38 replies to this topic

[Don W]

It’s the same as nearby Sedona and Flagstaff (and Lowell Observatory).

 

Bortle 4

[Augustus]

I was really dismayed driving down 17 between those 2 and seeing a faint glow from Phoenix

[MEE]

How did you figure it out?

[Don W]

https://www.lightpol...2l0eSI6Ijg1In0=

[MEE]

The Bortle rating of a location is not determined by a color coded map. It’s determined by what you SEE, according to John Bortle himself:
https://skyandtelesc...arkSkyScale.pdf

Local lights & transparency (dust/haze/smoke) affect sky darkness.

This other method was incorrectly started years ago and it spread.

Best just to say you live in the (name the color) zone.

Then mention the Bortle level of each night, based upon your naked eye observations.

It’s not a perfect scale, but it does have lots of good points.

Let’s respect John’s original intent.

[moefuzz]

Bortle 1 is 22.00? I was under the impression that 21.90 and up would or should be Bortle 1??

[Illinois]

Bortle 1 is 22.00? I was under the impression that 21.90 and up would or should be Bortle 1??

It said 21.75 to 22.00 is Bortle 1 but for me is 21.90 to 22.00 is Bortle 1 .  22.00 is perfect dark sky but rare is very small area in this Earth.  

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[moefuzz]

It said 21.75 to 22.00 is Bortle 1 but for me is 21.90 to 22.00 is Bortle 1 .  22.00 is perfect dark sky but rare is very small area in this Earth.  

Exactly!

 

When I followed the OP's (Don's) link to his general location on the map and clicked around a bit,

-just outside of Don's  Cottonwood AZ home the window popped up to show me Bortle 3 as 21.89

 

???

 

Bortle 3 @ 21.89 ??? somethings not right here

 

So that tipped the radar off and I wondered if Don/OP is aware that the skymap

~may be~ reporting Bortle differently than those who thought they knew what MPAS actually was.

 

 

 

 

 

I may be Dazed and Confused but I still know what Bortle 1 is,

Or at least I thought I did?

 

What's going on here?

 

 

moe

 

 

 

.

Edited by moefuzz, 15 June 2025 - 11:56 AM.

[moefuzz]

It said 21.75 to 22.00 is Bortle 1 but for me is 21.90 to 22.00 is Bortle 1 .  22.00 is perfect dark sky but rare is very small area in this Earth.  

BTW, thanks for posting the chart to the thread as this is a great visual tool/confirmation for those of us who thought we knew what Bortle actually was.
 

 

As far as "rare" goes, I'm an hour from 21.90 skies and just a little further out puts me into 21.95+, this depending on stability etc.

 

And to the naked eye, the difference between 21.90 and ~21.98 is Very Obvious once you've been there and done that.

 

Way up here in the north, Our 3000 foot altitude and very nice dry air

is fantastic in fall thru winter (no wildfire smoke, no all night twilight -just crystal clear steady naked eye seeing)

 

 

moe

 

 

.

Edited by moefuzz, 15 June 2025 - 11:58 AM.

[moefuzz]

.

 

Just for interests sake ,

Here's an old chart that I use which also confirms  Bortle 1 starts at 21.75.

Although, like some others here, I still kinda consider 21.90 as the real Bortle 1,

 

-your mileage may vary

 

 

 

 

 

 

moe

 

.

Edited by moefuzz, 15 June 2025 - 12:00 PM.

[David Knisely]

That is nice.  I long ago kind of gave up on the Bortle "rating", as the problems and inconsistencies in the descriptions for the various levels made it a somewhat less than useful method of describing how good an observing site is on a given night or what its potential for how good a site it may or may not be.  I prefer to just note in my logbook what the site is and my best visual zenith limiting magnitude on a given night when I am out and leave it at that.  For describing to others what a site I have been to might be like, I tend to fall back on my own 6-level descriptive system of rating the light pollution at a given observing site.  If I want something more scientific, then citing an SQM reading definitely would be the way to go.  Clear skies to you.

 

Edited by David Knisely, 15 June 2025 - 10:05 AM.

[Starman1]

Bortle 1 is 22.00? I was under the impression that 21.90 and up would or should be Bortle 1??

There is no difference visually in a telescope between 21.7 and 22.0 because the same pristine site will vary that much from night to night.

The Bortle scale does NOT have more than a very general association with any SQM readings.

