50 replies to this topic

[kingsbishop]

The zenith is still very bright indeed when the Sun is 14 degrees below the horizon. Here's a series of measurements that I took with my SQM at Stellafane:

{SKYBRT sqm 9:00} 18.45 Sun -10.7
{SKYBRT sqm 9:05} 18.75 Sun -11.4
{SKYBRT sqm 9:10} 19.40 Sun -12.2
{SKYBRT sqm 9:15} 20.00 Sun -12.9
{SKYBRT sqm 9:20} 20.50 Sun -13.7
{SKYBRT sqm 9:26} 20.90 Sun -14.5
{SKYBRT sqm 9:30} 21.10 Sun -15.1
{SKYBRT sqm 9:35} 21.20 Sun -15.8
{SKYBRT sqm 9:40} 21.25 Sun -16.5


Extrapolating strictly mathematically, this curve should reach 21.29 at 9:50 with the Sun 18 degrees below the horizon, and remain nearly flat after that. In fact my SQM read 21.46 at 1:30 am, but I assume that's due to the decrease in artificial light pollution as lights are turned off late at night.

surely those readings would be much darker if you were in true dark skies.

[Tony Flanders]

surely those readings would be much darker if you were in true dark skies.

Yes, of course. But since they were taken over quite a short time span, it's reasonable to assume that the artificial component of the skyglow remained fairly constant during the series. With a little elementary math, we can deduce what they would have been if the baseline skglow had been 22.00 rather than 21.30:

 

New = -2.5 * log₁₀(0.1^Old/2.5 - 0.1^21.3/2.5 + 0.1^22.0/2.5)
 
Sun      Old       New
-10.7    18.45    18.49
-11.4    18.75    18.80
-12.2    19.40    19.49
-12.9    20.00    20.17
-13.7    20.50    20.78
-14.5    20.90    21.33
-15.1    21.10    21.65
-15.8    21.20    21.82
-16.5    21.25    21.91
 
On the rare occasions when I'm at a genuinely dark site, I have better things to do than take long sequences of SQM readings! I don't normally expect to do a lot of "serious" observing at Stellafane.
 
In any case, I was responding to the claim that the zenith is effectively dark when the Sun is 14 degrees below the horizon. Any way you slice it, the zenith is still very bright indeed at that point. Not bright enough to wipe out the Milky Way entirely; in fact the Milky Way is readily visible at the end of nautical twilight, with the Sun 12 degrees below the horizon. But bright enough to erase most of the Milky Way's detail, and to render the telescopic views of most other galaxies pretty dismal.

Edited by Tony Flanders, 23 February 2024 - 06:36 AM.

[kingsbishop]

Yes, of course. But since they were taken over quite a short time span, it's reasonable to assume that the artificial component of the skyglow remained fairly constant during the series. With a little elementary math, we can deduce what they would have been if the baseline skglow had been 22.00 rather than 21.30:

New = -2.5 * log₁₀(0.1^Old/2.5 - 0.1^21.3/2.5 + 0.1^22.0/2.5)

Sun Old New
-10.7 18.45 18.49
-11.4 18.75 18.80
-12.2 19.40 19.49
-12.9 20.00 20.17
-13.7 20.50 20.78
-14.5 20.90 21.33
-15.1 21.10 21.65
-15.8 21.20 21.82
-16.5 21.25 21.91

On the rare occasions when I'm at a genuinely dark site, I have better things to do than take long sequences of SQM readings! I don't normally expect to do a lot of "serious" observing at Stellafane.

In any case, I was responding to the claim that the zenith is effectively dark when the Sun is 14 degrees below the horizon. Any way you slice it, the zenith is still very bright indeed at that point. Not bright enough to wipe out the Milky Way entirely; in fact the Milky Way is readily visible at the end of nautical twilight, with the Sun 12 degrees below the horizon. But bright enough to erase most of the Milky Way's detail, and to render the telescopic views of most other galaxies pretty dismal.

what would the sqm readings be if it were pointed to the horizon where the sun set for each sun altitude below the horizon?

Edited by kingsbishop, 23 February 2024 - 04:46 PM.

[Starman1]

what would the sqm readings be if it were pointed to the horizon where the sun set for each sun altitude below the horizon?

An SQM-L, for safety, and to give a good reading, really cannot be aimed below 30°, as the 20° version has a bit of response out to 25° off axis.

Obviously, the brightness so measured would exceed that of the zenith.

And it would not read as dark as the zenith, ever, because of horizon lighting.

