47 replies to this topic

[kingsbishop]

Is it possible to see any twilight when the sun is 17.9 degrees below the horizon because it seems like too much of a coincidence that twilight ends when the sun is exactly 18 degrees below the horizon?

[Waddensky]

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.

[Astrojensen]

17.9° might be pushing it, but I know from experience that it's possible, when the Sun is around 17° below the horizon.  

 

 

Clear skies!

Thomas, Denmark

[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.

in the graph the darkest is 14 degrees and I have seen twilight when the sun is 14 degrees below the horizon unless if it’s talking about zenith brightness and not brightness on the horizon.

[kingsbishop]

17.9° might be pushing it, but I know from experience that it's possible, when the Sun is around 17° below the horizon.


Clear skies!
Thomas, Denmark

have you ever experimented what the sun altitude below the horizon was when you couldn’t see any twilight on the horizon.

[Urban Uraniborg]

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

Edited by Urban Uraniborg, 16 February 2024 - 01:13 PM.

[Tony Flanders]

17.9° might be pushing it, but I know from experience that it's possible, when the Sun is around 17° below the horizon.

In this particular case one degree makes a very big difference. The Sun's glow isn't subtle at all when it's 16 degrees below the horizon.

[Starman1]

Is it possible to see any twilight when the sun is 17.9 degrees below the horizon because it seems like too much of a coincidence that twilight ends when the sun is exactly 18 degrees below the horizon?

It depends.  I find that measuring the darkness of the sky with a wide SQM drops during twilight and levels out at around the 18° point.

BUT, at a truly dark site, you can tell the western horizon is brighter than the eastern horizon until the sun passes its nadir and starts back up.

Then the eastern horizon appears brighter than the western horizon.

I've seen this many many times--there is no time of night when the horizons are completely dark when you are fully dark adapted.

The sun has some influence all night long.

 

Of course, we don't look at the horizon, or we shouldn't anyway.

[Starman1]

have you ever experimented what the sun altitude below the horizon was when you couldn’t see any twilight on the horizon.

Yes, as implied by my previous post, the sun has to pass the nadir by some degrees before the western horizon is completely dark to the dark-adapted eye.

[luxo II]

It's purely a convention. Down low near the horizon the sky is never as dark as it is at higher elevations anyway. This fish-eye view of the sky shows the phenomenon. https://simple.wikip...Gegenschein.jpg

 

And there is the ant-solar point - or gegenshein, also visible.

 

Other issues include oxygen discharge (faint green glow) in the upper atmosphere, aurorae, and the zodiacal light.

[JohnTMN]

Is it possible to see any twilight when the sun is 17.9 degrees below the horizon because it seems like too much of a coincidence that twilight ends when the sun is exactly 18 degrees below the horizon?

NO! you have to wait until it's 18.3 below horizon. Unless there is a mountain or tree's in the way,,

[Octans]

It depends.  I find that measuring the darkness of the sky with a wide SQM drops during twilight and levels out at around the 18° point.

BUT, at a truly dark site, you can tell the western horizon is brighter than the eastern horizon until the sun passes its nadir and starts back up.

Then the eastern horizon appears brighter than the western horizon.

I've seen this many many times--there is no time of night when the horizons are completely dark when you are fully dark adapted.

The sun has some influence all night long.

 

Of course, we don't look at the horizon, or we shouldn't anyway.

That sounds like the zodiacal light from sunlight scattered off dust grains around the solar system rather than Earth's atmosphere. Even though it's normally thought of as just the triangular wedge of light near the ecliptic, it really covers the whole sky and is brighter closer to the direction of the Sun and ecliptic, but can visibly extend very far from either.

Edited by Octans, 17 February 2024 - 02:18 PM.

[Tony Flanders]

That sounds like the zodiacal light from sunlight scattered off dust grains around the solar system rather than Earth's atmosphere.

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.

[kingsbishop]

It depends. I find that measuring the darkness of the sky with a wide SQM drops during twilight and levels out at around the 18° point.
BUT, at a truly dark site, you can tell the western horizon is brighter than the eastern horizon until the sun passes its nadir and starts back up.
Then the eastern horizon appears brighter than the western horizon.
I've seen this many many times--there is no time of night when the horizons are completely dark when you are fully dark adapted.
The sun has some influence all night long.

Of course, we don't look at the horizon, or we shouldn't anyway.

but is that caused by zodiacal light?

[luxo II]

but is that caused by zodiacal light?

No it is caused by atmospheric scattering.

 

If you look at the image I linked previously the glow is visible right round the horizon. That it doesn't rotate overhead with the stars - it's fixed with respect to the horizon - does rather mean it is an atmospheric effect, not beyond.

 

And as starman describes one side is brighter than the other. But to really see this well you need to find a dark mountaintop site with a good 360 degree horizon, and sit up there all night.

I've done that a couple of times long ago, at Mt Kaputar.

Edited by luxo II, 18 February 2024 - 10:13 PM.

[kingsbishop]

No it is caused by atmospheric scattering.

If you look at the image I linked previously the glow is visible right round the horizon. That it doesn't rotate overhead with the stars - it's fixed with respect to the horizon - does rather mean it is an atmospheric effect, not beyond.

And as starman describes one side is brighter than the other. But to really see this well you need to find a dark mountaintop site with a good 360 degree horizon, and sit up there all night.
I've done that a couple of times long ago, at Mt Kaputar.

is it like what the app Sky Guide shows because I can notice it on the app when I turn it to night.

[luxo II]

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.

Edited by luxo II, 19 February 2024 - 12:07 AM.

[Starman1]

but is that caused by zodiacal light?

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.

[JohnTMN]

is it like what the app Sky Guide shows because I can notice it on the app when I turn it to night.

Uhm, maybe,,turn off the "app", and go outside and look up?

It gets better when you stay out there, and keep looking up after dark.

Just trying to help, honest.

Edited by JohnTMN, 19 February 2024 - 02:33 AM.

[luxo II]

Sharman the sky where he’s observing is Bortle 6-7 - not a chance of seeing Zodiacal light or the gegenschein.

Edited by luxo II, 19 February 2024 - 04:58 AM.

[kingsbishop]

Sharman the sky where he’s observing is Bortle 6-7 - not a chance of seeing Zodiacal light or the gegenschein.

I attempted trying to see it over the ocean in a bortle 4 still couldn’t see it.

[kingsbishop]

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.

I only saw zodiacal light once when I went to a bortle 3-2 site i only saw it near the horizon i suppose you need good eye sight to see higher than 45 degrees.

[Inkswitch]

I only saw zodiacal light once when I went to a bortle 3-2 site i only saw it near the horizon i suppose you need good eye sight to see higher than 45 degrees.

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".

[birger]

Depending on season, it would likely be very hard to distinguish from Zodiacal light and/or airglow. I have been annoyed by a -16 degree Sun once, but only when looking near the western horizon.

 

Unless your skies are very dark, I doubt there is much different toward the zenith and anti-solar horizon when the sun is -14 degrees. Cameras of course are much more sensitive, making the whole sky look blue with a sun at -16 degrees.

[Tony Flanders]

Depending on season, it would likely be very hard to distinguish from Zodiacal light and/or airglow. I have been annoyed by a -16 degree Sun once, but only when looking near the western horizon.

 

Unless your skies are very dark, I doubt there is much different toward the zenith and anti-solar horizon when the sun is -14 degrees. Cameras of course are much more sensitive, making the whole sky look blue with a sun at -16 degrees.

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.