NO ONE HAS SUCCESSFULLY LINKED SQM READINGS TO BORTLE CLASSES.

One cannot say 21.4 is Bortle 3 as 21.4 may contain elements of 5 different Bortle Classes depending on transparency and altitude of the site and proximity of large cities.

SQM readings are objective and measurable, but they only measure darkness, not transparency or seeing.

The Bortle Scale is a subjective evaluation of the night sky which varies from night to night.  Being subjective, it is mostly relevant to the person making the evaluation.

The Color Zones are measurements from space and while good at suggesting a direction to go to get to darker skies, they are poor at predicting local light problems.

NELM is subjective and will vary from person to person at the same site on the same night.

 

The 4 scales only very roughly correspond, and there is a lot of disagreement.

If you must use a Bortle Scale, and I do not recommend it, try:

https://en.wikipedia...ki/Bortle_scale

*the NELM figures are ridiculous and depend 100% on visual acuity.

 

Or, use the nomogram in post #10.

 

I suggest using the SQM measurements found on these charts:

https://djlorenz.git...erlay/dark.html

Drill down in to see your site.  Assume the actual SQM reading will be a tad brighter than the figures suggest, but will give you a more accurate way to rate sites.

 

For describing your site to others, in order from very good (top) to poor (bottom):

SQM

Color Zone

NELM

Bortle

Edited by Starman1, 15 June 2025 - 10:24 AM.

[moefuzz]

....

 

...Or, use the nomogram in post #10.

 

I suggest using the SQM measurements found on these charts:

https://djlorenz.git...erlay/dark.html

Drill down in to see your site.  Assume the actual SQM reading will be a tad brighter than the figures suggest, but will give you a more accurate way to rate sites.

 

For describing your site to others, in order from very good (top) to poor (bottom):

SQM

Color Zone

NELM

Bortle

I think the true value of Bortle is in that it's more universally used amongst the amatuers who come out for the odd star parties  as mpas and sqm usually brings that deer in the headlights look.

 

And I'm glad you liked the chart I posted in post 10

 

 

moe

 

.

[Starman1]

I think the true value of Bortle is in that it's more universally used amongst the amateurs who come out for the odd star parties as mpas and sqm usually brings that deer in the headlights look.

 

And I'm glad you liked the chart I posted in post 10

 

 

moe

 

.

I've posted that chart a few times myself over the years.

The main problem with the Bortle Scale is that too many people think it's an objective measurement, not knowing that the same site may vary over 3 classes or more depending on conditions.

[Tony Flanders]

Bortle 1 is 22.00? I was under the impression that 21.90 and up would or should be Bortle 1??

If we are talking about John Bortle's definitions, it's pretty universally recognized that the main difference between Class 1 and Class 2 is transparency rather than skyglow. In other words, a Bortle 1 site is just a Bortle 2 site on a night of good transparency. On the other hand, nobody except John Bortle himself thinks that all the criteria that Bortle uses to distinguish between classes match up as described in John Bortle's article. And that's particularly true for the 1 vs. 2 distinction.

 

If we're not talking about John Bortle's definitions, then all bets are off; you can define Bortle 1 vs. 2 any way you like, and nobody has grounds to disagree with you.

 

Assuming identical transparency, the difference between a sky with zenith SQM-L reading of 22.00 and one with a zenithal SQM-L reading of 21.90 is barely detectable. However the difference between a 22.0 mpsas sky with great transparency and a 21.0 mpsas sky with average transparency is huge.

[moefuzz]

That is nice.  I long ago kind of gave up on the Bortle "rating", as the problems and inconsistencies in the descriptions for the various levels made it a somewhat less than useful method of describing how good an observing site is on a given night or what its potential for how good a site it may or may not be.  I prefer to just note in my logbook what the site is and my best visual zenith limiting magnitude on a given night when I am out and leave it at that.  For describing to others what a site I have been to might be like, I tend to fall back on my own 6-level descriptive system of rating the light pollution at a given observing site.  If I want something more scientific, then citing an SQM reading definitely would be the way to go.  Clear skies to you.

 

BigIndianBlackMambaMilkyWayCN#4-1024x.JPG

Well you've certainly got nice dark skies in your favour.

 

The problem we have up here is in all night twilight (sunset June 21 is after 11 pm) which doesn't let up til Sept and that's when the dry & warm west winds help keep the overnight skies at almost zero humidity and consistently warm/steady thru the early morning hours (altitude ~3000 feet).

