Is there anything particularly dangerous about viewing the sun directly? When I was dumb and young I'd look at the sun for seconds at a time without any problems developing so far. Is the danger that an eclipse encourages you to stare at the sun without encountering any pain?
I stared at a partial eclipse as a child without eye protection for several periods of about 10-12 seconds (remembering that I used to occasionally look at the regular sun for 3-4 seconds without apparent injury). Unfortunately, this gave me an afterimage of the eclipse in one eye that lasted for six months (!) -- although I was fortunate that it didn't last even longer than that.
The optometrist I consulted the day after the eclipse said that there didn't appear to be significant permanent damage to my eye, but the subsequent six months of the afterimage, and not being sure whether or not it would last my whole life, were certainly no fun. Overall, I feel like I got very lucky, and I wouldn't have been surprised if the afterimage had lasted for decades rather than months. Maybe it would have if I had continued looking for just a few more seconds!
My impression is that the risk of different kinds of damage is complicated, but there are several things that can go very wrong. One is that the sun during an eclipse is much more interesting than usual, so it's so much more tempting to look longer than momentarily; another is the UV exposure that people have mentioned (where there may still be enough UV to cause damage even at times when the visible light is weak enough not to cause significant discomfort). UV is a big factor that means that we shouldn't trust our intuition about whether something may harm our eyes. In industrial settings, too, people sometimes get significant eye injuries because they're staring at UV (or IR) sources that don't subjectively feel dangerous or painful.
(Edit: taneq in this thread also mentions a problem about dynamic range where your pupillary contraction may not be enough to protect you.)
While permanent damage might not happen quite as quickly as some people may have been led to believe, and some risks may be slightly exaggerated in certain accounts, there's just no reason to take the risk of not having proper eye protection, especially when you can't really judge from the level of discomfort or lack of discomfort whether damage may be happening. As far as I know, every eclipse leads to emergency follow-up optometrist visits, and I can testify that it's no fun to be the patient in one of those visits waiting to hear the news about whether the aftereffects are likely to be permanent.
As a child, I got to see a perfect full solar eclipse at mid day on a clear sky. I took the glasses off for a few seconds when the sun was perfectly behind the moon and it was amazing. It felt very comfortable to look at, no afterimage at all; except in my memory... The memory is still there after 15+ years.
When the sun is fully eclipsed, you can look at it without damage.
When it's not, you can't. But it may not appear "as bright" when it's partially eclipsed, it may be "easier" to stare at it, without reflexes kicking in to stop it, and do permanent damage to your eyes.
> When I was dumb and young I'd look at the sun for seconds at a time without any problems developing so far.
Hopefully this was during hazy sunsets. Or maybe you just have mega-strong pupils that can contract more than normal and that's why you're not blind.
> Is the danger that an eclipse encourages you to stare at the sun without encountering any pain?
Exactly. Human eyes aren't very good at handling ultra-high-dynamic-range scenes. During an eclipse, your surroundings are somewhere between twilight and night-time in brightness, so your pupils dilate. The small un-covered sliver of sun is still as bright as it is during the day, and can damage your eyes without triggering a painful 'too-bright' response. It's somewhat similar to the danger posed by a laser.
Just to add to the other replies, there's another thing very different about a partial eclipse vs. when you looked at the sun as a kid. You were looking at a full sun, not a smidgen of a sun. There was so much light hitting your eyes that they stopped down naturally, giving you some protection.
And there was so much bright light that you really couldn't look at it for very long without involuntarily looking away or closing your eyes. But of course if you kept looking at the sun no matter how much it hurt, you would damage your eyes.
In a partial eclipse, most of the sun is covered, and there is a lot less light reaching your eyes. So they don't stop down as much as they would for a normal full sun.
However, that fraction of the sun that is visible is just as bright as before! With such an interesting sight to look at, you will be much more tempted to stare. And with your pupils wide open, there is even more sunlight hitting that small area of your retina than there would have been in normal conditions.
Now if you are fortunate enough to experience the total eclipse, that is another matter entirely. It is perfectly safe to look at the solar corona during totality, even with binoculars! The corona is only about as bright as a full moon. The sun itself is completely blocked by the moon.