 

[The SQM should not be aimed below 70° from the horizon, as it will read falsely too dark if the horizon cuts off the field.]

[kingsbishop]

An SQM-L, for safety, and to give a good reading, really cannot be aimed below 30°, as the 20° version has a bit of response out to 25° off axis.
Obviously, the brightness so measured would exceed that of the zenith.
And it would not read as dark as the zenith, ever, because of horizon lighting.

[The SQM should not be aimed below 70° from the horizon, as it will read falsely too dark if the horizon cuts off the field.]

so what mpsas do you predict the horizon is just after astronomical twilight excluding Zodiacal light?

[kathyastro]

Good eyesight helps, but more important is transparency.  When transparency is good, I can trace the zodiacal light into the zodiacal band for quite a ways and my eyesight is not the best.  I have never traced it all the way, probably up to about 60 degs.

 

When I was young (12-15 yrs old) I used to see zodiacal light all the time and didn't know what I was looking at.  The old timers called it "false dawn".

Definitely transparency is the main factor.

 

I went to a presentation about zodiacal light.  The speaker told us to make sure we were outside with no lights for half an hour to get our eyes properly dark-adapted if we wanted to see it.  So the next clear moonless night, I stepped out the front door, intending to look for the zodiacal light later.  Within the first minute, I looked to the west, and there it was, totally obvious!

 

So I can confidently say that, under Bortle 2 skies with excellent transparency and no nearby light sources, the zodiacal light is easy to see with only minimal dark adaptation.

[CrazyPanda]

It's just a definition. When the Sun is at least 18 degrees below the horizon, no visible parts of the atmosphere are illuminated by the Sun, so the zenith is as dark as it gets. The exact value depends on a lot of things of course, but it's easier to say 18 degrees than a more accurate value that differs each night and for each location.

 

Take a look at this graph for example.

Not sure if the zenith is as dark as it will get at 18 degrees. With my SQM-L, I see up to a 0.3 MPSAS drop from astronomical night (18 degrees) to a couple of hours later depending on transparency and time of year.

 

At my club's observing site one spring, one of the members said we had entered astronomical twilight and the sky was as dark as it was going to get. I measured the zenith at 21.0 MPSAS. By the time we were all headed home around 1AM, the sky was reading 21.3 MPSAS. I've seen this every time I'm out.

 

The darkest part of the night always falls roughly around the sun's most southerly position.

[Starman1]

so what mpsas do you predict the horizon is just after astronomical twilight excluding Zodiacal light?

There is never a time when there is NO Zodiacal light, merely times when it is at a much lower angle.

Extinction at a high altitude site with superbly clear air can reach as low as 0.12 magnitudes per atmosphere, or about 1.2 magnitudes at the horizon compared to the zenith.

Add in Zodiacal light, atmospheric glow, and a lower altitude, and the extinction can easily exceed 2-3 magnitudes at the horizon.

How that exactly translates to mpsas at the horizon, I'm not sure.

Every ALL-sky image I've ever seen shows the horizon MUCH brighter than the zenith, even at light-polluted sites.

So I suspect 2-3 magnitudes is not a bad estimate.

[Starman1]

Not sure if the zenith is as dark as it will get at 18 degrees. With my SQM-L, I see up to a 0.3 MPSAS drop from astronomical night (18 degrees) to a couple of hours later depending on transparency and time of year.

 

At my club's observing site one spring, one of the members said we had entered astronomical twilight and the sky was as dark as it was going to get. I measured the zenith at 21.0 MPSAS. By the time we were all headed home around 1AM, the sky was reading 21.3 MPSAS. I've seen this every time I'm out.

 

The darkest part of the night always falls roughly around the sun's most southerly position.

Here in SoCal, a lot of exterior lights get turned off between midnight and 2am.

I reliably get my darkest SQM reading of the night about that time, though it does depend on the altitude of the Milky Way.

[N-1]

It was June(1997) a week past solstice when I saw Continuous Astronomical Twilight. The location was 50° N Ontario, Canada. So right at that 17°below the horizon. I stayed up all night hoping to see the Aurora for the first time. That is what I thought I was looking at, at first, ….”must be the Aurora far to the north.” Then I waited hours longer to see it turn into dawn. It was then I knew the tools of cartography would be most useful for astronomy and began studying. 
I would say the possibility of see CAT at 49° N near solstice is possible with the horizon.  The CAT that I saw stretched over 1° above the horizon at midnight. However the Hudson Bay beyond the horizon to the north may have been a reflective favorable factor. 

 

Clear Continuous Astronomical Twilight!!