 

On truly clear nights when we venture into our favourite near perfect 22.00 territory the sky is so full of stars that it becomes difficult to find many constellations, and at the far site we go to it can edge right up to 22.00.

 

This is what I was referring to when I stated that there's a difference between ~21.8x and edging on 22.00 as at our alternative 21.8x~ site you can still make out constellations with the naked eye.

 

I honestly don't know how people in the ~relativity low lying and humid eastern US cope but I also realize that there are exceptions to the rule and not all areas have dense cool and humid nights

.

Up here we have basically no moisture in the air. Winter is particularly dry edging on desert like humidity at temps below -20.

Only problem is that January and February can and do dip to the -40's but with luck when the sun comes up we'll make it all the way up to -35.

 

Summer is 24 hour twilight from early April thru early Sept so it's kinda a wash.

The latitude does allow for Noctilucent clouds But I haven't seen any this year due to over night clouds and the odd showers, June is turning out to be somewhat unpredictable this year.

 

 

Pictured below, The Grainy Screen Cap of a Crappy Cell Phone Video is from several years ago at around the 2nd or 3rd week in May at 2:30 in the morning and sadly, neither the cell phone nor the screen capture does justice to how bright, blue, orange and white the Noctilucent clouds were.

 

-The sky was actually quite light and the high latitude Noc Clouds looked similar to what clouds look like after sunset but before navigational twilight sets in, lots of sunlight reflecting upwards what  with the sun at less than 14 degrees below the horizon (at 2 or 3 AM).

 

To the well adjusted naked eye they are very bright and so was the sky.

 

And I know everybody is crazy about Aurora but Noctilucent Clouds are just spectacular and personally knock it out of the park due to the short window of opportunity and the specifics of latitudes of where you can and can't see them.

 

For me, If I were only a few more degrees north I would be out of the zone of opportunity for Noctilucent clouds.

 

 

 

 

 

 

Cheers

 

 

moe of the north

Edited by moefuzz, 16 June 2025 - 08:45 AM.

[Starman1]

Moe,

 

I've been at pristine high altitude sites and seen 21.85-22.05 skies and never once have I been unable to pick out the constellations because there were too many stars.

The "I couldn't identify constellations" trope is an exaggeration, just like the "it was so dark I couldn't see my hands or feet" statement.

Both issues disappear after a minute in the dark.  I grant you, though, that the appearance of the sky is very different when there is NO light pollution at all.

 

Count me as jealous that you can easily get to such sites!  For me, even being in the west of the US, it's a 4.5 hour drive to get to a 21.7-21.8 site and about 10 hours to get to 22.0.

I usually settle for 21.2-21.35.

 

First and second magnitude stars are even brighter in such pristine skies and they help identify a few constellations immediately and all the rest fall in line.

On one such excursion, Equuleus was so bright I thought it had magnitude 2 stars in it, but my charts say:

α Equulei (Kitalpha): The brightest star in the constellation, with a visual magnitude of 3.92.
δ Equulei: The second brightest, with a visual magnitude of 4.49.
γ Equulei: A variable star with a magnitude ranging from 4.58 to 4.77.
β Equulei: A star with a visual magnitude of 5.16.
6 Equulei: A companion star to Gamma Equulei, with a magnitude of 6.1.
ε Equulei: A triple star system with a primary star of magnitude 5.4.

On that night, I used my Telrad to point directly at M15, which was quite visible to the naked eye as a fuzzy spot.

 

When the Great Square of Pegasus looks like a star cluster inside the square because there are so many stars, you know you're seeing deep!

If you can see only two stars within the square, you are reaching visual magnitude 4.6 and your skies are probably light polluted. If you spot eight stars you are hitting magnitude 5.5, and 13 stars 6.0. A tally of 31 to 37 stars on a dark, moonless night means an exceptionally good naked-eye limit of 6.5. At Athens Observatory in the mid-19th century, Julius Schmidt counted 102 stars; he was seeing down to magnitude 7.4!

 

P.S. I've seen noctilucent clouds often, but they're caused by the rocket exhaust of launches from Vandenburg, LOL.

Edited by Starman1, 15 June 2025 - 01:37 PM.

[moefuzz]

Moe,

 

I've been at pristine high altitude sites and seen 21.85-22.05 skies and never once have I been unable to pick out the constellations because there were too many stars.

