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How do you know what bortle you are in

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#26 jaraxx

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Posted 23 November 2019 - 07:50 PM

It's pretty simple.
If I can't see my own galaxy, I'm in a bad Bortle.
if I can see my own galaxy, I'm in a fair Bortle.
If I can see another galaxy, I'm in a good Bortle.

I can’t see it naked eye but I can see M51 and a very faint hint of the lane between the two galaxies on a night of good seeing in a 8” reflector from my house. And I can make out Andromeda as well as the Pleiades with averted vision naked eye. The former has to be very good seeing and the latter even down to below average to poor seeing. I am in a Bortle 7 18.49 SQM zone. Which was nice because I thought I was in an 8. I used the above mentioned LP map. So not sure this is a good way to say what kind of a zone you are in...

jeez, you folks are a tough audience - just trying for a bit of humor here... I never expected a group such as this would take a suggestion to actually go outside and just look up seriously. Why, there's not even any decimal places in my "measurement technique"...


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#27 jaraxx

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Posted 23 November 2019 - 08:01 PM

Actually, I find M31 quite a bit easier to see than our own Milky Way. With a bit of effort, I can make out M31 naked-eye from my local city park, about 4 miles from the center of Boston, Massachusetts (USA). But I've never seen even a hint of the Milky Way there.

 

To get a rudimentary view of the Milky Way, I need to be about 10 miles from the center of Boston, by which point M31 is bold and bright. I would rate those sites somewhere between Bortle 5 and 6.

 

By Bortle's standard, any place where you cannot see the Milky Way in considerable detail, and where M31 isn't instantly obvious to the unaided eye, is very poor indeed. That cutoff is about Bortle 5, halfway in the Bortle Scale.

Tony's comment is kinda interesting. I too have seen M31 in the city (it was pretty much overhead) when I couldn't even come close to seeing the Milky Way. And I have had the occasion to see the Milky Way out in the country while M31 wasn't visible naked-eye. 

 

Someone who isn't trapped under a rainy sky at 38 degrees and drinking 10% Imperial Stouts will have to figure this out.



#28 SeaBee1

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Posted 23 November 2019 - 08:21 PM

Heck, I can't see the MW or M31, or much of anything in my sky naked eye. Literally a handful of stars... I tried to find my Bortle scale number, but failed... so I just call mine Bortle 20... I can go anywhere and it is better than at my backyard...

 

And I still get to see stuff...

 

Good hunting!

 

CB


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#29 airbleeder

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Posted 23 November 2019 - 08:29 PM

It's pretty simple.
If I can't see my own galaxy, I'm in a bad Bortle.
if I can see my own galaxy, I'm in a fair Bortle.
If I can see another galaxy, I'm in a good Bortle.

jeez, you folks are a tough audience - just trying for a bit of humor here... I never expected a group such as this would take a suggestion to actually go outside and just look up seriously. Why, there's not even any decimal places in my "measurement technique"...

   I laughed, but you didn't hear me.


Edited by airbleeder, 23 November 2019 - 11:39 PM.

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#30 jaraxx

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Posted 23 November 2019 - 08:30 PM

Towards a modified Bortle Scale # 2:

 

If you can't see the constellations because there are too few stars it's Bad Bortle.

If you can see the constellations its Mid Botle

If you can't see the constellations because there are too many stars it's Good Bortle.

 

And if you can't see the constellations because there are too many stars and there is no light anywhere and you look down and see your own very faint shadow, why, you've past Bortle. WELCOME TO HEAVEN.


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#31 birger

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Posted 24 November 2019 - 04:48 AM

I was at a Bortle 3/4 site about a month ago. I could easily see M31 with the naked eye (it was even obvious just after I got out of the car), and M33 could be noticed with averted vision, and was very easily seen in 8x40 binoculars. However, the Milky Way was not that impressive. Not visible through Perseus or Auriga, and not too obvious through Cygnus. Limiting magnitude was about 6 or so, but I didn't really try to see how far I could go. Temperatures were just below freezing.

 

So this evening of observation fills some of the criteria of Bortle 3, but others only reach Bortle 5. I think the problem was humidity. A nearby city with a mere 4,000 inhabitants brightened the southern sky like a metro and made the Milky Way through Aquila hardly noticeable.

 

If you live in a place where humidity is a major problem, then the Bortle scale might be very misleading at times.


Edited by birger, 24 November 2019 - 04:48 AM.

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#32 Araguaia

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Posted 24 November 2019 - 05:10 AM

 

 

If you live in a place where humidity is a major problem, then the Bortle scale might be very misleading at times.

 

Humidity, smoke, or haze on a dry night.  

