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Zero-G

How to gauge light pollution in my area

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Anyone know what the most commonly used measurement of LP is and how I can get a measurement/rating for my area? I'd just like to know how my area fares in the grand scheme.

Also I've heard about a LP overlay for Google maps but can't find it - does it really exist??

Any help much appreciated as always ;)

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There is a google maps overlay I think, but I can't find the book mark here

You could try this interactive map, it describes what you can expect to see in easily understandable terms.

UK Light pollution Map

People seem to refer to having magnitude 4.5 (example) sky's which I think relates to the dimmest objects you can see with you naked eye (I'm sure there is a better definition of this).

There is a device for measuring sky quality as well, but it's not cheap (well depends on what you think of as cheap I guess).

Tyr

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ps my location shows on that map as magenta to red in colour meaning pretty bad.

Going by the description of Red "constellations and others stars appears. In a telescope, Some Messier object began to be observable." is pretty accurate for my back garden.

Tyr

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hi

the Orion link one seems a bit strange [i live in the south west of glasgow 4 miles from citycentre and i can make out 9 stars of orion [the belt, betleguese and pye3? ie the head of the bow]but i have definite lp] to the north i can see the whole of ursula major but only polaris on ursula minor ,and andromeda is faint, mirach being the only decent star i can see . my point is that lp isn't the be all and end all ,I make do with what i can see and if i get a chance i go out to the country and see a bit more ;) .

Edited by dtr42

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Here's another piece on estimating Naked Eye Limiting Magnitude:

Naked Eye Limiting Magnitude: Redux Dark Sky Diary

and from the same author:

The Bortle Scale: A Flow Chart Dark Sky Diary

I shall have to give this a try to see how it compares to other results. Based on the stars I can see I reckon the NELM for here is somewhere close to 6.0. I can pick out the mag 5.85 star halfway between Betelgeuse and Bellatrix, for instance. Someone with younger eyes than mine might do better.

James

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Some good resources there folks, cheers! NELM and the Bortle scale seem to be the only things close to standardised measurement. No google LP overlay though? I'm beginning to suspect it was available at one point but was then pulled...

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The Avex map is an approximation that assumes a certain amount of light pollution associated with a given level of population and uses a function to model spread from the centres. Very effective it is too.

If you want to know how to measure light pollution where you are then register with MyDarkSky (link below) and we'll send you instructions.

Using a Sky Quality Meter is the gold standard as far as these things go but they're not cheap. Worth it if you're going to going to do some serious surveying but not if you just want to figure out how your own back garden compares.

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All of those seem to tally for me.

I use a couple of whole constellation NELM charts and record it for my observing log. Depending on transparency my own sky estimates are mag 3.0 on a bad night to mag 4.5 on a good night. 4.25 average most nights. I have had mag 5.0 on exceptional nights. I may be estimating a bit low.

The SPA Pegasus one gives me mag 5

The Orion one seems a bit odd because I am between 4 and 5 on this scale.

The Need less LP map gives me mag 4.8 and Bortle 7.

The Avex one I am in the yellow to the south west of Gloucester.

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Interesting to know how the Avex map is done. I've been wondering why it showed one of the local villages quite brightly when, to the best of my knowledge there isn't a single street lamp there. I bet the only significant source of LP there is the lighting on the church.

James

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Interesting to know how the Avex map is done. I've been wondering why it showed one of the local villages quite brightly when, to the best of my knowledge there isn't a single street lamp there. I bet the only significant source of LP there is the lighting on the church.

James

Interesting. We've found that (thus far) it correlates quite well with our SQM and NELM readings, but yeah - it's still a model. I've got a PDF from the chaps who developed it which explains it all. One of these days I'm going to get round to translating it into English.

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Slightly off topic but if your not happy with the skies in your location (most of us could find better I would guess) - this is worth getting for a fiver - got my spring nights planed already in Mid West Wales

http://www.amazon.co.uk/Philips-Dark-Skies-Britain-Ireland/dp/0540086126/ref=sr_1_fkmr0_1?ie=UTF8&qid=1330587392&sr=8-1-fkmr0

Edited by stevetynant

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NELM, SQM and Bortle all have their pros and cons: none offers absolute accuracy.

In judging NELM, everyone's eyes are obviously different, and there is no generally agreed rule on whether vision is direct or averted, or whether the faintest star should be continuously seen or merely glimpsed. These factors can make a big difference.

With Bortle, there is (I think) insufficient gradation between conditions in the middle of the range.

The SQM assumes perfect transparency, and is affected by any light in the sky (e.g. the Milky Way); what you get is an average over a large area of sky, not the brightness of the background sky between stars (which is what professional astronomers measure). So it needs to be used sensibly. You can point an SQM at a totally cloudy sky and get what looks like a wonderful dark-site reading.

The SQM costs about £100 and I'm very pleased that I got one: I take a measurement every time I observe. My back garden (which I never use for observing) is about 18 to 19, my dark site is usually around 21.2. My nightime garden sky is equivalent to twilight at the dark site.

There's a formula for converting between NELM and SQM but it's based on a theoretical model, and all models have their limitations. According to the model, my dark-site reading should equate to NELM 6.3. In practice I never see beyond 6, though younger eyes would doubtless do better.

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Cheers Cath, good find ;)

Clearly you can get right into the intricacies of detailed measurement with the various methodologies...

Lots of good content shared and all much appreciated.

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