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Sky seems darkest from midnight until sunrise

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#1 CapPic

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Posted 01 November 2012 - 06:04 PM

Where I live, the sky doesn't feel dark for quite some time after the sun goes down. In fact, it doesn't feel dark until after midnight. This darkness persists until the sun comes up.

Is this the effect of light pollution and people finally turning off lights as it gets later? Or is there some other explanation?

#2 Tony Flanders

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Posted 01 November 2012 - 06:49 PM

Where I live, the sky doesn't feel dark for quite some time after the sun goes down. In fact, it doesn't feel dark until after midnight. This darkness persists until the sun comes up.

Is this the effect of light pollution and people finally turning off lights as it gets later?


Yes, it's almost entirely due to light pollution. It's not true at a pristine location, though airglow may wane a little as the night progresses.

I think the biggest single factor is that people stop driving. Automobile headlights constitute a huge chunk of all light pollution.

The effect can be quite dramatic, by the way. At typical suburban and semi-rural locations, the sky can be twice as bright at 9 p.m. as at 2 a.m.

#3 FirstSight

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Posted 01 November 2012 - 07:01 PM

Another significant factor where the light dome of a metro area is visible is that a significant-enough portion of commercial lighting in many areas switches off (at least partially) sometime between 11pm and midnight to make a noticeable difference in sky darkness. At one of our club's primary observing sites SW of the Raleigh-Durham-Chapel Hill area, the reduction in LP from that direction is often quite dramatic about that time, and I noticed the effect as well (though not quite as dramatic) while observing with fellow CN member JayinUt an hour SW of Salt Lake City at Vernon, Utah.

#4 Tony Flanders

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Posted 02 November 2012 - 04:55 AM

Another significant factor where the light dome of a metro area is visible is that a significant-enough portion of commercial lighting in many areas switches off (at least partially) sometime between 11pm and midnight to make a noticeable difference in sky darkness.


Right, business lights go out continuously from about 9 pm to midnight. Automobile dealers are undoubtedly the biggest individual light sources. You can tell when their lights go out for miles around.

The other truly huge individual contributions to light pollution -- the only ones that have grown significantly over the past decade in my part of the world -- are lighted outdoor ball fields.

But all of those are out by 11 pm or midnight. The continuing improvement after that can only be due to car headlights.

#5 JayinUT

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Posted 02 November 2012 - 07:51 AM

An article from July 2010 from Cornell discusses the impact of light pollution over time at Kitt Peak. In the article it states:

"We also noticed a significant decrease in the sky brightness towards Tucson as the night progressed. Figure 5 shows the sky spectrum in 2010 February at both three hours after sunset and nine hours after sunset. A comparison of the sky spectrum taken in 2009 October at two hours and eight hours after sunset shows a similar result. Since all of the observations were taken when the sun was greater than 18 degrees below the horizon, we believe this change is due to households and businesses switching off their lights rather than the progression of twilight."

Here is a link to the article which is a good read. It confirms that for this region, the authors believe the reduction was due to both businesses and households turning off their lights.

#6 FirstSight

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Posted 02 November 2012 - 11:24 AM

The other truly huge individual contributions to light pollution -- the only ones that have grown significantly over the past decade in my part of the world -- are lighted outdoor ball fields.


I personally have deeply conflicted interests about lighted outdoor ball fields. Refereeing soccer games provides the discretionary income that has enabled me to purchase every item in my considerable collection of scopes, eyepieces, and other astro equipment. But a significant portion of those games (and among the best-paying ones) are high school varsity games and a portion of the adult games I work. In fact, on Wednesday night I was center referee for a late-afternoon boy's varsity playoff game that had to be moved from its originally scheduled site to another because of the risk of insufficient daylight to finish should the game go to overtime. The adult night games I do go from 8pm until 11pm.

One thing I have noticed is that many of the newer, better-financed school facilities use lighting that, while far from benign to the night sky, is nevertheless much more efficiently directed and shielded (in part for long-range economic savings, in part to mitigate objections from neighboring residential communities about light trespass).

Nevertheless, I cannot help but think about the conflict every time I step into a lighted facility to do a game. Even on the clearest nights only one or two of the brightest stars and perhaps Jupiter are visible at best from the nearby vicinity of a lighted field.

#7 derangedhermit

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Posted 02 November 2012 - 05:56 PM

Nevertheless, I cannot help but think about the conflict every time I step into a lighted facility to do a game. Even on the clearest nights only one or two of the brightest stars and perhaps Jupiter are visible at best from the nearby vicinity of a lighted field.

As long as the lights are shielded to cut off unwanted light, and the fields are not lighted when not in use, I see no problem. It's unreasonable to expect amateur astronomy or a simple preference for a dark sky to everywhere override all other human activities.

The ball fields in a city park near my house have a promising observing field behind them. I went to scout it a couple nights ago. The gates to the park are locked at night, and the lights are on at all 8 (unused) fields. I think I need to talk to the city.

#8 Tony Flanders

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Posted 02 November 2012 - 08:17 PM

As long as the lights are shielded to cut off unwanted light, and the fields are not lighted when not in use, I see no problem. It's unreasonable to expect amateur astronomy or a simple preference for a dark sky to everywhere override all other human activities.


Yes, it's unreasonable to expect amateur astronomy to override all other interests.

On the other hand ballfield lights certainly are a major problem even when they're properly shielded. And that's not true just for amateur astronomers; it's also true for all the people who might just happen to look up and notice the stars -- but never will because they're not visible.

So there's definitely a conflict here.

As a parent, I sympathize with the kids who want to play sports. On the other hand, I can't help thinking that perhaps life was better in the good old days when people didn't try to cram 18 hours of activity into their 16 waking hours each day.

That's especially true for kids, who lives are grossly overscheduled and overpressured.

#9 Daniel Guzas

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Posted 03 November 2012 - 12:21 PM

Living in Boston there is a HUGE difference when there is a ball game at Fenway.

Game on, the sky to the south west is ablaze in white. Game off, huge difference. I must take a photo some time. Comsidering its a historis ball park I think everyone is just accustomed to it. But it is something they should consider.

Those are really powerful lights. Fortunately they aren't on every night and are dark all winter.

#10 George N

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Posted 04 November 2012 - 10:52 AM

Living in Boston there is a HUGE difference when there is a ball game at Fenway....... Fortunately they aren't on every night and are dark all winter.


Of course if you live were I do, in ski country, the ski resorts have their mountain sides lit at night, and those bright lights are shining down on artificial snow even when there generally is none. There is one ski center in northeast PA that can be seen for many miles when driving on I-81 thru the mountains.

#11 George N

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Posted 04 November 2012 - 10:57 AM

One effect that may be unique to my area (but I’ve also seen it at Stellafane) is river valley fog. At least 85% of the population (and lights) in my area are concentrated in two river valleys, and they fog up at times (usually around midnight, or a little later). Once the low areas (and most of the lights) are fogged over, it gets much darker out in the hills and mountains.

#12 Daniel Guzas

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Posted 04 November 2012 - 01:55 PM

Living in Boston there is a HUGE difference when there is a ball game at Fenway....... Fortunately they aren't on every night and are dark all winter.


Of course if you live were I do, in ski country, the ski resorts have their mountain sides lit at night, and those bright lights are shining down on artificial snow even when there generally is none. There is one ski center in northeast PA that can be seen for many miles when driving on I-81 thru the mountains.



Ugh ski areas at night! That would be a huge issue up at my cabin! Thankfully night skiing is not the norm up in the mountains. I didn't think of that. I can only imagine the white lights reflecting off that white snow right up into the sky! What a bummer!

At least your summers should be dark :smirk:






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