How much brighter is the Sun when viewed from space?

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How much brighter is the sun from space than it is when viewed from Earth?
How much brighter is the sun when seen from space than it is when viewed from Earth?
 

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  • #2
Charles Link
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I can't quantify this at the moment, but qualitatively, in the visible, there is not a tremendous difference when looking through the atmosphere with the sun directly overhead. However, during sunset and sunrise, where the light has a much longer atmospheric path, the Rayleigh scattering reduces the intensity considerably, and especially scatters the shorter wavelengths, making for a sun of much lower intensity, and with significantly more red (longer wavelength) getting through than the shorter wavelengths. It is anticipated that for directly overhead, the shorter wavelengths (violet and blue) would be reduced somewhat from what would be observed viewing from space, outside the Earth's atmosphere.
 
  • #4
DaveC426913
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Do you mean objectively or subjectively?

The sky is quite bright - plenty enough to stop your pupils down to near minimum, greatly reducing the apparent brightness of the Sun.

In space, surrounded by black, your pupils could be open quite wide.

Catching a glimpse of the Sun when your eyes are fully dilated, it could appear - at least in theory - as much as 16,000 times brighter:

"..., most estimate that our eyes can see anywhere from 10-14 f-stops of dynamic range.."
https://www.cambridgeincolour.com/tutorials/cameras-vs-human-eye.htm

One f-stop doubles/halves the light.

214 = 16,384
 
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  • #5
sophiecentaur
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Summary:: How much brighter is the sun from space than it is when viewed from Earth?

How much brighter is the sun when seen from space than it is when viewed from Earth?
It's difficult to know exactly what you mean by your question but, if you are just concerned with visible light then we can say that the difference will not be great. (Ball park figure) There is a so called window in the atmosphere which let's the optical (visible) part of the EM spectrum through with very little attenuation. There's no surprise that we evolved with the ability to detect those particular wavelengths because there is more energy around for our eyes to detect. Some wavelengths are absorbed very strongly and there would be very little to 'see', even if we had appropriate sensors. In the visible region the light is hardly absorbed at all but it is scattered.
Assuming you just want to know about visible light then what do you see when you look up? You see a blue(ish) sky and you see the Sun (avoid looking directly at it, of course or you can literally burn holes in your retina!) The blue(ish) sky is there because of (more blue than red) light that has been scattered from rays of light that arrive at different places on the surface so the rays from the Sun that arrive at your eyes will also have lost some of their power, scattered in other directions.
If you compare how much brighter the ground looks, compared with how bright shadows look, that will give you an idea of how much of the direct sunlight is scattered and arrives indirectly. Look at the 'dropper' tool in Photoshop (or any other app) and you'll see that the RGB values in the shadows are about 1/3 to 1/4 the values of the same objects in full sunlight with a cloudless sky. That sort of supports the figure in
30% give or take.
because, without all that scattered light (i.e. in space), the shadows would have Zero brightness and all the light would fall only on the ground outside the shadows. The effect on the ground outside shadows is a good representation of the effect on 'how bright' the Sun would look (but DON'T LOOK!) up there and down here.
 
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  • #6
Drakkith
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Catching a glimpse of the Sun when your eyes are fully dilated, it could appear - at least in theory - as much as 16,000 times brighter:

"..., most estimate that our eyes can see anywhere from 10-14 f-stops of dynamic range.."
https://www.cambridgeincolour.com/tutorials/cameras-vs-human-eye.htm

I'm not sure how that works. Just going off of pupil size, the intensity of light in the image is directly proportional to the unobstructed area of the pupil. Since the pupil varies from roughly 2-7 mm in diameter, the difference in area is only about 12x, which is not even close to 16,000x. Can you enlighten me here, Dave?
 
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  • #7
sophiecentaur
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not even close to 16,000x
There is also an 'automatic gain control' - otherwise known as Dark Adaptation. The large figure quoted by @Drakkith is 'of interest' and it also applies to turning on the bedroom light in the middle of the night. We would assume that the OP implied not looking at the Sun at night. (Haha)
If the pupil contraction could give enough stops, we wouldn't need to take special measures to observe the Sun - just ramp it up slowly and we could look directly without knackering our retina.
 
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  • #8
Charles Link
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I'm not sure how that works. Just going off of pupil size, the intensity of light in the image is directly proportional to the unobstructed area of the pupil. Since the pupil varies from roughly 2-7 mm in diameter, the difference in area is only about 12x, which is not even close to 16,000x. Can you enlighten me here, Dave?
I read the "link" from @DaveC426913 that says 10-14 f stops of dynamic range is what the human can see. Perhaps the terminology they use is poor and misleading. I think @Drakkith has the f-stop much more accurately quantified.
 
  • #9
DaveC426913
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I'm not sure how that works. Just going off of pupil size, the intensity of light in the image is directly proportional to the unobstructed area of the pupil. Since the pupil varies from roughly 2-7 mm in diameter, the difference in area is only about 12x, which is not even close to 16,000x. Can you enlighten me here, Dave?
Yeah. In retrospect I may have conflated two things.

The article suggests humans can see 10-14 f-stops of range, but, thinking about it, that is not the same thing as the f-stop measurement of the pupil alone.

Good catch.

*for the purposes of the following math, I am going to decree that
- 10-14 stops == 14 stops
- the area of a 7mm pupil == 3.5 f-stops (12x) greater than a 2mm pupil

At a given fixed pupil size, humans can detect quite a range of exposures (essentially like looking at the contrast in a still shot - wherein your pupil does not change size).

Let's suppose with some given pupil diameter, humans can detect lights and darks that span, say, 10.5 stops. i.e. the difference between the darkest thing in that scene and the brightest is 10.5 stops before bright objects are indistinguishable and dark patches are indistinguishable.
1581118309379.png



Now the pupil can come into play, adding another 3.5 stops - for a total of 14 stops range.

1581117550254.png

With the pupil, brighter brights and darker darks can be discerned (just not at the same time), bringing our total range up to 14 f-stops.

1581118583515.png

Essentially that would be where the article got its 10-14 f-stop range from.

But - our fixed pupil range of 10.5 f-stops will not change the appearance of the brightness of the sun - so it doesn't count.

The only factor then that changes the appearance of brightness is the pupil f-stop - which incrased light gather by 12x.

So yeah, the sun may appear about 12x brighter if you look at it with your night vision engaged.

(Though frankly, that seems quite low. If you've ever had drops put in at the Optometrist, you know that it is impossible to open your eyes more than a slit in broad daylight - never mind looking at the sun!)
 
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  • #10
Drakkith
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So yeah, the sun may appear about 12x brighter if you look at it with your night vision engaged.

(Though frankly, that seems quite low. If you've ever had drops put in at the Optometrist, you know that it is impossible to open your eyes more than a slit in broad daylight - never mind looking at the sun!)

Well, imagine an entire daylight scene that's 12x brighter than normal. That's not an insignificant amount of light. There's a reason your pupil closes down most of the way during daylight!
 
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