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You know.. at some places of Earth, the night sky looks like this: (right?)Milkway galaxy seen from Earth

However, you might definitely know that, at some other places, the night sky looks like this:Night sky of a city. No stars to be seen
(source: themarkeworld.com)

Have you seen any stars from the last one? So, I was wondering, what are exactly the right conditions, such that the night sky will be like in the first image? To be able to see plenty of stars? Evidently needs to have no clouds. But, what else?

It needs thin atmosphere? No city lights? No suspended pollutants? Needs that certain molecules are not there? No light scattering? No light pollution? So, what are the some of the important conditions that has to happen? And why?

Some remarks:

Now, this answer blames on light pollution from cities. However, during a general blackout some time ago, which sizable areas had no light (over 700Km radius), sure, I could see more stars than before, but never such a great sky. So, maybe light pollution has its contribution, but it is the only factor? Besides, this answer itself acknowledges in the end, that this depends on several factors. And so, I guess I wish a more complete answer.

Glorfindel
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Physicist137
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2 Answers2

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There are a few factors affecting the clarity of the sky:

  1. Clouds

Obviously, they block out the stars.

  1. Surrounding lights

Basically, since the light from the stars is very faint(it comes from really far away). The light from cities, streetlamps, and so on, kind of "washes" it out. While you may not be looking directly at the light source, it is scattered in the atmosphere. This means that, similar to the blue sky during the day time, wherever you look light is coming at your eyes and fainter stars are hidden. That's why, you can only see stars in rural areas these days.

QuIcKmAtHs
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You pretty much hit all the points. This is why Chile and Hawaii make such great locations for telescopes since they have areas that are high up so the atmosphere is thinner and are far from civilisation whish reduces light and atmospheric pollution. Chile especially is very dry due to the Andes mountains which means cloudy nights are very rare.

The reason you want a thin atmosphere and few atmospheric pollutants is because this reduces the amount of radiation that is absorbed and scattered by our atmosphere meaning we can detect more frequencies and with greater accuracy (even with our eyes). However as previously mentioned the perfect conditions don't exist on earth which is why we have space telescopes.

cal
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