How light polluted is your Sky?

[pyrasanth]

Are there any simple tests to gauge how polluted your viewing location is. I would guess you would use naked eye limiting magnitude stars then equate this to some form of scale.

Let me know & it would be useful to post your approximate location & your light pollution level.

[Beulah]

I've got a dark sky meter - last measured on 18th April 2014 at 10:30pm: NELM 6.2  (21.20 mpsa) from zenith from my front garden, in between the neighbours' halogen discos...that night was quite dark and quite clear; a true rarity.

Most of the time is can be as low as 5.5 - 5.7. Most of my 'readings' are done with the mark 1 eyeball and finding the dimmest star I can see; the dark sky meter showed the darkest my patch of the sky can be, which isn't often!!!

[ronin]

Usually it is simply counting stars that you can see in the square of Pegasus or the square oif Hercules.

What always bugs me if they never say if the corner stars count or not.

You can use Ursa Minor in a similar way.

I tend to be philosophical and if I can see the square of Pegasus or Hercules then that's a good start.

[oldpink]

I can only see the brightest of stars with my naked eye
Orion I can see the main outline but thats about it very light polluted

[RikM]

In a talk att Cotswold AS last night, Guy Hurst of The Astronomer said that in a recent study, the UK currently has the most light polluted skies in the world

It certainly seems that way. My limiting mag is about 4.5 as an average to about mag 5.0 on a very good dark night. I can occasionally make out the Milky Way and sometimes the Andromeda galaxy is possible with the naked eye, but I do know exactly where to look. It would actually not be that bad if I could shield some local sources of light pollution that makes proper dark adaptation impossible. Even with these difficulties, I have seen some challenging objects.

[D4N]

That depends a lot on how dark adjusted your eyes get, and how good your vision in general is.

Being able to see Andromeda is a good start.

My LP is all natural atm, long summer days :s

Sent from my iPhone using Tapatalk

[scarp15]

In a talk att Cotswold AS last night, Guy Hurst of The Astronomer said that in a recent study, the UK currently has the most light polluted skies in the world

Probably going to get even worse with current policy for developing new Garden Cities.

[acey]

The best way to assess sky quality is instrumentally, e.g. using a Unihedron Sky Quality (SQ) Meter. If used properly, this is very reliable. There are various online calculators purporting to equate SQ reading with limiting magnitude, but the model they use is somewhat unreliable.

To do it by eye, the best way is to use the method of star counting over a designated area - there are various charts and regions that can be used. Ursa Minor is convenient because it's always above horizon and at a fairly constant height in the sky. Square of Pegasus is popular but not always visible - ideally you want to be observing stars near zenith (i.e. directly overhead), where you can see the faintest stars (because air is thicker and the sky is brighter as you go down towards horizon).

Most people judge their limit pretty crudely - to the nearest whole number or maybe half-integer. That's a bit like rounding everyone's height to the nearest foot or half-foot. And when you factor in personal variation and accuracy of assessment, you realise that magnitude estimates are in most cases not particularly reliable. Really you would want to be doing it to the nearest fifth of a magnitude or better (which is not easy). Or else accept that it's just a crude estimate. There are various projects attempting to gather sky quality data from the public. One interesting venture is the "Loss of the night" app. If you're interested in assessing your sky quality and producing useful scientific data then give it a try:

https://play.google.com/store/apps/details?id=com.cosalux.welovestars

To assess magnitude limit there is no hard and fast rule, but most people would do it using binocular vision (i.e. two eyes rather than one), direct or averted vision (most would use the latter - it will show the faintest stars), and viewing as close as possible to zenith. It is a matter of judgment whether to look for specific stars whose position (and magnitude) are known in advance, or else to detect random faint stars then check on their identity and magnitude afterwards. The former is more susceptible to false positives (you convince yourself you've seen it when maybe you haven't). It is also a matter of opinion whether to take the limit from stars that are definitely and continuously visible, or to allow for momentary glimpses of extremely faint stars. When you hear reports of very high limits (7+) at very dark sites, it's quite possible that this is for stars momentarily glimpsed rather than steadily visible. In my paper on magnitude limits (link below) I argue that scintillation may be a significant factor at some dark-sky sites (e.g. Mauna Kea), making very faint stars momentarily visible. There are also various tricks that will boost a person's threshold, e.g. blocking off the background sky (looking at stars through holes in a black screen, or through gaps between leaves of a tree, or through a sight tube, etc).

