During the recent eclipse there was an enormous amount of publicity about the dangers of looking directly at the partially-concealed Sun. As I understood it, the issue is that looking directly at the Sun (for more than a few seconds) is dangerous at any time, but that ordinarily it’s painful to do this so it’s not really relevant. The issue comes up during eclipses because the partial blocking makes it easier to do it.
The question I have (actually my daughter, who asked me about it) is that there are many other times when the Sun is partially blocked. Sometimes it’s partially blocked by cloud cover, by trees, buildings etc., and it would seem that in such situations the exact same issue would prevail. And yet, you never hear about it. Why is that?
It depends on what you mean by “partially covered”. With an eclipse, part of the Sun is completely covered, and part of it is not covered at all. The parts that aren’t covered at all are just as dangerous as they always are. With, say, cloud cover, though, you’ll get all of the Sun partly covered. The parts behind a cloud will be anywhere from completely safe to dangerous-but-less-dangerous-than-normal, depending on how thick the clouds are.
In the situations you mention, people are not (generally) looking to see what crazy-ass thing the sun is doing.
In other words, “don’t look at the sun” is a rule 1) we generally follow instinctually and 2) don’t generally feel a strong urge to break.
A partial eclipse combines strong impulses to say “eff it!” to both 1 and 2.
Looking at the sun on a clear day is like picking up a really hot cast iron pot with both hands and dropping it one second later. Looking at the sun on a cloudy day is like picking up a pot (with both hands) which is warm, not hot, and holding it for ten seconds. Looking at an eclipse is like picking up a really hot cast iron pot with just one finger, for ten seconds. That one finger will get a third degree burn.
On a clear day, if you were standing in such a way that the corner of a building was obscuring 95% of the sun and you stared at the remaining 5% for a half a minute. You’d burn a tiny portion of your retina very badly, just as badly as looking at a solar eclipse. Looking at a sunset is not nearly as dangerous. Our species evolved looking at sunrises and sunsets daily and our nervous system is calibrated to handle that level of danger and pain. Partial solar eclipses are rare events that we aren’t evolved to handle.
On the bright side, burning a tiny sliver in your retina would be unlike to blind you completely, so you’d still be able to avoid predators and find food. It’s likely that your ability to reproduce before you die would be adversely affected. But now in Modern Civilization, it could definitely affect your ability to read and write and hold down a job.
FWIW, when I referred to a cloudy day, I meant a day with partial clouds where the sun shines clearly through some of the breaks. It’s fairly common.
Right. But if it’s as dangerous as all that in those other cases, then it should be something that gets more publicity because it’s so common. Say a guy is lying back in the shade of a tree and can see small amounts of sun through the leaves. Or there’s a break in the clouds and someone looks up to see it and perhaps points it out to someone else. These things are so common and it’s hard to imagine that millions of people are routinely damaging their eyes by doing things like this.
It’s staring at the sun which is dangerous. Most people lying on the ground, looking up through the trees are unlikely to stare at the sun streaming through the gaps. It’s still too bright, and there’s no reason to.
The biggest danger from a total eclipse is actually just after totality ends. Because 1) your pupils are dilated which lets more light into your eyes and 2) you’ve just spent a couple minutes looking at the blocked sun. Suddenly you get a very bright light, but it doesn’t seem to be bright enough to trigger your reflex to look away, at least not immediately. But it can cause damage if you don’t look away and put on your eclipse glasses.