Your caution is well advised, but the sun doesn't instantly burn your retinas. You do get a tiny bit of time to look away before any damage is done.
I viewed the 1979 total eclipse with binoculars, and when totality ended and the diamond ring and Baily's beads started to appear, I lingered a few seconds longer than I should have, and still no harm was done. I didn't suffer any temporary or permanent vision damage, even viewing those through the binoculars.
I haven't gotten it from the sun, but I got a minor case of it from welding with the incorrect lenses (it may seem fine to use gas welding goggles for MIG welding, but it's really not). It's really not pleasant, your eyes feel all scratchy and too large for their sockets.
> It's really not pleasant, your eyes feel all scratchy and too large for their sockets.
I've had hints of that from welding when using a proper mask, but inadvertently not wearing a dark enough shirt (worst case is a white t-shirt - can bounce the UV up under the mask; it's not as bad as "arc eye", but can still be uncomfortable afterward).
Other welding safety lessons:
1. Be very careful welding on zinc-coatings (galvanized steel); have plenty of ventilation, and don't inhale any of the fumes (a fan to blow the fumes away can help). You can get zinc-poisoning, which feels like a very bad flu-like symptoms. I'm sure it can be fatal, but mostly you'll feel really terrible afterward, and there's no cure except to wait it out. Better - remove the galvanization (grinding or with pool acid), or use a respirator under your mask (specifically one rated for it - there's a particular one 3M makes that if you research this topic, people recommend).
2. Less known: Never weld on steel that has been cleaned using chlorine-based solvents (like certain kinds of brake cleaner). There's a tale out there from a guy who did it, and lived to tell about it. Basically, the intense heat creates a change in the chemical to form phosgene gas; even a small amount can kill you. This poor soul managed to live, but not without some major after-effects that will cause him trouble the rest of his life:
Note that not all brake cleaners have chlorine (they are banned in California, for example), but this is one of those things that if you've never heard of it, but you find yourself welding (especially if you also do your own brake fluid changes or brake jobs, or other auto work) - and you clean your parts (hey, let's use brake cleaner - that'll do it!) - you wouldn't think there'd be a problem - until it's much too late.
Get the word out on this last one, too - not many people know about it (most welders know about welding on galvanized steel).
There's a short term condition called arc eye (https://en.wikipedia.org/wiki/Photokeratitis). Basically a sun burn on your eyes that heals after a day, and supposedly doesn't really have long term damage. I have experienced it several times (both from welding and hiking in the snow), and haven't noticed any major changes in my vision since.
So did I, usually in while bored in class, so a window between me and the sun (blocking UV, for what that's worth). I'd stare until the sun developed an outline in my vision, then look elsewhere until the sunspot in my vision wore off, then I'd do it again.
But there are documented cases of eye damage, so the danger is real.
I too have stared at the sun for far longer than is recommended (30s+), and had no issues, but it certainly is a risk:
> In a study after an eclipse in Turkey in 1976, around 10%of [sic] those with damage had permanent visual loss to the extent that they were not able to read a car number plate at 25 yards (23 metres) with the affected eye or eyes.[0]
IIRC, an eclipse is particularly dangerous because the lack of light doesn't trigger your pupils to constrict as much or a "too-bright" response to look away, but the visible portion is still sufficient to cause damage.
My grandmother (at age 70) had a crescent-shaped burn on the back of her eye (edit: ie. retina) from viewing a partial eclipse as a child. She couldn't "see" the burn, likely because her brain adjusted to it.
Exactly. The light doing damage isn't necessarily the visible light that gives you the impulse to avert your eyes, it's the UV. In one study, 50% of the people looking at the eclipse had caused permanent damage to their eyes (sans any precautions or averting their eyes).
My understanding is that the human eye has pain receptors that respond to wavelengths in the visible spectrum. As the moon occludes the sun, the level of visible light drops below the level where prolonged viewing causes pain.
But visible light is not the only frequency. Sunlight also contains UV and IR wavelengths. The human eye does not have pain receptors that respond to these frequencies.
The end result is that during an eclipse it becomes more likely that a person will fries their eyes due to the presence of UV and IR wavelengths.
