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Bortle 1 vs. 2.

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#1 Bluefish

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Posted 15 February 2024 - 10:25 AM

I visited a bortle 2 sky last october in Northern Wisconsin, and a bortle 1 sky in Big Bend last week.  Both under new moons.  Both according to many websites (clearoutside.com, cleardarksky.com and the GOES satellites were perfect conditions.  No clouds at any level, low humidity, no dust.  Solid blue across the board.

 

It’s probably a result of my brain, but maybe you can help.  I found the Bortle 2 sky in Wisconsin  more interesting.  Meaning, I think I saw more stars.  It seemed more amazing. Northern wisconsin was at 600 ft. Above sea level, Big Bend was 4500 ft.  Big bend has lower humidity and is desert, Wisconsin was forest, except out over the lake.

 

Could this be a result of an October sky with Andromeda at the zenith with Perseus and its surrounding areas blasting?  In Big bend, andromeda was about 35 degrees and setting in the NW.  The forest of Wisconsin blocked much of the views behind us to the west, whereas in Big Bend, it was mostly horizon to horizon except to the South with a mountain blocking views.  I just didn't see much close to the horizon in BB but in wisconsin I remember seeing stars right off the dark lake to the East (Orion rising).  Interestingly,  I believe I was seeing a very strong zodiacal light in BB after sunset.

 

Could this be mind trickery or possibly very similar conditions where there was just more to see in an October sky?  I don't know my sky, so not sure which season shows the most stars.  This was all naked eye and binoculars only, no SQM meter, but I assume both were 22 (ish) since wisconsin was off season where most all locals are gone.  Interestingly, I found Big Bend fairly populated now around Terlingua, without a forest, every bulb is seen for miles.  We were far outside that area of local lights in that area.

 

Thanks for your help.


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#2 KBHornblower

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Posted 15 February 2024 - 10:37 AM

I am venturing a guess that dust around Big Bend was reducing the transparency.  John Bortle in his visual prime would have rated the Wisconsin sky as superior on these occasions.  Be wary of online sources taking his name in vain on the basis of some supposedly fixed model.


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#3 Jon Isaacs

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Posted 15 February 2024 - 10:38 AM

How did you determine the Bortle rating? 

 

A Bortle rating is a specific set of criteria that an individual observer uses to rate a site on a particular night.    Bortle raings do not come from the internet charts.  This page covers John Bortle's rating system:

 

https://wapps.umt.ed...k-Sky Scale.pdf

 

This thread has some good stuff in it:

 

https://www.cloudyni...g-skies-by-sqm/

 

Jon


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#4 Bluefish

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Posted 15 February 2024 - 10:45 AM

Hi Jon,  I just go off the map since I’m not really qualified to make judgements on what I’m seeing.  Wisconsin just seemed darker?  


Edited by Bluefish, 15 February 2024 - 10:49 AM.


#5 Sandy Swede

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Posted 15 February 2024 - 11:50 AM

How did you determine the Bortle rating? 

 

A Bortle rating is a specific set of criteria that an individual observer uses to rate a site on a particular night.    Bortle raings do not come from the internet charts.  This page covers John Bortle's rating system:

 

https://wapps.umt.ed...k-Sky Scale.pdf

 

This thread has some good stuff in it:

 

https://www.cloudyni...g-skies-by-sqm/

 

Jon

Thanks for posting these links, Jon.



#6 moefuzz

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Posted 15 February 2024 - 12:18 PM

Higher altitude gets you further away from the dreaded humidity that causes poor unsettled skies. That probably isnt a concern this time of year but come summertime the higher altitude will offer clearer more settled seeing. ... I live within an hour or so of both a bortle 1 and a bortle 2 site but generally head for the higher altitude of bortle 2 as it is also closer.

Last Sept on a cool well settled night we saw the flaming star nebula naked eye. Amazing clear settled conditions.



#7 icomet

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Posted 15 February 2024 - 12:32 PM

I'd like to know where you were standing in Wisconsin, seeing as the surface of Lake Superior 

is 601 ft. above sea level. 

 

Just saying. 

 

Clear Skies. 



#8 Bluefish

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Posted 15 February 2024 - 12:55 PM

Yes, I’m off by 23 feet.

#9 Sebastian_Sajaroff

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Posted 15 February 2024 - 01:07 PM

Bortle scale is mostly about darkness and transparency, not on how interesting are the targets we’re observing.
For example, as a naked eye observer, I’d prefer a Bortle 3 view of Cygnus to a Bortle 1 view of Hydra.
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#10 icomet

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Posted 15 February 2024 - 01:30 PM

Yes, I’m off by 23 feet.

I'll take a guess. Cornucopia?

 

Clear Skies. 



#11 Keith Rivich

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Posted 15 February 2024 - 08:14 PM

I'm guessing its the different environments. In Wisconsin with the trees there is a lot of contrast between the sky, which is bright, and the trees, which are dark.  Out in the wide open the sky will appear brighter then it actually is as there is no real contrast comparison. Another factor may be the summer milky way is still visible in October while in February its replaced by the fainter winter milky way.  

