Eli5: How does light pollution affect our ability to see stars?


When there is less light pollution, you can see loads of stars, but when there is a lot of light pollution, you see much less stars. What is the science behind this?

In: 3

Close and brighter light source overwhelms the distant one.

We technically can for the most part see the moon all day long but because the sun is so bright it obscures the moon.

If the air were perfectly clear, light pollution wouldn’t affect you. But since air isn’t perfectly clear, the nearby light reflects off of the particles in the air, creating that “glow” you see around light sources or over cities. That glow drowns out the distant stars, because the glow is brighter than the tiny amount of starlight that manages to make its way to Earth from that far away.

The ultimate example is how the sun prevents us from seeing the stars during the day. Essentially, our air is slightly blue and you can see this if you shine a light through it. Human light pollution is much dimmer than the sun but there’s still an effect.

Another factor is the limited range of brightnesses that our eyes can see at any one time. In a dark environment, chemical changes in our eye increase sensitivity, allowing us to see much dimmer things. This can take 20 to 30 minutes but a single glance at a bright light will ruin the effect and your dark adaptation has to start again. Even if nearby lights in the city didn’t brighten the sky, they’d still stop your eyes from adjusting fully to see dim objects.