The "I couldn't identify constellations" trope is an exaggeration, just like the "it was so dark I couldn't see my hands or feet" statement.

Both issues disappear after a minute in the dark.  I grant you, though, that the appearance of the sky is very different when there is NO light pollution at all.

 

Count me as jealous that you can easily get to such sites!  For me, even being in the west of the US, it's a 4.5 hour drive to get to a 21.7-21.8 site and about 10 hours to get to 22.0.

I usually settle for 21.2-21.35.

 

 

 

You know that Is something I've always thought about, does it really end at 22.00?

 

Our near site is an hour and in a slightly different direction we get to truly pristine skies in an hour and a half.

 

I think the altitude makes a big difference even though we're only at the 3000 foot level.

This is right at the point were the western prairies start to step off  as just a few hours east and we've already lost 1000 feet.

We're sorta right on the edge of the high prairie step looking east and gently but steadily downward.

 

Cheers

moe
 

[Starman1]

You know that Is something I've always thought about, does it really end at 22.00?

 

Our near site is an hour and in a slightly different direction we get to truly pristine skies in an hour and a half.

 

I think the altitude makes a big difference even though we're only at the 3000 foot level.

This is right at the point were the western prairies start to step off  as just a few hours east and we've already lost 1000 feet.

We're sorta right on the edge of the high prairie step looking east and gently but steadily downward.

 

Cheers

moe
 

22.0 is a calculation based on illumination profiles.  Several SQM users have seen 22.04-22.06.  Beyond that, though, it usually indicates the view of the sky is blocked by trees or a building.

See: How Dark Can the Night Sky Get

https://www.astropix...g/skybrite.html

[Don W]

All I know for sure is that now after 44 years of observing when people ask what my Bortle is I can answer with confidence that it is 4!!

[Starman1]

All I know for sure is that now after 44 years of observing when people ask what my Bortle is I can answer with confidence that it is 4!!

Except when it's not.  The charts you linked to notoriously underestimate the brightness of the sky.

Most "Bortle 4" skies so determined map out to B5 or B6 when someone actually compares the sky to the Bortle Scale descriptions.

Now, maybe your site is B4, but you won't know until you look.  And the site's skies will vary over time, depending on the transparency of the air, the solar cycle, 

and your experience at gauging the descriptors.

So I suggest you print out the scale and then look at the sky when the moon is not above the horizon.

Make sure it is past the end of astronomical twilight (this time of year, 100 minutes after sunset at 40°N).

[Don W]

Frankly, I don’t give a hoot what my bortle or SQ whatever is. 
 

I stick my head out and if the sky looks good, I observe. If not, I watch America’s home videos.

[dan_hm]

The guy who runs that sight is actually a big critic of the Bortle scale.  He recommends using the VIIRS overlay.

[David Knisely]

moefuz wrote:

 

You know that Is something I've always thought about, does it really end at 22.00?

 

Well, going back to my college days with my Astronomical Instrumentation Techniques course in the mid 1970's, the "ballpark" figure my professor cited was 22 mag/sq-arc-sec, based on small area ground-based photometry.  There were some measurements made on high altitude balloons which had V-band figures up to a full magnitude fainter than the 22.0 figure, but since we live on the ground, the 22.0 seems to be a good guidepost for how dark a really good dark sky site probably can get.  That having been said, as a site (Nebraska Star Party location) which I personally consider to be at least potentially in my "pristine" category, I have only seen a few nights when we actually got a 22.0 reading.  Otherwise, most reading from that site (Merritt Reservoir Dark Sky Park in northwest Nebraska) tended to be in the 21.4 to 21.8 range.  Aside from that, we have people with good eyesight that have seen stars with averted vision down to magnitude 8.1 (verified), and I once hit 7.8 from there, although I can't reach quite that level anymore.  Phenomena like the discreet airglow bands have been seen with the unaided eye, and the Gegenschein is obvious on a good night, with the zodiacal band often being faintly visible.  My local dark sky site isn't that good, but it isn't bad either, as it can allow me to occasionally get down to a ZLM of 6.7 to 7.1 or so on a really good night.  All I need to know is that my sites are definitely darker than what I experience from my driveway even on a dark moonless light, and are good enough to keep me going out to them.  Clear skies to you.  

[Don W]

At my age averted vision is kind of iffy. So I use averted imagination.