 

An SQM-L probably would given you your answer. 


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#12 weis14

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Posted 15 February 2024 - 08:43 PM

Hi Jon,  I just go off the map since I’m not really qualified to make judgements on what I’m seeing.  Wisconsin just seemed darker?  

It probably was.  What others are saying is that the Bortle rating of a sky isn't fixed.  It can change in a matter of minutes.  Dust in Big Bend probably reduced the transparency significantly making it dark, but with poor transparency.  Humidity, dust, smoke, pollen all contribute to what you can see. 

 

I've never observed in northern Wisconsin or Big Bend, but in northern Michigan I've seen the summer Milky Way cast a shadow.  That was a special night.  I've observed dozens of times at the same site since and while the light pollution hasn't changed, I've never had skies that perfect there again.  



#13 Jon Isaacs

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Posted 16 February 2024 - 08:57 AM

Hi Jon,  I just go off the map since I’m not really qualified to make judgements on what I’m seeing.  Wisconsin just seemed darker?  

 

As Keith said, different times of the year, different environments.  If you using Light Pollution Maps.info for your Bortle ratings, Bortle Class 1 measures 22.00 mpsas.  Bortle class 2 covers 21.90mpsas to 21.99 mpsas.  There is really no difference if these numbers had been measured just because of night to night differences in the sky.  Add in the errors in the LPM estimates, the supposed in difference is meaningless.

 

A true Bortle rating involves observing certain objects and phenomenon to estimate just how dark the sky is at a particular location and moment in time.  

 

Please do not use the Bortle rating unless you have actually determined it by observation.  It just causes confusion.. 

 

Jon


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#14 MEE

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Posted 16 February 2024 - 10:37 AM

Bortle 2 vs Bortle 1 (actual observed conditions) can typically be judged by:

1. The visibility of the zodiacal band (faint extension of the zodiacal light) - in Bortle 2, it extends down to about 45 degrees; in Bortle 1, it extends almost to the horizon

2. The width of the Milky Way: the MW will appear broader (not longer) in Bortle 1 conditions vs Class 2

#15 KBHornblower

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Posted 16 February 2024 - 11:30 AM

As Keith said, different times of the year, different environments.  If you using Light Pollution Maps.info for your Bortle ratings, Bortle Class 1 measures 22.00 mpsas.  Bortle class 2 covers 21.90mpsas to 21.99 mpsas.  There is really no difference if these numbers had been measured just because of night to night differences in the sky.  Add in the errors in the LPM estimates, the supposed in difference is meaningless.

 

A true Bortle rating involves observing certain objects and phenomenon to estimate just how dark the sky is at a particular location and moment in time.  

 

Please do not use the Bortle rating unless you have actually determined it by observation.  It just causes confusion.. 

 

Jon

John Bortle lost about half a magnitude in his limit for stars in going from his personal class 1 to class 2.  I would be surprised if brightening the sky by 0.1 magnitude or less would cause that much loss with the stars.

 

Once again, I think some of these people are taking his name in vain while using questionable data and models.


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#16 Tony Flanders

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Posted 16 February 2024 - 03:21 PM

Those map and apps that rashly attempt to estimate Bortle classes usually choose Bortle 2 for sites where the estimated zenithal artificial skyglow is between 1% and 10% as strong as the natural skyglow. In other words, at the very worst sites estimated to be Bortle 2, the zenith should be only 10% brighter than it would otherwise be. That's for all practical purposes undetectable by the human eye.

 

It's true that down low, looking directly toward the brightest light dome, the difference is going to be quite a bit bigger than that. Nonetheless, it's a fair bet that for sites rated either Bortle 1 or 2, artificial light pollution is going to be a very minor consideration, and which site is better is going to depend on other factors, such as typical transparency and how much of the sky is visible.


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#17 Nerd1

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Posted 16 February 2024 - 07:15 PM

I've seen it so dark at Elephant mountain, you could literally not see your hand in front of your face. Elephant mountain is about halfway between Alpine and Big Bend. Maybe Terlingua is ruining BBs skies. 



#18 Astro-Master

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Posted 17 February 2024 - 02:20 AM

In July of 2021 at 9,500ft at 2:00am I stepped out of my motorhome to a sky filled with so many stars all the way down to the horizon.  The Milky Way was blazingly bright overhead casting my shadow on the ground.  MY SQM (not a SQM-L) was reading 21.95 MPSAS.  The Light Pollution map showed I was in a black zone.

 

I looked for a skyglow on the horizons, but there was none!  The transparency was simply amazing at 9,500ft, the Milky Way in Cygnus was resolved into a myriad of tiny points of light, and in between the stars wherever you looked the sky was pitch black, I thought to myself this is about as good as it gets unless you're in outer space.

