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SQM-L readings and dark skies


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Hi all

Is there an official scale/text showing the lowest SQM-L reading that might be considered a ‘dark sky’ ?

I am writing a short report for our Parish Council to highlight our rural sky quality and the impact of local light pollution in some places. I consider the skies here reasonably dark (not dark sky park quality tho). A light pollution website suggest local readings should be around 21.6 but my readings hover around 21.2.

Thanks

Gary

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There is this:

http://www.darkskiesawareness.org/img/sky-brightness-nomogram.gif

(click to get image since http images are no longer displayed embedded)

Taken from this page:

http://www.darkskiesawareness.org/nomogram.php

It is a rough guide but can serve purpose. Bortle 1 and 2 could be considered truly dark skies and I think that they start at SQM 21.5.

Difference between 21.6 and 21.2 in bortle scale is significant - Bortle 2 vs Bortle 4 (well on edge with Bortle 3 - but still Bortle 4).

There is this page as well that lists approximate SQM ranges:

https://en.wikipedia.org/wiki/Bortle_scale

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Thanks vlaiv, I've seen these pages before. 

Using the Bortle Scale I have assessed as Bortle 4 but not far off 3 tbh, 3.5+ if there was such a thing !

I don't really like the Bortle scale, I can see how it may be useful but to me it seems subjective and perhaps broad in places. I prefer the SQM-L it is more objective and there’s less risk of user bias creeping in. Not sure how the websites calculate things but 21.6 does seem quite a bit higher than my usual 21.2 readings (I did get a high of 21.4 once). I'm guessing they are estimates.

The wiki page suggests the sky here is bortle 4 and therefore rural/suburban transition. It is actually very rural here, I wouldn't class it as suburban transition, but again it is all down to how you interpret things I guess.

Thanks

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2 hours ago, Spaced Out said:

A light pollution website suggest local readings should be around 21.6 but my readings hover around 21.2

Locations never have one, fixed, SQM value. They change with all sorts of factors. Mine varies by over 0.5 magnitudes, depending on transparency (i.e. how much light is reflected compared with how much passes through and out of the atmosphere), sunspot cycle and probably lots of other things apart from the Moon.

So I wouldn't get too tied up with what other websites say it should be. I suspect that all they do is look at the population of towns and villages and make a guess based on nothing more than their distance away.
What is interesting from your all-sky image is that the brightest LP to the south isn't as bad as the effect the dimmer LP from the east has.

Edited by pete_l
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36 minutes ago, pete_l said:

Locations never have one, fixed, SQM value. They change with all sorts of factors. Mine varies by over 0.5 magnitudes, depending on transparency (i.e. how much light is reflected compared with how much passes through and out of the atmosphere), sunspot cycle and probably lots of other things apart from the Moon.

So I wouldn't get too tied up with what other websites say it should be. I suspect that all they do is look at the population of towns and villages and make a guess based on nothing more than their distance away.
What is interesting from your all-sky image is that the brightest LP to the south isn't as bad as the effect the dimmer LP from the east has.

Thanks, that's good to know that readings can vary that much, most of my readings are around 21.2 but I have had 21.4 once.

In the field the LP to the south is a pain, it is from Newcastle upon Tyne and surrounding towns (maybe 25-30 miles away), it washes out everything low in the southern sky.

To the east is straight out over the North Sea, no obvious artificial light sources in that direction, it is the darkest horizon here but I see airglow that way fairly regularly.

To the north is a small dome of LP from a couple of small(ish) villages (about 5 miles away), and to the north-west and south-west is some local LP from our village.

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Hi yes the SQM-L is the best tool to have and will require a succession of readings over a clear transparent night at a particular location to gauge an average magnitude read out. Even then it can vary, as mentioned from the same location on alternate nights. Early morning is usually best to gauge an optimum reading if conditions are applicable. I have roamed many potentially dark sky locations in Northumberland. These include the Breamish Valley just on from Ingram (very good for taking readings),  on the same approach, a location near to Thrunton Crags, west over at Cawfields on the Wall and a favourite at Battle Hill. Oh yes also at  / near Stonehaugh. I agree with your perspective on light domes from Tyneside (also Byth etc. Tyne Valley), north is very dark, east and west - depending on where you are located is also dark. My own readings are also typically 21.2 - 21.4, sometimes edging to 21.5, yet not at all the given dark sky online charts implying 21.8 -9 mag. However higher readings are definitely possible at most of the mentioned locations, in optimum conditions. Kielder achieves 21.6 - 21.7 for certain, the very darkest spot I understand is near to Deadwater Fell.  My own darkest reading is close to Carter Bar on the border at 21.5 mag, taken early on in the evening and would easily had surpassed this had sea fret not intervened. Anywhere north from Rochester on the A68 will be very good for gauging readings within the 21.5 - 21.7 margins.  

