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How useful is the Bortle scale ?


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According to Clear Outside, I image in Bortle 5 sky.

Under no circumstances am I ever able to see the Milky Way from my back yard. This would indicate that my conditions are more like a Bortle 7/8.

I imagine that because I am surrounded by houses/street lights etc and these make a considerable difference to my specific viewing conditions and add at least 2 to the Bortle scale in my back yard.

 

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Not sure how this works but I am supposed to be Bortle 6 but can easily see the Milky Way, in fact its so dark sometimes I cant even see my feet.

Alan

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Posted (edited)

I’m in Southampton. Last year ClearOutside had me in Bortle 8. Now it says Bortle 7. The sky definitely hasn’t got any darker! And I’ve never been able to see the Milky Way.

I have been to a few places that are labelled Bortle 5 (although not with a telescope) and haven’t had luck with making out the Milky Way, although maybe I wasn’t there on the right night. However, I’ve seen it from Bortle 4 but it was faint and you had to be well adapted to the dark. I’ve been to one Bortle 2 site and it jumped out like a sore thumb, even without being use to the dark.

The Bortle Scale is a guide and as such I find it does a good job.

Edited by PeterStudz
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PS - in my Bortle 7 back garden time of night makes a huge difference - eg 2am is significantly darker than mid-night. Also area of the sky. For me E—SE is best or right overhead. But anywhere near Southampton Docks is appalling for obvious reasons. 

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Posted (edited)

I live in the middle of a housing estate in a market town and according to clear outside I have Bortle 5 skies.  My dad lives in the middle of the countryside surrounded by fields and, apparently also has Bortle 5 skies.  Definitely much darker at my dad's. Makes no sense to me! 

Edited by SmokeyJoe
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It is useful - but it's not the whole story.

I've been able to spot MW at zenith in Bortle 7/8 location (edge of 7 and 8 according to 2015 data - probably Bortle 8 now). I've also detected M31 with averted vision.

This only happens on very transparent nights. Transparency is big part of equation as it impacts brightness of the target as well as actual LP levels (how much ground LP scatters in the air).

Key is dark adaption and shielding from direct LP sources like street and house lights.

Best way to see it is to pick transparent night, and just sit relaxed in the chair shielded from LP sources. Spend about half an hour - 45 minutes just relaxing and looking at the stars to get well dark adapted. It also helps if MW is overhead in that moment - so aim for it to be close to zenith.

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The reported sky qualities are very rough estimates and the real life situation varies a lot, since you mention street lights nearby, its safe to say that the general measurements carry no weight for you then. If you want to really know what quality of skies you are under, you need to measure it yourself by either visually, using a sky quality meter (unihedron SQM for example) or by letting a software like ASTAP measure it for you using one calibrated subexposure.

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CO reports that I have SQI 21.66 and Bortle 4, nearly 3. The Milky Way is structured with lots of filaments running off, and glitters all the way to the horizon. On one night / early morning when I went to shut down the obsy about 4 o/c the sky was pitch black, Auriga glittered with clusters and M33 was visible naked eye, not averted, so I would estimate more like middle Bortle 3. I've certainly seen the star cluster in the middle of the rosette direct naked eye, and I think the nebula itself glimpsed averted. M44 is easy routine direct vision, not even a test.

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I think there's a danger when using such estimation scales, that they can have a negative effect on the observer, perhaps giving them a defeatist mentality. 

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I would imagine that the data is less reliable now and possibly the impact has changed.  The switch the led lights will alter the effect (since 2015 there has been a large roleout, at least in Scotland).  Also, and I could be wrong, but the satellites they use for the measurements have different sensitivity to led I think.

If you go on one of the online maps it's quite fun to look at the North sea.  You can see decommissioning by the removal/reduction in the light pollution.  At least in the UK sector.

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I may be interpreting this all wrong (often am) but I believe Mr Bortle provided the scale as a way to evaluate and categorise the particular sky from an observers point of view (literally).

I think the apps that tell you what your Bortle level is, should only be a very rough guide to what you might expect. The skies from my back garden are Bortle 6 to the S and W and Bortle 5 to the N and E. My front garden is Bortle 9, due to the street light directly above!

The original article:

https://www.nps.gov/subjects/nightskies/upload/BortleDarkSkyScale-2.pdf

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It is very much an estimate even with the satellite data, the thing to remember is the measurements are only taken at specific times, are not sensitive to all wavelengths, and are often out of date. Plus satellites only look down at large areas whereas you are looking up from a very specific one. Personally, I don't take much notice of it anymore. My house and my friends' farm 10 minutes away are both Bortle 4 areas, very different when you look up at the sky from the farm!

