10 replies to this topic

[kingsbishop]

I have always been curious since at -18 degrees is when the sky is around it’s natural darkest on a clear night at a dark site but what about when it is overcast, rainy or stormy what would be the sun altitude for it to be pitch dark I assume it would be higher than -18 degrees

I know it is dependent on how much cloud cover and rain there is but what is the approximation?

[KBHornblower]

I would expect the cloud cover to dim the light from the stars, the lingering twilight and the airglow by the same factor.  That should make the the brightness level off when the Sun reaches -18, just as with a clear sky.  Of course this is assuming no light from the ground.  The least bit of light pollution would mask the last traces of twilight.

[triplemon]

My best guess: -9 degrees.

Why ?

In photography, guidance for a sunny day is 100,000 lux and overcast 1000 lux, 100x less. But thats the light shining on the scenery, which would not be the exact same as the sky brightness. But lacking a better data point lets assume normal overcast clouds would absorb about 99% of the light passing though.

 

100x is 5 astronomical magnitudes, so at a SQM of 17 mags above the clouds it should be below those clouds as dark as naturnal sky brightness of 22 mags. And yes - thats still not "pitch dark" like in you can't see anything.

 

And here is the sky brighness at Paranal in Chile and some other observatory in the northerm hemispehre. Subtract 90 degrees to convert zenith distance into sun angle below the horizon:

Edited by triplemon, 08 December 2024 - 04:20 AM.

[kingsbishop]

My best guess: -9 degrees.

Why ?

In photography, guidance for a sunny day is 100,000 lux and overcast 1000 lux, 100x less. But thats the light shining on the scenery, which would not be the exact same as the sky brightness. But lacking a better data point lets assume normal overcast clouds would absorb about 99% of the light passing though.

100x is 5 astronomical magnitudes, so at a SQM of 17 mags above the clouds it should be below those clouds as dark as naturnal sky brightness of 22 mags. And yes - thats still not "pitch dark" like in you can't see anything.

And here is the sky brighness at Paranal - the ESO southern observatory. Subtract 90 degrees to convert zenith distance into sun angle below the horizon:

I have heard from astronomers that when it is overcast in a truly dark site when the sun is -18 degrees below the horizon or more it is like covering your eyes with blankets you cannot see absolutely anything but is that true?

[WillR]

Lol, this is the very definition of idle curiosity. 

[kingsbishop]

Lol, this is the very definition of idle curiosity.

you need to wonder about the what if’s if you are into astronomy which there are so many otherwise what is the fun

[triplemon]

I have heard from astronomers that when it is overcast in a truly dark site when the sun is -18 degrees below the horizon or more it is like covering your eyes with blankets you cannot see absolutely anything but is that true?

Yes, I have been camping in deep forest locations in the north cascades NP. On an overcast night it gets truely zero visibility dark. The only thing you see is electrostatic discharges of pretty much anything nylon rubbing nylon.

Its pretty confusing, likely even more so if you are used to do things at night without flashlights, being fully dark adapted.

Edited by triplemon, 08 December 2024 - 06:59 PM.

[kingsbishop]

Yes, I have been camping in deep forest locations in the north cascades NP. On an overcast night it gets truely zero visibility dark. The only thing you see is electrostatic discharges of pretty much anything nylon rubbing nylon.

Its pretty confusing, likely even more so if you are used to do things at night without flashlights, being fully dark adapted.

but what sun altitude below the horizon does the sun have to be to be able to see absolutely 0 on overcast

[ABQJeff]

but what sun altitude below the horizon does the sun have to be to be able to see absolutely 0 on overcast

Well -18 degrees to be safe that it is absolutely zero and no twilight has a chance of leaking thru.  And I assume we are only talking New Moonand at a Bortle 1 site....otherwise there will be plenty of other light (my home skies are brighter on cloudy nights than clear nights!).

[kingsbishop]

Well -18 degrees to be safe that it is absolutely zero and no twilight has a chance of leaking thru. And I assume we are only talking New Moonand at a Bortle 1 site....otherwise there will be plenty of other light (my home skies are brighter on cloudy nights than clear nights!).

yep we are but i wonder if it absolute 0 at -17 degrees

[triplemon]

I read some pappers that put the abslolute sensitivity of human eyes at around -25 mags. Makes aense to me as I feel like 1.5mm exit pupil makes views " fieldstop dark" under truly dark skies.
So with the data from my previous posts that would be at -20 mags above the clouds or about -12 deg sun elevation.