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Why is the sky not dark even on "dark" nights?

Ouroboros

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Ouroboros

Ouroboros

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I was observing at around midnight in north Cornwall just before and around the time of the last new Moon. I could see some light on the horizon from Bude about 4 miles away to the south, and from some of the surrounding small hamlets and villages. But apart from that I was 'in the sticks' with no local artificial light whatsoever.

Yet between the stars and away from the Milky Way the sky was not truly dark - even looking out to sea to the North West. (I was near the South West Coast path) There was even enough light for me to see my surroundings.

Where does that light come from? I can only think of starlight, scattered Sun light in the atmosphere and light pollution albeit at a low level. Any other source?

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brantuk

brantuk

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The milky way is overhead so you are looking through least atmosphere. Any light in the area will be reflected from molecules in the atmosphere, and you're looking through a lot more atmosphere the lower your scope is elevated.

The insolation tab on this little planetarium will show you the best times for darkness throughout every day for the whole year :)

http://www.skyviewcafe.com/skyview.php?version=4

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hemihaggis

hemihaggis

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probably just slight light pollution, plus your eyes will adjust to what little light there is. I live literally in the middle of knowhere, and there is near zero light pollution, yet after 20 minutes or so, i can see tree's etc

Define " middle of nowhere " ........

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ollypenrice

ollypenrice

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I live in a site in south eastern France chosen entirely for astronomy and with a zenith scoring about 21.6 on the SQ meter. (You might get 22 in the Atacama desert.) We're at 3000 feet, to boot. However, the night is not totally dark and, once adapted, you can move about with care. I think that starlight, and notably the Milky Way, does light up the sky. It's very true, too, that any humidity lifts and scatters light pollution and the quality of the darkess is greatly enhanced at lower elevations when it is very dry.

Olly

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Macavity

Macavity

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Irrespective of "weather", many locations seem to be guaranteed a thin overlay of spread-out aircraft contrails? During daylight hours, on most days, "custard" skies, seem to outnumber truly BLUE ones - with hard-edged white clouds. On the other hand, I recall the latter indicate presence of abundant water vapour? :)

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acey

acey

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Even in parts of the world that are completely free from man-made light pollution there is natural "skyglow" caused by various factors. In UK, even at the darkest sites, skyglow is mainly down to distant man-made light (possibly beyond the horizon) being scattered by atmospheric molecules. The OP observed 4 miles away from a town of about 10,000 people, which would have been the main contributor to scattered light at the zenith.

Skyglow - Wikipedia, the free encyclopedia

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Bart

Bart

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I wondered this very same question recently, it seemed to me that when I first got a scope in January, the skies seemed darker. I'm in a fairly rural location also. Maybe I'm more demanding now in what I expect to see.

Bart

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jkwhinfrey

jkwhinfrey

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I've noticed that I'll start an observing session in darkness, and towards the end I'm berating the fact the it's so light I feel I could read a book out here!

It's not true of course, but my night vision's certainly improved by then, and the once black(ish) sky is now grey. I proved this to myself by switching on the lights in the kitchen and then going back out to fall over everything in the garden whilst packing up :)

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Ouroboros

Ouroboros

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Even in parts of the world that are completely free from man-made light pollution there is natural "skyglow" caused by various factors. In UK, even at the darkest sites, skyglow is mainly down to distant man-made light (possibly beyond the horizon) being scattered by atmospheric molecules. The OP observed 4 miles away from a town of about 10,000 people, which would have been the main contributor to scattered light at the zenith.

Skyglow - Wikipedia, the free encyclopedia

Thanks for the interesting link. Air glow due to the emission of photons due to photochemistry in the atmosphere hadn't occurred to me.

You're right, Bude is a pain and for me rules out observing towards the southern horizon.

But towards the North is above North Devon - an acknowledged dark sky area - so I was surprised how bright the sky appeared in that direction.

Mind you, we've had a lot of rain of late so the air is very moist. It'll be interesting to see conditions this winter if we have some colder, drier weather.

