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Why is the light reduced by a huge amount during an eclipse but the UV is still at dangerous levels?

by jl6 9y ago

deadmetheny 9y ago
Because the moon is in front of the sun. The brightness is reduced during partial coverage to the point that you won't reflexively look away anymore, but the UV is still there and still causes eye damage.
- deadmetheny 9y ago
  So why am I being downvoted for answering a question, exactly?
  - jstanley 9y ago
    I didn't downvote you, but the reason some did is probably because we want to know why the UV is still there. Merely restating that the UV is still there contributes nothing.
amluto 9y ago
As I understand it, the relevant factors are the brightness per unit solid angle (aka surface brightness), the aperture (area of your pupil). The degree of danger for a given time spent starting at the sun is proportional to the product. The surface brightness of the sun is the same during a partial eclipse: each bit of sun is just as bright as if there was no eclipse -- there's just less of the sun visible. This is true in visible and UV, so the ratio of UV to visible light is unchanged. The problem is that everything gets darker, so your eyes dilate, which increases the danger. Also, partial eclipses are neat, so you're more likely to stare. During totality, it's a whole different ballgame. You can't see the bright part of the sun, and all that's left has much less surface brightness. I think it's roughly comparable to the moon.
saltcured 9y ago
There's no appreciable wavelength-specific filtering going on in an eclipse since the moon has no atmosphere. (The corona does have a different spectra than the surface of the sun, but that's not the issue here. The corona is dim enough to look at.) The problem is that there is so much invisible energy in proportion to visible energy. So blocking a large portion of the sun's disk, so your eyes no longer detect too much visible light, still allows too much total energy which fries your retina when focused into a very small area by your eye's lens.

Why is the light reduced by a huge amount during an eclipse but the UV is still at dangerous levels?

by jl6 9y ago

Why is the light reduced by a huge amount during an eclipse but the UV is still at dangerous levels?

Because the moon is in front of the sun. The brightness is reduced during partial coverage to the point that you won't reflexively look away anymore, but the UV is still there and still causes eye damage.

deadmetheny 9y ago
So why am I being downvoted for answering a question, exactly?
- jstanley 9y ago
  I didn't downvote you, but the reason some did is probably because we want to know why the UV is still there. Merely restating that the UV is still there contributes nothing.

amluto 9y ago

As I understand it, the relevant factors are the brightness per unit solid angle (aka surface brightness), the aperture (area of your pupil). The degree of danger for a given time spent starting at the sun is proportional to the product. The surface brightness of the sun is the same during a partial eclipse: each bit of sun is just as bright as if there was no eclipse -- there's just less of the sun visible. This is true in visible and UV, so the ratio of UV to visible light is unchanged. The problem is that everything gets darker, so your eyes dilate, which increases the danger. Also, partial eclipses are neat, so you're more likely to stare. During totality, it's a whole different ballgame. You can't see the bright part of the sun, and all that's left has much less surface brightness. I think it's roughly comparable to the moon.

saltcured 9y ago

There's no appreciable wavelength-specific filtering going on in an eclipse since the moon has no atmosphere. (The corona does have a different spectra than the surface of the sun, but that's not the issue here. The corona is dim enough to look at.) The problem is that there is so much invisible energy in proportion to visible energy. So blocking a large portion of the sun's disk, so your eyes no longer detect too much visible light, still allows too much total energy which fries your retina when focused into a very small area by your eye's lens.