Stellarium Bortle index.

[DaveS]

Is anyone else puzzled by the values it gives?

When I let it use my location from the database it shows up 9 with the sky totally washed out, something I only see if there's been a long run of dry weather under high pressure, allowing the London clag to build up, possibly with a moon.

When I try and adjust the settings manually to match what I actually see, it comes up between 7 and 6, but not consistantly, eg to see all the "paralleagram" of Lyra needs 6, but at that level Sagitta and Delphinus should also show up, but while I can just make out all the parallagram stars, I can't quite make out Delphinus and Sagitta. So, Bortle 6.5? I don't know. Looking at Umi would appear to agree with 6, but it looks like it varies across the sky. The Milky Way also shows up at 6, which it shouldn't, though I do see a hint of structure to the sky in that area, but that may only be because I know it should be there.

Putting in what I remember seeing 20 - 25 years ago from Acton, 5 miles further in gave me Bortle 5. And I gave up because of the light pollution! Argh!

[alexwolf]

We use Bortle scale from here: http://en.wikipedia.org/wiki/Bortle_scale

[pete_l]

There seem to me to be many, many inconsistencies in the Bortle descriptions (for example: the idea that the Zodiacal light can be bright enough to trigger the colour receptors in the eye, yet we know that even much brighter nebulae like Orion don't ever show colour - and that whether we see it in colour changes with sky background brightness).

It also doesn't tally with the NELMs for city / rural environments, is unclear whether the NELM is meant to be at the zenith, or the point of observing the subject in question (e.g. Andromeda) and takes no account of an observer's visual acuity.

It's also worth noting that the visual limiting magnitude can change by over ½ of a magnitude¹ depending on where we are in the solar cycle - even if everything else remains constant.

Now that we have access to SQMs, it doesn't seem very useful any more.

[1] Atmospheric Extinction and Night-Sky Brightness at Mauna Kea - http://adsabs.harvard.edu/full/1987PASP...99..887K

[DaveS]

Yes, I've seen the Wiki article. My puzzlement was over what Stellarium was showing and saying what I should see vs what I could see..

The thing about the Sagitarous star clouds casting shadows (Bortle 1) made me laugh. From here they barely rise above the horizon.

[acey]

I agree that the Bortle indicators are in many cases quite strange - I'm not aware of anyone ever having reported seeing colour in the zodiacal light, and wouldn't expect it to be seen even from space (it's white, after all, as well as being extremely faint). Personally I have no use for the Bortle scale; it is just as much subject to personal variation as limiting magnitude, so the latter is good enough as a subjective judgement of sky quality. SQ readings are better.

I don't know how Stellarium does it, but I would guess that it takes the limiting magnitude and gives a corresponding Bortle number, as is done for example in the Wiki entry. Those magnitude recommendations are, I think, overly optimistic to a great degree. In my opinion this further reduces the usefulness of the scale.

If you estimate a limiting magnitude then there's no need to convert that to a Bortle number, which just constitutes a loss of precision.

[alexwolf]

Are you can propose better solution for light pollution?