 

From the same benighted place, on different "clear" nights in the dry season, I have held  M33 in direct vision, seen it only fleetingly at the edge of averted vision, or not seen it at all.  So my Bortle must vary between 1 and 5!



#33 Allan Wade

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Posted 24 November 2019 - 07:28 AM

With the Bortle scale, many people discuss M33. It’s not such a good marker here in Australia as it only reaches around 30 degrees altitude. Many nights I’ve strained with averted vision to see it. Then on one night I wondered what that really bright patch of light in the north was. It was M33 blazing away in direct vision. The easiest I’ve ever seen it in either hemisphere. 

 

So my site doesn’t generally conform to some of the Bortle scale markers. I use my wide SQM and believe that’s a better indicator of sky conditions. When someone quotes an SQM reading I have a much better understanding of their observing conditions than if they try providing a Bortle number.



#34 Arcticpaddler

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Posted 24 November 2019 - 01:05 PM

 

So my site doesn’t generally conform to some of the Bortle scale markers. I use my wide SQM and believe that’s a better indicator of sky conditions. When someone quotes an SQM reading I have a much better understanding of their observing conditions than if they try providing a Bortle number.

I think that SQM readings have some of the same problems that the Bortle Scale does.  I live in an area with very little light pollution (Bortle 3 by any criteria), but have seen some exceedingly dark nights when there was smoke in the atmosphere from distant wildfires, and could only see five stars.  Being outside was like being in a dark closet.   SQM readings do not take into account transparency, but only sky brightness.


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#35 dr.who

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Posted 24 November 2019 - 05:31 PM

It's pretty simple.
If I can't see my own galaxy, I'm in a bad Bortle.
if I can see my own galaxy, I'm in a fair Bortle.
If I can see another galaxy, I'm in a good Bortle.
jeez, you folks are a tough audience - just trying for a bit of humor here... I never expected a group such as this would take a suggestion to actually go outside and just look up seriously. Why, there's not even any decimal places in my "measurement technique"...


Sorry mate. The humor went over my little bean head...
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#36 rzgp33

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Posted 24 November 2019 - 09:19 PM

My app IOS "Good To Stargaze" tells me I'm 4.5



#37 j.gardavsky

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Posted 25 November 2019 - 05:12 AM

It's pretty simple.
If I can't see my own galaxy, I'm in a bad Bortle.
if I can see my own galaxy, I'm in a fair Bortle.
If I can see another galaxy, I'm in a good Bortle.

jeez, you folks are a tough audience - just trying for a bit of humor here... I never expected a group such as this would take a suggestion to actually go outside and just look up seriously. Why, there's not even any decimal places in my "measurement technique"...

It is as simple as with The Weather Stone for qualifying the weather,

 

JG



#38 NYJohn S

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Posted 25 November 2019 - 09:34 AM

Actually, I find M31 quite a bit easier to see than our own Milky Way. With a bit of effort, I can make out M31 naked-eye from my local city park, about 4 miles from the center of Boston, Massachusetts (USA). But I've never seen even a hint of the Milky Way there.

 

To get a rudimentary view of the Milky Way, I need to be about 10 miles from the center of Boston, by which point M31 is bold and bright. I would rate those sites somewhere between Bortle 5 and 6.

 

By Bortle's standard, any place where you cannot see the Milky Way in considerable detail, and where M31 isn't instantly obvious to the unaided eye, is very poor indeed. That cutoff is about Bortle 5, halfway in the Bortle Scale.

This surprised me. I always consider my sky Bortle 5 but now I'm beginning to wonder. I can see the Milky Way overhead with no problem but have never seen M31 naked eye. It certainly isn't obvious so I assumed it wasn't possible. Last night I gave it a shot and with great difficulty I managed to see it with averted vision. It was so faint I had to go back and forth with 10x50 binoculars to confirm. I was using Nu Andromedae to confirm the position and that required averted vision to see. It's a mag 4.53 star and I have seen a magnitude deeper than that so not the best conditions. I'm surprised anyone can see M31 in skies much closer to a city. Maybe my eyesight just isn't good enough. With light pollution it's not only faint but very small naked eye. I've seen it in Borlte 3 skies naked eye and the larger size help me spot it. 



#39 Araguaia

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Posted 25 November 2019 - 10:36 AM

Last night, a rare clear night in the rainy season, I saw 5 galaxies by naked eye.  And still the sky was not as good as it gets.  I could see only 3 galaxies of Stephan's Quintet through the scope.  On a good night I can see all five.



#40 Starman1

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Posted 28 November 2019 - 03:25 PM

My dark sky site is 21.68 SQM but a Bortle class 4...

