stars. This is because the angular resolution of the telescope is around $1.22\lambda/D$, where $\lambda$ is the wavelength and $D$ the telescope diameter. A 10cm telescope can give you an angular resolution approaching 1 arcsecond (atmospheric conditions permitting) and thus a 3rd magnitude star has a similar brightness to the daytime sky through such a telescope.

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  • $\begingroup$ A simple way to look at exactly the right spot is to start looking at dawn when the star or planet is still easy to see and keep looking as the sky gets brighter and brighter. I've done this with Jupiter a few times. You can also use a telescope on an equatorial mount as a pointing device (I don't have access to one, so I'm limited by the need to get inside to eat breakfast). $\endgroup$ – Count Iblis Jul 5 '16 at 17:52
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Photons interacting or not with each other is pretty much irrelevant for the purposes of this question; in most situations you can basically assume light doesn't interact with itself.

The reason you can't see stars (except one!) during the day is very simple: the atmosphere is too bright. In space this doesn't happen, so you can see stars during the "day" (which is all the time). The atmosphere drowns out the light from the stars, not because photons collide, but because detectors (our eyes) are not sensitive enough to distinguish the light coming from stars. As CurioseOne says in their comment, the brightest planets and stars can be seen during the day if you know where to look.

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  • $\begingroup$ Thank you for reply! May you please explain this more? the atmosphere is too bright $\endgroup$ – lucas Jul 5 '16 at 3:54
  • $\begingroup$ ...just as you can be talking to your friend in a train station, but then a train comes roaring by, and you can no longer hear your friend talking... $\endgroup$ – Steve Byrnes Jul 5 '16 at 14:44

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