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I have always lived near a large city. There is a stark contrast between the picture linked below for example, and what I see with the naked eye.

Sometimes I can see a few stars here and there, but usually they can be counted on my fingers. I'm looking up at the night sky right now, and there isn't a single visible star!

Question: What creates this large gap in what I can see?

Here's the photograph I was talking about: http://apod.nasa.gov/apod/ap120123.html

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5 Answers

up vote 13 down vote accepted

This is a simple and clear issue, with a unique answer. I see other replies mentioning weather conditions, dark adaptation and so on. That's just so much hand waving, given that the first thing you said was "I've always lived in somewhat large cities".

The core problem here, by a very wide margin, is light pollution if you live in a large city. This is the one factor, above everything else, that affects your ability to see the stars.

Here's a light pollution map:

http://www.jshine.net/astronomy/dark_sky/

The white zones are the worst, and they are in the middle of the cities. Black zones are the best.

Here's a somewhat better (but not perfect) comparison of a dark sky versus light polluted sky (your picture was taken with a very long exposure that doesn't look very realistic):

enter image description here

The dome of light above the city is very visible if imaged from afar:

enter image description here

Long exposure pictures in cities will reveal the orange skyglow, which is the main reason why you can't see the stars - it's like noise masking off the faint light from the distant objects:

enter image description here

Light pollution affects primarily the observations of faint objects, such as nebulae or distant galaxies. Bright objects such as the Moon, the big planets, or some of the bright stars, are not affected by light pollution.

Using a telescope with a large aperture alleviates the effects of light pollution to some extent, but it cannot work miracles. A dark sky is always better.

Usually a 1 hour drive away from the city will bring you in a place with dark sky, free of light pollution - but it depends on several factors. In such a place you should be able to see the Milky Way with your naked eye. The Andromeda galaxy also is visible with the naked eye if the sky is dark enough.

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+1 for demonstrating the problem with pictures. –  Warrick Apr 26 '12 at 16:42
    
Yes, that picture used a high exposure setting. But the first two pictures you've included are very different! And I was looking everywhere for a pollution map like to one you linked to. This is exactly the answer I was hoping for! Thanks. –  Hassan Apr 26 '12 at 18:53
    
@Hassan Go camping far away from any lights and cities. Stay in complete darkness for 20...30 minutes. Then look up. You'll never forget the view. :) –  Florin Andrei Apr 28 '12 at 0:20
    
I would posit that you don't need to stay in complete darkness for 20-30 minutes for this to work. One of my most memorable views of the night sky before I started pursuing astronomy seriously was on a road trip at night in the winter time. We got out of the car to check on someone who had landed themselves in a ditch, and lo, not only a plethora of stars and the milky way, but an obvious difference in the colour of some of the stars. It doesn't help to look up on a clear night, either. Not everyone is so blessed with that regularly. :) –  Ernie Dec 13 '12 at 16:42
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A number of factors:

  • Weather conditions will obviously affect your ability to see the stars. Try on a perfectly clear night.
  • Let your eyes become adjusted to the dark for at least 30 minutes for optimal seeing.
  • Due to the nature of the human visual system, the rod cells concentrated in the outer parts of the eye (peripheral vision) are more sensitive to light than cone cells. This means that you can see stars more easily by not looking directly at them.
  • The image you linked to was obviously taken by a camera which is able to take in more light over an extended period of time than our eyes. The camera can capture more light which means it can see more stars.
  • If you really want to see the stars, get out of the city and find a nice dark area, lay down, and observe.
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Adjustment of the eyes doesn't work if there is lots of light sources like neon signs and street illumination. But +1 for the idea of getting out of the city. –  xmjx Apr 25 '12 at 7:04
    
The main issue is light pollution, given that the OP said they always lived in the city. Not sure why you mentioned the other ones before it - while certainly real, they are by far secondary to light pollution in large cities. Everything else is inconsequential in such places. –  Florin Andrei Apr 26 '12 at 15:34
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I live in a 5 million people conurbation in Germany and even there I can see stars. From my balcony. Heck, the light pollution isn't even bad enough to /not/ see the ring nebula in an 8" reflector. So unless you live right under a neon sign, you should be able to see some stars.

In typical street illumination and under a clear sky you will still see around 5 - 10 of the brightest stars and planets (e.g. Rigel, Betelgeuse, Venus, Jupiter). If you manage to find a "dark" spot - like a back alley - you might see much more. On my balcony (again: 5 million people conurbation) I can see many constellations - also the darker ones. You could use Google Sky Map or similar software to find some stars.

What you will never see in a city, though, is the milky way as on the picture you quoted because it is not bright enough.

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What color is your sky during the day? If it is deep blue, then you should see lots of stars at night even in a large city. If your daytime sky is very light blue though, or worse, pale grey, white, brown, or orange, then you have a problem. City light polution will light up this haze at night and make it impossible to see any stars. Haze isn't due to weather alone, it can come from air polution, pollen, or dust. If these conditions are common or persistent in your area, then seeing stars might take patience. Try a night after a day with an unusually blue sky. Try just after a rain storm has cleaned the air. Try when the wind comes from a different direction, maybe bringing clearer air.

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Thanks for your insight. I actually didn't know about the color of the sky. At the moment, it's 100% gray from cloud cover. On a sunny day though, I'd say it's fairly deep blue, at least at times. The city I live in is pretty spread out. –  Hassan Apr 25 '12 at 19:04
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The picture you showed is a long exposure, which shows many more stars than are visible to the naked eye anywhere on Earth. I currently live on a farm in rural Canada where I can see several thousand stars on a clear night, but not anything like the photograph. I used to live in downtown Toronto, population 6 million, but even there I could see perhaps a hundred stars any clear night. Part of the problem with people who say they can't see any stars is that they don't spend enough time in the dark: it takes up to 20 minutes for the human eye to become fully dark adapted after exiting a brightly lit area.

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Yes, that's probably a major issue, since I don't remember waiting for a long period of time for the stars to appear. I guess it never occurred to me. I'll try it (as soon as it stops raining). –  Hassan Apr 25 '12 at 23:13
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protected by Qmechanic Sep 18 '13 at 13:14

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