<p>The city sky is not black. It is so bright it &quot;washes out&quot; the stars.</p>
<p>You discern objects because of contrast between them and their background. Think of a polar bear against the snow. All you see is the eyes and the nose because the bear doesn't contrast with the background.</p>
<p>If you looked at a polar bear in the forest, no such problem,</p>
<p>The city night sky is almost as bright as the stars (measured in lumens per solid angle) so there is no contrast.</p>

CC BY-SA 4.0
2021-11-05T12:34:48.360Z
Also note that many published pictures of the night sky use sensitive equipment, long exposures, and post-processing to bring out colors the human eye may not be able to discern on-site.
notovny
CC BY-SA 4.0
2021-11-05T12:53:34.293Z
Might be worth adding (for clarity) that the washed-out night sky is still much darker than  a dimly-lit room, e.g. .
Carl Witthoft
CC BY-SA 4.0
2021-11-04T19:32:19.703Z
2021-11-04T19:32:19.703Z
Woody
<p>I have seens pics of the night sky in areas that naturally have little population like deserts, cold regions and forests and the night sky is slighly bright and colored by shades of blue, red, purple and yellow with many bright stars in the sky. However in a big city like Rio de Janeiro where I live the night sky is boring it is always black and do not have any stars. Why?</p>

<amateur-observing><night-sky><light-pollution>
Why does light pollution make the night sky completely black without stars?
user44346
# Why does light pollution make the night sky completely black without stars?

(Asked by: username_0 on 2021-11-04)

I have seens pics of the night sky in areas that naturally have little population like deserts, cold regions and forests and the night sky is slighly bright and colored by shades of blue, red, purple and yellow with many bright stars in the sky. However in a big city like Rio de Janeiro where I live the night sky is boring it is always black and do not have any stars. Why?

**Tags:** amateur-observing, night-sky, light-pollution


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## 1 answers

### Answer by username_1 on 2021-11-04. Score: 7

The city sky is not black. It is so bright it "washes out" the stars.

You discern objects because of contrast between them and their background. Think of a polar bear against the snow. All you see is the eyes and the nose because the bear doesn't contrast with the background.

If you looked at a polar bear in the forest, no such problem,

The city night sky is almost as bright as the stars (measured in lumens per solid angle) so there is no contrast.

> **Comments:**
> **username_2**: Also note that many published pictures of the night sky use sensitive equipment, long exposures, and post-processing to bring out colors the human eye may not be able to discern on-site.  
> **username_3**: Might be worth adding (for clarity) that the washed-out night sky is still much darker than  a dimly-lit room, e.g. .  

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