-Micah

I sat on a 1,000-metre-peak in the far south of New Zealand in December of 2014, looking towards Antarctica, trying to capture (visually and photographically) any sort of twilight at local midnight with the Sun around 20° below the horizon. This would have confirmed tales I had heard from locals that it doesn't get truly dark there at the height of summer. The finding: Nope. No twilight. Sun well and truly gone for over 2h. However, there was a twist: Something did appear to illuminate the horizon. It was unevenly distributed (so clearly not twilight), very low and very slow-moving (if moving at all). Examination of the photos later showed that this was the auroral oval at Kp=2. Beams maybe a few degrees high were captured. Mental note - I need to dig out that data and make it into a timelapse/star trail composite. It was a special night. 
 

Edited by N-1, 25 February 2024 - 07:59 PM.

[Tony Flanders]

At my club's observing site one spring, one of the members said we had entered astronomical twilight and the sky was as dark as it was going to get. I measured the zenith at 21.0 MPSAS. By the time we were all headed home around 1AM, the sky was reading 21.3 MPSAS. I've seen this every time I'm out.

21.0 mpsas minus 21.3 mpsas equals 21.25 mpsas, which is far too much to attribute to lingering glow from the Sun. If there is any lingering skyglow with the Sun 19 degrees below the horizon it would be on the order of 25 mpsas or fainter.

 

As Don says, the difference is undoubtedly due to the fact that a bunch of lights got turned out in the interrim. In my experience the artificial component of skyglow almost always drops at least 30% between 10 pm and 1 am.

[kingsbishop]

21.0 mpsas minus 21.3 mpsas equals 21.25 mpsas, which is far too much to attribute to lingering glow from the Sun. If there is any lingering skyglow with the Sun 19 degrees below the horizon it would be on the order of 25 mpsas or fainter.

As Don says, the difference is undoubtedly due to the fact that a bunch of lights got turned out in the interrim. In my experience the artificial component of skyglow almost always drops at least 30% between 10 pm and 1 am.

but the sky is never 25 mpsas so then it’s impossible to see any glow when the sun is -18 degrees or more below the horizon.

[kingsbishop]

I wouldn't trust an app with this, you need to see the real thing in a seriously dark location. You won't see it in Sydney since the sky is dominated by light pollution anyway.

I saw it at Mt Kaputar and the effect is also visible in images I took at Mt Cook in NZ, which is likewise pretty dark - the sky is brighter near the horizon.

where do you think the nearest place is where i can see it?

[N-1]

Could be early after twilight and quite late before morning twilight, but zodiacal light isn't going to impact the brightness of the horizon when the sun is at its nadir.

Airglow, on the other hand, is.
There are also light bridges connecting the Zodiacal light with the gegenschein, and in summer, these intersect the horizon at a shallow angle at midnight, both in the NE and the NW.

[timelapser]

I suspect the residual twilight glow when the sun is around -18 deg will vary a fair bit depending on cloud conditions along the line of sight towards the sun, and especially on dust/smoke conditions.  High-altitude volcanic/forest fire aerosols can have a big effect on twilight.

 

The way to minimize any potential contamination from zodiacal glow is to observe towards the north at local midnight from a latitude of 48.5 deg or higher in June (or south/December for the southerners), when the sun would be around -18 deg.

[kingsbishop]

It's just a definition. When the Sun is at least 18 degrees below the horizon, no visible parts of the atmosphere are illuminated by the Sun, so the zenith is as dark as it gets. The exact value depends on a lot of things of course, but it's easier to say 18 degrees than a more accurate value that differs each night and for each location.

Take a look at this graph for example.

when you say visible parts do you mean horizon too?

[kingsbishop]

Good point. In real life, when you look toward the horizon directly above the Sun, you're going to see both light scattered off the atmosphere and light scattered off interplanetary dust. There's no way to distinguish them visually, so how do you know when scattered light has become undetectably dim.
Obviously sunlight scattered off the atmosphere doesn't actually stop when the Sun travels more than 18 degrees below the horizon; it just gets ever more attenuated. And no doubt soon drops far below the level of the zodiacal light and airglow.

can you tell by the colour Because natural twilight seems to be more orange than Zodiacal light

However Zodiacal light is in space and the scattering would make it orange so i may be wrong

[Starman1]

can you tell by the colour Because natural twilight seems to be more orange than Zodiacal light

However Zodiacal light is in space and the scattering would make it orange so i may be wrong

At pristine, transparent, 22.0 sites, the Milky Way is silver-white, the Zodiacal light is yellow, and the final bits of sky glow are a very deep blue to blue-purple.

