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I was in the desert the other day, in an area that was free of light pollution.

While it was easy to see the stars above us, I noticed that near the horizon, even when there is no distance light, we can see almost no stars.

Is this normal? What is causing this? Is it air pollution that covers the Earth and is denser when looking at the horizon? Is it just the density of the atmosphere itself?

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  • $\begingroup$ Are you in any part of the world that could have atmospheric contamination from fires? e.g. Western wildfire smoke is contributing to New York City's worst air quality in 15 years $\endgroup$
    – uhoh
    Aug 15 '21 at 11:03
  • $\begingroup$ @uhoh No, no fire/smoke contamination. $\endgroup$
    – riorio
    Aug 15 '21 at 11:07
  • $\begingroup$ The effect can reach thousands of kilometers. I understand if you don't want to mention a location, but it really may factor in strongly. The effect happens even for clear air, it just becomes more dramatic when the air has particulates. Volcanic eruptions can fill the entire planet's atmosphere with attenuating particles, the Earth even cools measurably. $\endgroup$
    – uhoh
    Aug 15 '21 at 11:13
  • $\begingroup$ As James K points out in his answer, this is a generic and universal effect of the Earth's atmosphere; the basic physics of it was worked on in the 18th Century. Things like light pollution and other forms of pollution can make it worse, but it's always there. $\endgroup$ Aug 15 '21 at 12:08
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    $\begingroup$ More simply: you've probably noticed that the Sun is fainter when it's very close to or on the horizon. Same phenomenon. $\endgroup$ Aug 15 '21 at 12:09
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When you look towards the horizon you are looking through a much greater thickness of air. The air does absorb some light. Dense air near surface absorbs more, and if you look towards the horizon you are looking thought a great distance of dense air.

It is not "pollution" per se, though atmospheric aerosols and smoke can exacerbate the effect. Water vapour also absorbs light, and of course any haze or mist will absorb more.

If there are wildfires, dust or air pollution, the "extinction" of stars will be greater. Smoke can travel very long distances (I've known smoke from wildfires in Spain to have a very noticeable effect in the UK). In deserts, there can be windblown dust that has a similar effect. However, even with pristine air, the extinction of stars near the horizon will still occur.

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    $\begingroup$ @uhoh Pollution could exacerbate it, but the phenomenon will be apparent even in pristine air. $\endgroup$
    – Ryan_L
    Aug 15 '21 at 16:01
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    $\begingroup$ @Voile I don't see how that's relevant. Absorption due to both natural and anthropogenic particles (e.g. wildfires, volcanos, pollution) can be a huge factor. Comments of the form "Nobody cares" don't have any effect on this fact. $\endgroup$
    – uhoh
    Aug 16 '21 at 8:54
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    $\begingroup$ Just returning to the question, I have already mentioned "pollution" (in the form of aerosols and smoke) But please look again at the original question. The misconception is twofold: "Extinction of stars is only due to light pollution.", and "Extinction of stars is due to air pollution". Now light and air pollution do cause stars to be less visible, but the crux of my answer is that extinction will occur even without it. It is not pollution of itself that causes this, but the longer sight line through dense air near the horizon. In the desert, a particular issue is windblown dust. $\endgroup$
    – James K
    Aug 16 '21 at 9:20
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    $\begingroup$ Note that the atmosphere doesn't need to absorb any light to obscure the stars; scattering works too (example: thin cirrus). I suspect scattering is still dominant over absorption in pristine air, but I haven't done the maths. $\endgroup$
    – gerrit
    Aug 16 '21 at 9:30
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    $\begingroup$ For some numbers, there's about 100km of air between the ground and space when looking straight up (0-degrees from zenith). When looking to the horizon at 70-degrees there is about three times as much air and by 80-degrees from zenith you're looking through about six times as much air mass compared to looking straight up. That's a huge difference. $\endgroup$
    – J...
    Aug 16 '21 at 17:20
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Some of the light will also get absorbed in the path. Even though there is no light in the desert, light coming from other distant cities ( even countries) will make more impact as the star is near the horizon compared to stars that are visible clearly. Its not like the air pollution is dense in Horizon, Horizon is just an illusion.

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    $\begingroup$ @uhoh Ha ! Thats some of the vital information to be added in the answer ! :) $\endgroup$ Aug 15 '21 at 11:12
  • $\begingroup$ en.wikipedia.org/wiki/Air_mass_(astronomy)#Calculation $\endgroup$
    – uhoh
    Aug 15 '21 at 11:14
  • $\begingroup$ "this is the scenario only if there is no light in the path-plane of the light coming from the star" -- what does that mean? $\endgroup$ Aug 15 '21 at 12:03
  • $\begingroup$ Extinction due to absorption and scattering of light along the path through the atmosphere is always a factor. $\endgroup$ Aug 15 '21 at 12:04
  • $\begingroup$ @PeterErwin I forgot to Edit that, I added that for a different factor (refraction), but as its impact is too small I removed it, but I forgot to delete some sentences associated with it and its one of them $\endgroup$ Aug 15 '21 at 12:21
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I believe the correct answer is perspective. It's the same reason the full moon can sometimes look gigantic when it is just rising above the horizon, but the same moon looks the size of a dime up in the middle of the vast night sky.

I think what happens is, by comparison to the whole universe that we can see when standing on Earth, a section of that space would have to "scrunch down" to maintain the correct view from earth. Something like this:

demonstrating the effect of perspective and why our horizon appears astralphobic

(blowing up the 1/36th chunk of starry sky also blew up the sizes of the stars so I just hit the really visible stars with a 1x1px and substituted in a black background to demonstrate the effect).

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    $\begingroup$ I don't think this is correct. I couldn't really follow your argument, but the Moon doesn't change apparent size when close to the horizon. The angular diameter of the Moon is always about 0.5°. Maybe I don't understand what you are saying $\endgroup$
    – Prallax
    Aug 16 '21 at 18:33
  • $\begingroup$ The moon doesn't change apparent size when close to the horizon? In Missouri it does. $\endgroup$
    – CWill
    Aug 16 '21 at 18:38
  • $\begingroup$ Ahahah ;) Next time you see the moon, try to compare it with a finger when having the arm extended in front of your eyes. You should be able to see that its size doesn't change. Also, check this out solarsystem.nasa.gov/news/1191/… $\endgroup$
    – Prallax
    Aug 16 '21 at 18:49
  • $\begingroup$ The same is true for the sun...but the article lacks a real explanation, which is disappointing. Perhaps it's as simple as our eyes being more dilated as an object near the horizon is less bright. $\endgroup$
    – rtaft
    Aug 16 '21 at 20:08
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    $\begingroup$ @CWill millions of people en.wikipedia.org/wiki/Moon_illusion $\endgroup$
    – Tim
    Aug 18 '21 at 7:47

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