Yeah, right.

21.68 is a really dark site, and you can see everything Bortle defines as Class 1 in a site that dark.



#41 ChristopherBeere

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Posted 29 November 2019 - 04:03 PM

I have no clue, but I see lots of peolpe describing their darkness or lack of darkness. measured in bortles. So how do you determine what is yours.

To be honest the Bortle scale isnt accurate and needs revising. Thats not to say it isnt useful as a general descriptor of an observing sites quality though. Thats why its become a standard after all.

 

Even photometric devices like the SQM dont really tell the whole story.

 

For me the zodiacal band is the best indicator of sky darkness. Under a remote high altitude desert sky it is over 10 degrees wide and is visible at altitudes below 15 degrees. It stretches from horizon to horizon on a good night at a world class site.

 

In the southern hemisphere the LMC light bridge is also a very good indicator. When the LMC culminates you can see the glow of the cirrus all the way down to Triangulam Australe on the horizon. Thats when you know you are under a proper dark sky.

 

Another good benchmark is the Gegenschien. It is over 40 degrees long and between 15-20 wide under 22+ MPSAS skies.


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#42 Tony Flanders

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Posted 29 November 2019 - 05:33 PM

To be honest the Bortle scale isnt accurate and needs revising. Thats not to say it isnt useful as a general descriptor of an observing sites quality though. Thats why its become a standard after all.

 

Even photometric devices like the SQM dont really tell the whole story.

 

For me the zodiacal band is the best indicator of sky darkness. Under a remote high altitude desert sky it is over 10 degrees wide and is visible at altitudes below 15 degrees. It stretches from horizon to horizon on a good night at a world class site.

 

In the southern hemisphere the LMC light bridge is also a very good indicator. When the LMC culminates you can see the glow of the cirrus all the way down to Triangulam Australe on the horizon. Thats when you know you are under a proper dark sky.

 

Another good benchmark is the Gegenschien. It is over 40 degrees long and between 15-20 wide under 22+ MPSAS skies.

Every time I try to revise the Bortle Scale, I twist myself into knots and give up. It's easy to complain about it, but not easy to improve it.

 

It's important to accept that any guide to sky quality -- whether based on subjective criteria like the Bortle Scale or objective measurements like the SQM -- is necessarily oversimplified and fundamentally flawed. Sky quality cannot be reduced to a single, linear scale. At the very best, it has three independent axes: skyglow, transparency, and seeing. And separating those, though possible in theory, is mighty hard to achieve in practice.

 

And that doesn't even take into the account that even at one single site, different sectors of the sky can have very different properties.

 

And it's impossible to make a subjective scale based on particular objects, such as M33 or the LMC, that works for all latitudes, or even most latitudes.


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#43 John Fitzgerald

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Posted 29 November 2019 - 06:09 PM

At my home site, at about 1,220 feet MSL, the sky darkness is very different in different directions.  To the NE, east, and south, and in the zenith, it is pretty dark.  The north, not as good.  The west and NW are quite polluted, but covered by trees and structures anyway. The SW is in between, about like the north.  At my dark site, almost 25 miles to the SE, the east, south, and zenith are as about as good as it can get in this part of the country at 2,420 feet MSL. There is a tiny light dome in the north, and small ones in the west and SW, behind the tree lines which extend up about 15 degrees or so.  On rare, transparent nights, it's hard to tell subjectively from Ft. Davis, TX skies, as far as sky quality.  On a coupe of good nights in very early spring, Canopus has been prominent near the S horizon.

 

Most sites have some variability, depending on local transparency, direction of any light domes, seeing, and elevation.  If I had an accurate SQM to measure with, my home site would probably vary by two magnitudes, depending on the aim direction.  The Exploradome is placed optimally to take advantage of the site.


Edited by John Fitzgerald, 29 November 2019 - 06:11 PM.


#44 Redbetter

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Posted 29 November 2019 - 06:28 PM

 

For me the zodiacal band is the best indicator of sky darkness. Under a remote high altitude desert sky it is over 10 degrees wide and is visible at altitudes below 15 degrees. It stretches from horizon to horizon on a good night at a world class site.

 

In the southern hemisphere the LMC light bridge is also a very good indicator. When the LMC culminates you can see the glow of the cirrus all the way down to Triangulam Australe on the horizon. Thats when you know you are under a proper dark sky.

 

Another good benchmark is the Gegenschien. It is over 40 degrees long and between 15-20 wide under 22+ MPSAS skies.