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14 minutes ago, scarp15 said:

 My own readings are also typically 21.2 - 21.4,

Thanks for the reply. I think there may be a little more LP down here on the coast compared to some of those inland/upland sites.

I have been doing an average of 4 readings between midnight - 1ish. I suspect that with perfect conditions 21.40 will be as good as it gets here. This did happen just once on a very clear calm night a little while back. The other 3 nights I've taken measurements they've generally always been between 21.18 and 21.25, so this has made me question whether the 21.40 was accurate or not, sounds like readings might vary a little bit due to conditions tho.   

Yep, that dome of skyglow from down Ashington/Newcastle way does my head in tbh !

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It can be surprisingly difficult to get completely away from, although on atmospherically dry nights, it becomes less noticeable. Newcastle Airport is a concern, extravagantly lit up. 21.00 Mag / NELM 6.00 is considered entry level for good quality dark skies. Northumberland Dark Sky Park status and dark sky tourist industry does promote, within the park boundaries, awareness for regionally appropriate lighting, reducing unnecessary light intrusion. You mention the coast and I agree, the closest, easterly location I have used as mentioned is near to Thrunton Crags, nearest Town is Alnwick, which itself is not a concern, but south east as you mention, Ashington, Newcastle is notable and may often record my lowest readings, at very best about 21.2. Still it is fairly sheltered, easy to reach from home and I have seen faint deep sky objects from here, such as the Medusa Nebula, plus due west and north is very good. 

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6 minutes ago, scarp15 said:

21.00 Mag / NELM 6.00 is considered entry level for good quality dark skies.

Do you have any sort of reference for this statement, it would be really helpful if possible. I'm trying to convince the parish council that our 21.2 skies are dark enough to really push awareness and protection within the local community.

7 minutes ago, scarp15 said:

Northumberland Dark Sky Park status and dark sky tourist industry does promote, within the park boundaries, awareness for regionally appropriate lighting, reducing unnecessary light intrusion.

Pretty much the same approach I am asking our parish council and hopefully neighbouring parish councils to adopt.

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This is an interesting article that you could bring to the attention of the parish council. Concerning nocturnal impacts on Entomology. Those of us who seek dark sky environs are sensitive to the negative impact and ecological harm that local light intrusion and the relentless creep of light pollution creates.

https://www.ncl.ac.uk/press/articles/archive/2019/01/streetlighting/

 

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44 minutes ago, scarp15 said:

This is an interesting article that you could bring to the attention of the parish council. Concerning nocturnal impacts on Entomology. Those of us who seek dark sky environs are sensitive to the negative impact and ecological harm that local light intrusion and the relentless creep of light pollution creates.

https://www.ncl.ac.uk/press/articles/archive/2019/01/streetlighting/

 

Thanks. I am an ecologist. I am just trying to get the local nature reserve recognised for its dark sky quality, so I'll be flagging up the many ecological benefits (alongside others) of protecting the area from LP. Plenty of research/evidence showing negative ecological impact caused by LP, yet, as you say, the LP creep is just relentless.

Luckily I live in a small rural community, I'm hopeful that if we can get the community engaged with this we might be able to make a difference at a local level and encourage neighbouring parish councils to do the same.

Edited by Spaced Out
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25 minutes ago, Spaced Out said:

Thanks. I am an ecologist. I am just trying to get the local nature reserve recognised for its dark sky quality, so I'll be flagging up the many ecological benefits (alongside others) of protecting the area from LP. Plenty of research/evidence showing negative ecological impact caused by LP, yet, as you say, the LP creep is just relentless.

Luckily I live in a small rural community, I'm hopeful that if we can get the community engaged with this we might be able to make a difference at a local level and encourage neighbouring parish councils to do the same.

Could I also suggest that you contact Richard Darn. Richard was a key advocate for instigating the Northumberland Dark Sky Park status and was also a co founder for organising Kielder Star Camp. He also posts on here, but I cannot remember his user name. 

https://darkskiesuk.org/about/

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4 minutes ago, scarp15 said:

Could I also suggest that you contact Richard Darn. Richard was a key advocate for instigating the Northumberland Dark Sky Park status and was also a co founder for organising Kielder Star Camp. He also posts on here, but I cannot remember his user name. 

https://darkskiesuk.org/about/

That's great thanks, I'll get in touch with him 👍

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