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Posted (edited)
On 15/05/2022 at 18:10, PeterStudz said:

I’m in Southampton. Last year ClearOutside had me in Bortle 8. Now it says Bortle 7. The sky definitely hasn’t got any darker! And I’ve never been able to see the Milky Way.

 

I'm in Southampton and noticed this too. If anything I'm convinced the sky has got lighter.

Edited by Adam1234
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I live in bortle 5,and can see the milky way fairly easy on a moonless night . In bortle 4 the dark lanes around Cygnus stand out like you cant miss them.. trying to look for the milky way on a moon night won't change the bortle scale..

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Since I've got into astronomy I've spent a lot of time at two very different places, different from a light-pollution viewpoint. One is 20 miles from central London, say Bortle 7/8. The other is about as far South West Ireland as you can get, nominally Bortle 3. I have an SQM-L meter, and have taken data readings on almost every possible clear night for the last 3 years or so. The attached graph shows this data plotted by SQM reading vs Sun altitude. The "upper limit" curves agree quite nicely with the lightpollutionmap.info Atlas-2015 modelled data, modelled from an observer's viewpoint based on the 2014 VIIRS satellite "look-down" intensity readings.

Magnus

LPdata202205.JPG

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10 hours ago, Adam1234 said:

I'm in Southampton and noticed this too. If anything I'm convinced the sky has got lighter.

I haven’t been doing this for long but I agree. Eg during the lockdowns I’m convinced that generally the sky was darker. And I guess that isn’t surprising. 

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3 hours ago, PeterStudz said:

I haven’t been doing this for long but I agree. Eg during the lockdowns I’m convinced that generally the sky was darker. And I guess that isn’t surprising. 

The Bortle scale is a useful estimate if checking light pollution maps for an observing site but it's used as a guide. One cannot take the estimate literally - unaccounted for local variables will come into play.

CO had me at Bortle 8 for a couple of years then upgraded me to 7. In that time my sky has undoubtedly brightened, so I take that reading with a massive dose of salt.

Regarding the sky being "darker" during the lockdowns, I suggest the sky brightness was likely the same as before. However, the very significant reduction in air pollutants due to less aircraft, less road traffic and business energy use likely improved transparency. I had some wonderful urban observing sessions during lockdowns because the sky was "crystal clear".

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I think the Bortle scale was developed in the American South West, with dryer climate. Saying the Sagittarius star clouds cast a shadow in Bortle 1 doesn't make sense when they barely rise above the horizon here. Also what suburbs have Bortle 5 skies? I lived in the West London suburbs and would have cheerfully killed for Bortle 5.

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Young suburbs at the very edge of a growing metro area adjacent to farms or a forest and having low density housing tracts with no commercial development can easily have Bortle 5 skies.  Mine started out that way 30 years ago.  I'm closer to Bortle 7 or worse in most directions now that we have tens of thousands of houses, many giant shopping centers, night lighted school fields, car dealerships, supermarkets, etc.

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im moving soon to a near bortle 3 area measures with my sqm 21.68 should be ace 😆 this is uk, spain fell through. 8 weeks left untill we move to mid wales

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23 minutes ago, faulksy said:

im moving soon to a near bortle 3 area measures with my sqm 21.68 should be ace 😆 this is uk, spain fell through. 8 weeks left untill we move to mid wales

Horsehead skies them mate, m33 easy naked eye. Not a lot out of amateur visual range tbf. Not one bit jealous though 🙄

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1 minute ago, bomberbaz said:

Horsehead skies them mate, m33 easy naked eye. Not a lot out of amateur visual range tbf. Not one bit jealous though 🙄

just looking on light pollution map. to the west nearest town is 90 km and south is 80 km which is brecon 🤣

just waiting for solicitors to pull there fingers out

4 km from the house is 21.82

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33 minutes ago, faulksy said:

im moving soon to a near bortle 3 area measures with my sqm 21.68 should be ace 😆 this is uk, spain fell through. 8 weeks left untill we move to mid wales

Sorry to hear Spain fell through Mike, and best of luck with the new move.

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1 minute ago, faulksy said:

just looking on light pollution map. to the west nearest town is 90 km and south is 80 km which is brecon 🤣

just waiting for solicitors to pull there fingers out

4 km from the house is 21.82

Have you taken tgese readi gs from a lp map, cos they are not most accurate. However if you can get 21.82, that is black, milky way casting a shadow dark!

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