In my paper, rather than present definite limits for specific SQ measurements (which isn't really meaningful) I predict the loss of zenith magnitude limit as a function of sky brightening. I find that for every increase of 0.25 SQ you should lose about 0.1 mag of threshold (for skies in the approximate range SQ 22 to SQ 20.5). If you can see stars to 5.5 mag at a site with SQ 20.5 (just dark enough for the Milky Way to be plainly visible)  then at a perfect site (SQ 22) you would expect to see stars to approximately 6.1 mag. This is assuming full dark adaptation at both sites. But the perfect site would be dark down to the horizon, so would have a far greater "wow" factor, with phenomena such as zodiacal light plainly visible, while the brighter site could have terrestrial glare or light domes near horizon which would affect dark adaptation. For SQ 19.25 a person's magnitude limit is predicted to be about 1 mag lower than it would be at a perfect (SQ 22) site. I give a table of values in my paper.

"Human Contrast Threshold and Astronomical Visibility" (to appear in Monthly Notices of the Royal Astronomical Society):

http://arxiv.org/abs/1405.4209

[cotterless45]

Our sky is

A for abysmal

B for blooming awful

C clear

D dreadful ,

Nick.

[coulthamst]

I live in inner city Leeds - approx 1.5 miles from the city centre. For me a reasonable night is a naked eye view of the two pole guardians. A really good night I can just see Alcor. Still living in hope of a naked eye view of more than three stars In UMi. Masochism becomes me.....

[DaveS]

Probably around Bortle 7 or 8. I *have* seen it edging as far as Bortle 6, but that was an exceptional night. 20-25 years ago here, Bortle 6 was commonplace, all stars of UMi visible, and a faint Milky Way.

[ChrisLX200]

It's a bit odd where I am a couple miles from a town center, if there is no mush in the atmosphere to reflect light pollution from said town then the skies a reasonably dark - particularly looking S and E towards the Pennines. On such nights the Milky Way is clear, M31 is easy naked eye and all stars in U.Minor visible. A little but of mistyness and that all goes to hell - like there's someone up there with a mirror shining it back down onto me and all the fainter stuff disappears. So I would have difficulty saying what is 'typical' seeing here.

ChrisH

[RikM]

It's a bit odd where I am a couple miles from a town center, if there is no mush in the atmosphere to reflect light pollution from said town then the skies a reasonably dark - particularly looking S and E towards the Pennines. On such nights the Milky Way is clear, M31 is easy naked eye and all stars in U.Minor visible. A little but of mistyness and that all goes to hell - like there's someone up there with a mirror shining it back down onto me and all the fainter stuff disappears. So I would have difficulty saying what is 'typical' seeing here.

ChrisH

That sounds similar to my situation Chris. A good night can be really good given that I am about 3.5 miles from the centre of Gloucester. But if the atmosphere is a bit hazy then all the LP gets reflected and it can be pretty dreadful.

[DaveS]

I've found that immediately after rain / wind the sky can be pretty dark, that's when I got the Bortle 6 sky, but as the days pass and clag builds up (This is London remember) so the skies get worse until it can be that nothing below mag 2 is visible.

