Nice Observing Site Rating System

[JamesK]

Apologies if this has already been posted, but I found this great method of rating your observing site, on the Sky and Telescope website.

SkyandTelescope.com - Saving Dark Skies - The Bortle Dark-Sky Scale

I've always thought I had a decent sky from my garden, my usual location but my Sky apperantly only rates in the 6th of 9 classes - a bright suburban sky:

Class 6: Bright suburban sky. No trace of the zodiacal light can be seen, even on the best nights. Any indications of the Milky Way are apparent only toward the zenith. The sky within 35° of the horizon glows grayish white. Clouds anywhere in the sky appear fairly bright. You have no trouble seeing eyepieces and telescope accessories on an observing table. M33 is impossible to see without binoculars, and M31 is only modestly apparent to the unaided eye. The naked-eye limit is about 5.5, and a 32-cm telescope used at moderate powers will show stars at magnitude 14.0 to 14.5.

There are however, class 4 (Rural-Suburban Fringe) that I can get to easily (public transport in under 45mins) used for good nights and special events. And I can only think of the Pennines and the Lake District within 50 miles of me that could maybe make the 3rd Class. Read the description for 1st class, it sounds amazing!

Would be intrested to hear your thoughts on the system and where most peoples locations come in on the scale.

[Rusty Strings]

Good link. My back garden seems to be a class 5/6, front garden class 7. Good job my house block out most of the LP from the town I overlook.

John

[acey]

I observe in Northumbria, one of the darkest parts of Britain. At my site there are no artificial light sources directly visible in any direction, and naked-eye limit is 5.9.

But there are light domes on the horizon, clouds are brighter than the sky, visible overhead, and M33 is not visible with averted vision.

So on the Bortle scale it comes out Class 5: suburban!

Andrew

[JamesK]

Hmmm, maybe this guy has very good skies for his area. I would call my area suburban, maybe suburban/rural but I don't think it's that bright. Maybe they have better transparancy in America?

Anyway, 200TH POST.

Woop.

[gliderpilot]

One key aspect is altitude above sea level. In Spain at 5000ft in the Pyrenees the sky was so clear and dark as you are above a fair proportion of the atmosphere (I haven't got my ISA tables to work it out)

I do wonder if observing from the us desert at a high altitude gives the author the best of both world that we cannot equal in the uk?

Mark

[acey]

Definitely.

British air is also moist: this reduces transparency and reflects light pollution. Many parts of the US have very dry air, as well as altitude. So even if there are light domes on the horizon, the sky may be very dark overhead.

When Podgson devised the modern magnitude system in the 19th century he scaled it so that magnitude 6 would match the faintest naked eye stars. This was how things had been done since Hipparchus invented the original system. American observers have long been saying that 6 is not really the limit, and it's more like 7 or more - this is what the Bortle scale adopts as its standard.

I've heard reports of UK observers able to see M33 naked eye (direct or averted), and maybe there are places in Britain that would qualify as Bortle 4, or even 3. Visual acuity obviously also affects judgement of limiting magnitude. But I doubt there's anywhere in Britain with a limiting magnitude beyond 7, which is what Bortle requires for a "truly dark" site of Class 2. As for the "observer's Nirvana" of mag 8 (Class 1) - dream on!

So IMHO, the Bortle scale is of little use to UK observers - except perhaps as a way of choosing holiday destinations. Limiting magnitude is a more precise way of estimating the darkness of the sky at a particular site.

I use the stars of Ursa Minor as my guide - they remain at much the same altitude all year round. Using a chart with the magnitudes of its stars, you can estimate your site's limiting magnitude quite accurately.

Andrew

[Rusty Strings]

I'd go with one, it is a rare night when I can see all of the stars in ursa minor, but on those nights the rest of the sky is glorious.

John

[acey]

How much is "all"?

There are seven bright stars in Ursa Minor, and they go down to magnitude 5.0. For even a passably dark sky you want to be seeing fainter than that.

Just next to Zeta Ursae Minoris is a star of magnitude 5.2; next to Eta is one of 5.5. Half-way between Eta and Zeta is one of about 5.9 - that's the one I look for.

I keep a magnitude sequence inside my observing book - I don't have it to hand. Only sequence I can find right now is at the top of this page:

Star Magnitudes

[JamesK]

Hmm... just a question. If you can see to mag 5.0 with direct vision, but to 5.5 with averted vision, then is your sky mag 5 or mag 5.5?

[acey]

My understanding is that the limit is conventionally for direct vision so the answer would be 5.0. It's also conventionally taken at the zenith, though personally I'm happy to make do with familiar stars lower in the sky, even if it means the resulting estimate is a fraction of a magnitude lower.

Stephen O'Meara says in his Messier book that according to tests done by Brian Skiff, with averted vision it's possible to see up to three magnitudes fainter than with direct - something I find frankly incredible, and certainly can't manage personally. And O'Meara's Hawaiian skies are magnitude 7!!

We're living in the wrong country.