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"tricked into thinking it's dark" is kind of misleading, because it is in fact darker, but for some reason unknown to me there the energy from the sun harms you

"tricked into thinking it's dark" is kind of misleading, because it is in fact darker, but for some reason unknown to me there the energy from the sun harms you as much. I thought the issue with looking at its sun was its brightness, but i guess it isn't?

mikeash 9y ago
The issue is how much light gets focused onto a small area of your retina. When the sun is partially eclipsed, the darkness makes your iris open up to let more light in. That then focuses more light from the visible part of the sun onto the same area of retina.
dungle6 9y ago
No it's not the brightness per se. What you perceive as "brightness" is the intensity of the part of the spectrum that is visible to you. But plenty of the non-visible spectrum has physiological effects, the infrared and UV spectrum. Most of the acute damage from the eclipse and the sun to eyes is not from UV as commonly believed but from the intensity of infrared that your eyes don't detect as "bright" but will cook them right well. This is also why even relativlity low intensity infrared lasers are so dangerous.
Stratoscope 9y ago
During a partial eclipse, part of the sun is hidden behind the moon. So the overall amount of light reaching the ground around you is indeed less, in direct proportion to the amount of sun that is eclipsed. For example, if 90% of the sun is covered by the moon, then only 10% of the usual amount of sunlight will light up the area around you. So it is quite a bit darker than usual. However, the portion of the sun that is still visible is just as bright as it would be on any other similar day. If you stare at that remaining portion of the sun, or aim a camera at it, it will cause the same damage as it would any other time, only in a smaller crescent-shaped area instead of a full circle. The danger to your eyes is even greater, because the overall darkness tricks your irises into opening up wider and letting even more light in than usual. You're pretty unlikely to stare at the sun for 30 seconds on a normal day. But people do that during a partial eclipse, and that's the problem. Similarly, people don't usually aim their cameras directly at the sun with the lens wide open and no solar filter - except during an eclipse. A total eclipse is quite a different thing, of course. During totality, the sun's bright photosphere is completely hidden, and only the much fainter corona is visible. This is only about as bright as a full moon, and it's perfectly safe to take pictures with any lens, and to observe directly with the naked eye - even with binoculars.
- mysterypie 9y ago
  That's one of the best explanations I heard about why and how the eye damage can occur. I'm curious how far can you carry that reasoning: If 99.9999999% of the sun were covered during a partial eclipse (down to a single stream of photons), does it damage one single rod on my retina as opposed to a crescent-shaped region? I'm guessing that a single stream of photons from the sun doesn't have enough energy to do any damage.
  - Stratoscope 9y ago
    Now that is an interesting question! Anecdotally, I viewed the 1979 total eclipse with binoculars (and no filters), and at the end of totality I continued to look through them for a few seconds to see the diamond ring and Baily's beads. It didn't harm my vision at all, even though there was some fair amount of direct sunlight coming through by then. But it was only a few seconds. The duration of sun exposure certainly is a factor.
- animal531 9y ago
  So I could shine a flashlight into my eyes while looking at the eclipse, and I'd be ok?
rbobby 9y ago
Your iris reacts to visible wavelengths and not ultraviolet wavelengths. So your iris is open much wider than it ought to be given the amount of UV present (especially given that UV is much more damaging than visible). If you see what I mean :)
Coincoin 9y ago
It is brightness, but brightness at a single focused point, not total brightness.

mikeash 9y ago

The issue is how much light gets focused onto a small area of your retina. When the sun is partially eclipsed, the darkness makes your iris open up to let more light in. That then focuses more light from the visible part of the sun onto the same area of retina.

dungle6 9y ago

No it's not the brightness per se. What you perceive as "brightness" is the intensity of the part of the spectrum that is visible to you. But plenty of the non-visible spectrum has physiological effects, the infrared and UV spectrum. Most of the acute damage from the eclipse and the sun to eyes is not from UV as commonly believed but from the intensity of infrared that your eyes don't detect as "bright" but will cook them right well. This is also why even relativlity low intensity infrared lasers are so dangerous.

Stratoscope 9y ago

During a partial eclipse, part of the sun is hidden behind the moon. So the overall amount of light reaching the ground around you is indeed less, in direct proportion to the amount of sun that is eclipsed. For example, if 90% of the sun is covered by the moon, then only 10% of the usual amount of sunlight will light up the area around you. So it is quite a bit darker than usual. However, the portion of the sun that is still visible is just as bright as it would be on any other similar day. If you stare at that remaining portion of the sun, or aim a camera at it, it will cause the same damage as it would any other time, only in a smaller crescent-shaped area instead of a full circle. The danger to your eyes is even greater, because the overall darkness tricks your irises into opening up wider and letting even more light in than usual. You're pretty unlikely to stare at the sun for 30 seconds on a normal day. But people do that during a partial eclipse, and that's the problem. Similarly, people don't usually aim their cameras directly at the sun with the lens wide open and no solar filter - except during an eclipse. A total eclipse is quite a different thing, of course. During totality, the sun's bright photosphere is completely hidden, and only the much fainter corona is visible. This is only about as bright as a full moon, and it's perfectly safe to take pictures with any lens, and to observe directly with the naked eye - even with binoculars.

mysterypie 9y ago
That's one of the best explanations I heard about why and how the eye damage can occur. I'm curious how far can you carry that reasoning: If 99.9999999% of the sun were covered during a partial eclipse (down to a single stream of photons), does it damage one single rod on my retina as opposed to a crescent-shaped region? I'm guessing that a single stream of photons from the sun doesn't have enough energy to do any damage.
- Stratoscope 9y ago
  Now that is an interesting question! Anecdotally, I viewed the 1979 total eclipse with binoculars (and no filters), and at the end of totality I continued to look through them for a few seconds to see the diamond ring and Baily's beads. It didn't harm my vision at all, even though there was some fair amount of direct sunlight coming through by then. But it was only a few seconds. The duration of sun exposure certainly is a factor.
animal531 9y ago
So I could shine a flashlight into my eyes while looking at the eclipse, and I'd be ok?

rbobby 9y ago

Your iris reacts to visible wavelengths and not ultraviolet wavelengths. So your iris is open much wider than it ought to be given the amount of UV present (especially given that UV is much more damaging than visible). If you see what I mean :)

Coincoin 9y ago

It is brightness, but brightness at a single focused point, not total brightness.