 

The problem with the zodiacal light/band/gegenschein (all manifestations of the same thing, illumination of dust particles along the ecliptic) is that they are highly dependent on the time of year, where they fall on the Milky Way background, and the observer's latitude/object's elevation above the horizon.  The gegenschein really doesn't end in dark sky.  It just transitions into the zodiacal band that connects to the zodiacal light.  The Milky Way largely masks whatever component is overlaid on it--and makes it more difficult to distinguish the parts as they approach/blend into it.

 

Relative visibility of the band and gegenschein are also heavily influenced by transparency.  When transparency is good and the components are well positioned they are easy to trace across a dark sky.  When transparency is compromised the gegenschein shrinks and becomes indistinct, while the band is almost entirely lost.  Any horizon effects (e.g. extinction/haze/less than perfect transparency) only make it more difficult, particularly when the ecliptic is at a shallow angle.

 

Using any single Bortle number to describe a site has problems because even at a pristine site transparency is a factor.  For borderline Bortle 1, Bortle 2 or 3 sites it is even more noticeable.  The site I use most frequently peaks above 21.7 MPSAS overhead on the best nights (little Milky Way interference, excellent transparency) and last month was as low as ~21.1 MPSAS for several sessions when there was a general dust/smoke extinction/very milky sky during late afternoon, but no particularly identifiable direct reduction of transparency (e.g. no obvious smoke haze, or thin high level cloud, light fog, etc.)  That is a range starting at about Bortle 1/2 transition at best on the darker side of the sky and overhead, and a weak Bortle 3 on those poorer nights with something like Bortle 4 on the brighter side of the sky. 


Edited by Redbetter, 30 November 2019 - 03:53 PM.


#45 Araguaia

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Posted 30 November 2019 - 05:30 AM

 

 

And it's impossible to make a subjective scale based on particular objects, such as M33 or the LMC, that works for all latitudes, or even most latitudes.

 

Or for most of the year.  I can see both M33 and the LMC from here, but only for a few months of the year.

 

Zodiacal band works for me, as it is always high in the sky here in the tropics.  The amount of structure visible in the Milky Way also works for half the year, when one side or the other is high up.  

 

Gegenschein... depends if it is superimposed on the Milky Way or not.  Again, its visibility is seasonal.


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#46 Feidb

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Posted 02 December 2019 - 09:29 AM

Bortle dee, Bortle dum. Don't care. When I go out, I either see it or I don't. Simple as that.


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#47 bunyon

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Posted 02 December 2019 - 10:21 AM

Any folks really worried about their skies need to read some of the posts above carefully. Don't get lost in trying to "rate" your skies. Go out and look at the sky. Over time, you may notice one of your observing sites is better than others but, unless there is a clear difference, go with the one easiest to access. And if you don't have access to a "great" site? Relax and go out anyway.

 

There are reasons to try to measure, down to second or third decimal place accuracy, the quality of a sky. But those reasons are mostly for cloudy nights when you've already read all the books within a mile walk. 


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#48 j.gardavsky

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Posted 02 December 2019 - 12:46 PM

Bortle dee, Bortle dum. Don't care. When I go out, I either see it or I don't. Simple as that.

On the 30th November the skies have cleared between the clouds for about 2 and a 1/2 hour,

just Bortle 5 instead of Bortle 4,

but I have nailed down the Sharpless Sh2-202 nebula south of Pazmino's St 23, through the 15x85 binoculars,

 

so,

Bortle dee, Bortle dum, Bortle hummeldumm

 

Best,

JG


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#49 airbleeder

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Posted 02 December 2019 - 04:21 PM

   I agree with Bunyon, just go out and observe. I really don't know what bortle zone I'm in or which zone my brother and sister in law are in, but I do know it's  a heck of a lot darker there than here.  It doesn't stop me from observing here when I can.

   I thought I saw M31 naked eye from my yard once. From my brother's in the N Georgia mtns, I know I see it, along with M33 and the Double Cluster. It makes me look forward that much more to my next visit with them, but it doesn't keep me from observing from my yard if an interesting object is up. Sometimes to just scan my light polluted, tree limited, little piece of sky. Only difference is that my sessions are probably going to be longer when under the darker sky. 

    Whether here or there, light polluted or dark site, one thing for sure, I've always felt better after observing than I did before. If I dwell on what I can't see, I might miss what I did see.


Edited by airbleeder, 02 December 2019 - 04:24 PM.

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#50 MikeMiller

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Posted 02 December 2019 - 05:14 PM

Your link actually linked to the wikipedia article, instead of to astrobackyard (which has significantly more detail).

 

https://astrobackyar...e-bortle-scale/


Edited by MikeMiller, 02 December 2019 - 05:15 PM.



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