Thanks everyone for their comments. I've read them all. :)

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cantab

cantab

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I've read that a sign of a really light pollution free site is that clouds appear darker than the night sky, rather than lit up as they do near towns, which implies that the sky itself has some natural brightness.

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Beulah

Beulah

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I've noticed that I'll start an observing session in darkness, and towards the end I'm berating the fact the it's so light I feel I could read a book out here!

It's not true of course, but my night vision's certainly improved by then, and the once black(ish) sky is now grey. I proved this to myself by switching on the lights in the kitchen and then going back out to fall over everything in the garden whilst packing up ;)

Dare I even say that even if I go inside a bright room and back outside again (like grabbing a quick coffee whilst the kettle is still hot...:)), some dark adaption is still retained. I have read that you have to wait at least another half an hour before dark adaption is reached..

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Kai

Kai

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But towards the North is above North Devon - an acknowledged dark sky area - so I was surprised how bright the sky appeared in that direction.

Mind you, we've had a lot of rain of late so the air is very moist. It'll be interesting to see conditions this winter if we have some colder, drier weather.

Remember to the North of Bude you have the lights of Bideford and Barnstaple (and to a lesser degree Wales) they will add to the skyglow

When the air is crisp the skies are very nice and certainly a treat if you suffer from light pollution.

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glowjet

glowjet

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From my earlier years I remember my holidays in the Welsh Black mountains area, where the Milky Way was so bright as to give off its own illumination, something I shall never forget now I, like many others, live in our light polluted environment :)

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laser_jock99

laser_jock99

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I live in a fairly dark area, five miles from the nearest street light, where you are initially struck by the darkness when you first go outside..but after an hour's observing, you can actually see Jupiter casting shadows!

I agree- even the very 'darkest' sites are never actually dark. By 'dark sky' astronomers really mean 'free of artificial light pollution'.

The best way I've found to guage a sites 'darkness' is to go there on a cloudy night. If you can see light on the undersides of the clouds (i.e. reflected from other sources e.g. streetlights many miles away) then the site is not truly dark. Atmospheric haze plays a big part as this can pick up light pollution fronm very distant sources due to it's altitude.

My own location (Mid Wales) is very dark but not LP free in all directions. If it's cloudy (and moonless) I have difficulty seeing where the hills stop and the sky begins. A couple of minutes of dark adaption and then you start to see the divide.

I made a video which clearly shows what happens when clouds drift over a dark sky site. First the sky is alive with stars, when the clouds arrive blackness prevails (best viewed full screen mode in darkened room)!

And the clouds roll in.... on Vimeo

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acey

acey

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Hemihaggis

Up near kielder, surrounded by nothing but a few low tree's and no houses for miles

I was observing in that part of the world last night, at a place where no man-made light source is directly visible, and can testify to what others have said about the role of dark adaptation in judging sky darkness.

I observed from moonset (about 2.30a.m.) until I saw dawn breaking (about 6a.m.), by which time my eyes were completely dark adapted. When I judged the sky to have grown too pale for useful observing I took a meter reading and found the sky brightness to be 19.9 magnitudes per square arcsecond. This was still darker than the majority of urban/suburban sites, with many stars still visible, but not the milky way.

Had I stuck my head out of my own suburban back door and looked at a light-polluted sky of equal brightness, it would have appeared completely black to my unadapted vision: I would have seen stars down to about magnitude 4 or 5 against a black sky. But with dark adapted eyes at a site largely free from light pollution, I saw those same stars against a blue twilit sky.

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  • 4 years later...
glennbech

glennbech

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Interesting topic. And I have a very recent experience. I dragged my gear to my parent's cabin last night and waited for the moon to set. The night sky was vert bright compared to what I am used to at that sight and I thought it was because of the moon. The milky way use to be visible at this site, but not yesterday. I put a few logs in the fire place and dozed off until 01 am, and went back out. The sky was better, but not good even with the moon below the horizon.  

I suspect, like Olly says, that humidity plays a big role. The temperature was -13c (9f), and my scope iced down. 

Can anyone explain relative humidity and dew point and what It means for us astronomers? 

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