After that, you have the natural "horizon lightening" known as "Extinction" that causes the sky below 45° to appear brighter than the sky at the zenith.

[kingsbishop]

At pristine, transparent, 22.0 sites, the Milky Way is silver-white, the Zodiacal light is yellow, and the final bits of sky glow are a very deep blue to blue-purple.
After that, you have the natural "horizon lightening" known as "Extinction" that causes the sky below 45° to appear brighter than the sky at the zenith.

when i was in a 21.9 sky when Zodiacal light was almost flat i had to use long exposures to see sunlight scattering after Astro twilight
But mind there was a small bortle 4-3 town in that direction so it could have been light pollution.

But i have been in a bortle 3 area and did a long exposure after Astro twilight and saw orange in that direction but it wasn’t really visible to my eye

[kingsbishop]

Not sure if the zenith is as dark as it will get at 18 degrees. With my SQM-L, I see up to a 0.3 MPSAS drop from astronomical night (18 degrees) to a couple of hours later depending on transparency and time of year.

At my club's observing site one spring, one of the members said we had entered astronomical twilight and the sky was as dark as it was going to get. I measured the zenith at 21.0 MPSAS. By the time we were all headed home around 1AM, the sky was reading 21.3 MPSAS. I've seen this every time I'm out.

The darkest part of the night always falls roughly around the sun's most southerly position.

if i do a long exposure i have seen twilight below -18 degrees even in a bortle 4 however when i look with my Naked eye i struggle to see twilight when the sun is around -17 degrees since it is hard to tell the difference between airglow, flat Zodiacal light and all other natural stuff but from what is have seen the change is brightness really slows down when the sun gets to around -17 degrees and then just keeps getting slowly darker until the sun rises on the other end

[Starman1]

If the sky is bright due to oxygen glow from the sun's radiation, then it will lessen for hours after sunset.

Also, nearby towns and cities have a lot of lights go out after 2am.

Many years of measuring sky brightness all night taught me that 2am is almost always the darkest time of the night.

It does get brighter after that point and the eastern horizon gets noticeably brighter than the western horizon after 3am, even when morning twilight doesn't start till 5am.

[wuschel]

At pristine, transparent, 22.0 sites, the Milky Way is silver-white, the Zodiacal light is yellow, and the final bits of sky glow are a very deep blue to blue-purple.

Thank you. I currently try to estimate in which bortle zone my observing spots are, and since I seem to have some issues with vision (trouble focusing stars with naked eye) I wanted to use zodiacal light.
Today I went out in the morning and I indeed saw it! It had a warm color, which confused me because online I read it should be white.
I am confident I saw ZL because 1) the sky was black 1.5 hours before and 2) the elongated cone stood at an angle of ~75° to the horizon, and it reached right into the beehive cluster (starting from Spica).

[kingsbishop]

If the sky is bright due to oxygen glow from the sun's radiation, then it will lessen for hours after sunset.
Also, nearby towns and cities have a lot of lights go out after 2am.
Many years of measuring sky brightness all night taught me that 2am is almost always the darkest time of the night.
It does get brighter after that point and the eastern horizon gets noticeably brighter than the western horizon after 3am, even when morning twilight doesn't start till 5am.

after Sunset the sky will get darker pretty quickly until the sun is around -17 degrees below the horizon and then it slows down until somewhere between -20 and -30 degrees then really slows down after that

But ofc cameras can pick up more than the eye so these observations are only from the human eye

[lanndonkane]

Circling back - when I went out last week, it said Astronomical night started at 11:30pm, which is when the sun was 18 degrees below horizon. However, it seemed to me that things weren’t fully dark, and the sky seemed MUCH darker when I packed up around 12:30. What could have caused this?

[Starman1]

Circling back - when I went out last week, it said Astronomical night started at 11:30pm, which is when the sun was 18 degrees below horizon. However, it seemed to me that things weren’t fully dark, and the sky seemed MUCH darker when I packed up around 12:30. What could have caused this?

• improvement in transparency of the air, lessening light scatter.
• reduction in atmospheric glow from the sun as the night progresses.
• local municipalities turning off outdoor lights.
• a nearby business with a lighted parking lot turning off its lights.
• a reduction in traffic on nearby roads.
• sun dropping farther below the horizon.  At a truly dark site, I am sure I can see when the sun passes the nadir by looking at the horizon light in the east and west.
• businesses open late finally shutting down.
• almost everyone going to bed and turning off house and porch lights.