 

I hope to revisit that place this Summer, I'm curious if the sky will be as good with the Sun near Solar Maximum and all the volcanic eruptions we have had in the last few years.


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#19 Redbetter

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Posted 17 February 2024 - 05:33 AM

With the base level of natural air glow fluctuating far more than light pollution during the past two years of solar maxiumum, there is not much of any conclusion to be drawn.  I've seen what should have been very good Bortle 2 nights in dark sky running a paltry 21.4 mpsas as a result (this even with some of the darkest blue I have seen overhead in the same day in clear skies--enough that we took photos of it.)  I doubt either of the night readings you experienced would have been near 22.

 

I seriously doubt this had anything to do with dust.


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#20 Keith Rivich

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Posted 17 February 2024 - 10:56 AM

I've seen it so dark at Elephant mountain, you could literally not see your hand in front of your face. Elephant mountain is about halfway between Alpine and Big Bend. Maybe Terlingua is ruining BBs skies. 

I have never experienced this when outdoors under the darkest of skies. Even at TSP before the oil fields populated the Permian I can always walk around at night without a flashlight, except for some of the more treacherous areas. The exception here is when its completely cloudy and the sky cannot contribute any light. Very dark then!

 

Inside the hotel or bunkhouse, yes. Its dark enough my eyes would start creating their own flashes and sparkles. Kind of weird. 


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#21 Jon Isaacs

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Posted 17 February 2024 - 11:49 AM

Those map and apps that rashly attempt to estimate Bortle classes usually choose Bortle 2 for sites where the estimated zenithal artificial skyglow is between 1% and 10% as strong as the natural skyglow. In other words, at the very worst sites estimated to be Bortle 2, the zenith should be only 10% brighter than it would otherwise be. That's for all practical purposes undetectable by the human eye.

 

It's true that down low, looking directly toward the brightest light dome, the difference is going to be quite a bit bigger than that. Nonetheless, it's a fair bet that for sites rated either Bortle 1 or 2, artificial light pollution is going to be a very minor consideration, and which site is better is going to depend on other factors, such as typical transparency and how much of the sky is visible.

:waytogo:

 

One thing I keep in mind:  A Bortle rating is for a particular moment in time.  One night, a particular site might be rated Bortle 1, another night, Bortle 3. 

 

And as Red pointed out with the current conditions, the skies are brighter than normal.

 

Personally, I just measure the sky brightness, guess at the transparency and seeing and enjoy the view.

 

Jon



#22 George N

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Posted 21 February 2024 - 12:52 PM

Forgetting about "light pollution" - as was mentioned above natural sky glow can make a difference - but maybe the impact of 'local' light too??

 

Then there's how much your view is impacted by local light. I find the sky (and telescope views) look darker when wearing an observing hood or just a 'baseball' cap with a brim.

 

I remember an old S&T (??) write-up by Walter Scott Houston noting that he could see dimmer stars when looking up thru a hole in the forest canopy versus walking out in the open (from one of his Mexico observing locations). He attributed the difference to blocking extraneous light. He suggested trying to observe a small area of the sky thru a cardboard tube (paper towel for example) versus 'uncovered' face - to see if you can see somewhat dimmer stars. Never tried it myself - but again, I do prefer using an observing hood, at least for many-hour observing at a dark sky site.


Edited by George N, 21 February 2024 - 12:53 PM.

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#23 WillR

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Posted 21 February 2024 - 04:24 PM

I'd like to know where you were standing in Wisconsin, seeing as the surface of Lake Superior 

is 601 ft. above sea level. 

 

Just saying. 

 

Clear Skies. 

Interesting and surprising. Googling it gave me 579-582' at the shore of the Lake Michigan. But point taken.

 

We tend to think of Wisconsin as flat and equate that with low. For example, the lowest point in Kansas is 679' above sea level. The entire state of Florida is less than 345' above sea level.



#24 moefuzz

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Posted 21 February 2024 - 05:58 PM

I normally reside at over 3600 feet and my 2 goto B1 and B2 sights are around 2800' and 3100' respectively.

This makes for well settled skies once the sun goes down as humidity is virtually nonexistent and the nearest city is well over 100 miles to the west.

During the summer the biggest difficulty is either the moon or the arson set fires that have become more and more persistent during the last dozen years.



#25 Redbetter

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Posted 22 February 2024 - 01:10 AM

Interesting and surprising. Googling it gave me 579-582' at the shore of the Lake Michigan. But point taken.

 

We tend to think of Wisconsin as flat and equate that with low. For example, the lowest point in Kansas is 679' above sea level. The entire state of Florida is less than 345' above sea level.

 

And the highest point in Kansas is "Mount Sunflower" at 4,039 feet.  It is barely a low rise on the plain about 2,000 feet from the Colorado border.  I lived in the same county for a couple of years as a kid.  The state is relatively flat, but more like a tilted plane, gently sloping down from west to east.




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