[Charic]

Stellarium's  pollution level only goes up to level 9. I'm about 11 if the scale could go higher?  Doesn't really  get dark this time of Year at 57° N  Thats not far from Inverness, Scotland. No astronomy for me for a few Months   

[sheebert239]

my area is pretty light polluted, for optimal viewing (deep sky objects / other than planets) I have to drive 20 minutes to my local observatory (I live in a large city)

[johnd]

Usually it is simply counting stars that you can see in the square of Pegasus or the square oif Hercules.

What always bugs me if they never say if the corner stars count or not.

You can use Ursa Minor in a similar way.

I tend to be philosophical and if I can see the square of Pegasus or Hercules then that's a good start.

What there's an Ursa Minor?

In my skies I struggle to see Hercules main square stars and I only see polaris in UrMi

[DaveS]

I usually manage to see three stars of UMi unless it's very low down in the orange clag.

[RichM63]

Hey I feel for you guys.

I know that I am unpolluted and only have to deal with atmospheric conditions.

For fun I use a Naked Eye Limiting Magnitude chart which gives me an indication of how good a night it is.

This also helps when I am explaining to or showing a visitor the sky through the scopes by not only helping them to understand what we are all on about when it comes to sky quality, but for me to gauge how they are seeing the sky.

Attached is the chart I use.

@acey, great post, but my browser chops the full paper down to just the opening para'.

limiting_mag.pdf

[ollypenrice]

My location was chosen for astronomy. The best zenith reading I've seen was 21.9. 21.6 is more common and is the same as Namibia, according to someone who knows both skies well. However, we lose out to the desert as we approach the horizons. Nowhere in mainland europe can expect truly dark horizons. That said, we can image down to our horizons if the target demands it. The LP comes from cities about sixty miles to our south.

Olly

[Scott]

It's a bit odd where I am a couple miles from a town center, if there is no mush in the atmosphere to reflect light pollution from said town then the skies a reasonably dark - particularly looking S and E towards the Pennines. On such nights the Milky Way is clear, M31 is easy naked eye and all stars in U.Minor visible. A little but of mistyness and that all goes to hell - like there's someone up there with a mirror shining it back down onto me and all the fainter stuff disappears. So I would have difficulty saying what is 'typical' seeing here.

ChrisH

sounds about right, not too many of those gooduns though .

[AndyWB]

It's a bit odd where I am a couple miles from a town center, if there is no mush in the atmosphere to reflect light pollution from said town then the skies a reasonably dark - particularly looking S and E towards the Pennines. On such nights the Milky Way is clear, M31 is easy naked eye and all stars in U.Minor visible. A little but of mistyness and that all goes to hell - like there's someone up there with a mirror shining it back down onto me and all the fainter stuff disappears. So I would have difficulty saying what is 'typical' seeing here.

ChrisH

I dunno, that matches what I find. Sometimes at home when I'm intent on the eyepiece I notice clouds coming in due the ambient brightness increasing - 'cos they're reflecting streetlight. Yet on one really clear night year (August, at the Perseid peak) I could see the Milky way in Cygnus, and that's from within a couple of miles of the middle of Reading.

All that said, I find that the best seeing I've had - as in, the stability of the atmosphere, as opposed to transparency - is when they sky has been a bit hazy. In my head it's now "It's hazy - think planets or doubles"

And I printed an Ursa Minor chart for this too... 

[DaveS]

It's not just us down here

http://www.bbc.co.uk/news/science-environment-27824565

Even Gaia gets light pollution

[dick_dangerous]

I don't really go in for measuring NELM or any of that - I'm not that into the numbers, I just play it by ear. Being in London the sky is usually pretty awful anyway.

I judge the sky with a number of subjective methods - the number of stars in Ursa Minor visible, the number in Pegasus, what constellations I can see in the Summer Triangle and, out of London on a moonless night, the appearance of the Milky Way.

Bushy Park varies from murky rubbishness, especially on the Eastern and Northern quadrants, to passable sky quality - on the rarest of occasions I've been able to catch a tenuous glimpse of the Milky Way. My attitude is that you can't change your sky, just make the best of what you've got and the results might just surprise you!

DD