Eclipsapalooza is next Monday (4/8/24). Viewing the partial eclipse -the time while the moon only blocks part of the sun - is dangerous to the human eye if seen without protective eyewear. Why is that so? Is viewing the sun during a partial eclipse more dangerous than viewing the sun directly when the moon isn’t between the viewer and the sun?
The light level has dropped sufficiently that it is no longer painful to look at it-but the damage can still be done. I heard many reports in the wake of the 2017 eclipse of people who had crescent-shaped scars on their retinas from looking at the partially-covered sun before totality.
No matter what, looking at the un-blocked Sun will always be more harmful than looking at the Sun that’s been mostly blocked by the Moon. Both are bad; just one more so.
There is so much light from the normal uneclipsed sun that it is painful to look at, and it’s very difficult to force oneself to look at it long enough to cause damage (although it’s possible if you very determined to do so). In contrast, an almost fully eclipsed sun is not painful to look at but is still capable of damaging your retina. There is no reflex that prevents you from looking at it long enough to cause damage.
When it’s at like 99%, that might be true. But even as much as 10% of the Sun exposed is still easily enough to trigger the reflex to look away.
The real reason that an eclipse is dangerous is because normally, most folks aren’t trying to look at the Sun. Even that reflex to look away can be overriden, if a person is determined (that’s how Galileo was able to make his observations of the Sun).
This also means, incidentally, that the eclipse isn’t dangerous to pets or infants, because they won’t know enough to know that there’s something strange going on with the Sun, and hence won’t be motivated to override their reflexes.
(I will confess that, at the last one, I did watch the diamond ring with my naked eye. I figured that potentially burning out that tiny spot on my retina was worth it.)
If you want to be really safe, take a sheet of an opaque substance, poke a pinhole in it, and use it to cast an image of the sun onto a screen.
My local newspaper suggested a sheet of aluminum foil. I found that that required at least 3 hands, which I don’t have. I used a 3 x 5 index card for the pinhole, and a paper plate for the screen, and they worked fine. Once a significant portion of the sun was blocked, my backyard was dark enough that I could see the image quite well.
In possible event I get to look at the 90% will have here in Philly, is my plan of using multiple folds of a mylar emergency blanket as an eye screen any good?
I suspect most of those were from looking at the sun after totality. This is the major danger time.
Once in totality, it’s OK to look at the blocked Sun without the eclipse-glasses and most people do. But at the end of totality, once part of the Sun is unblocked, it’s imperative to put the glasses back on immediately. However, there’s not enough light yet to trigger the look-away reflex, so many people continue to look at the Sun. That’s when retinal damage most commonly occurs.
Don’t trust any filter that isn’t designed for this. A jerry-rigged filter is likely to be too dark, not dark enough, or both at once (i.e., you can’t see the Sun through it, but it’s still letting through dangerous levels of UV or IR). If you don’t have actual solar filters, then either use a pinhole projector, or look at the reflection in a not-very-good reflective surface (such as muddy water).
Thanks.
Well bah. Now I have a sad.
Oh, and that said, you probably can get a pair of eclipse glasses for a buck or two at the Franklin Institute or any of the universities.
Thanks. My employer has a terrible time off policy and I don’t know if I’ll even get the chance to see the eclipse. But, I’m hoping.
Plus, isn’t a mylar blanket mostly opaque?
An article on eye damage from the last total eclipse:
Well, if you’re only trying to catch the 90something% of Philadelphia, you don’t need “time off”, just a five-minute break at the appropriate time when you can step outside. And since you’re not getting totality anyway, there’s a fair bit of flexibility on when that break can be.
Woo Hoo! Everything is coming up Cathode!
What about low-light pupil dilation? Does that come into play as well as the look away/squinting reflexes during an eclipse?
At any rate, it’s looking like I’m going to have a fantastic filter for the eclipse. Like 70% cloud cover fantastic. Which kind of sucks, because my home is within the path of totality.
In fact, isn’t that the major protection offered by welding masks? Not so much the lumens but the UV?
It’s everything, especially for TIG welding, which doesn’t produce much smoke to filter the light. According to this site, the visible light isn’t bright enough to be injurious, but if you are dumb enough to strike an arc without eye protection (e.g. accidentally tapping on the pedal before you’re ready), you’ll be flash-blind for at least a little while. You need to block a large portion of the lumens to be able to get the job done. OTOH, this 3M PDF says that over the long haul, blue wavelengths can damage your night vision.
Also, welding masks come in various “shades” depending on the kind of welding you’re doing. If you’re doing stick welding, you can use lower shade numbers, like 7-10. If you’re doing TIG welding, maybe 9-12.
If you have access to welding equipment, shade 12 is reportedly safe for viewing the eclipse, but shade 14 is supposed to be more comfortable. For reference, here’s how much light gets transmitted by the various shade numbers:
So yes, for shade 14, you’re getting about 3 millionths of the original intensity in the visible wavelengths, and about 1 millionth of the original UV intensity. Even shade 12 only transmits about 20 millionths of the visible waves and 4 millionths of the UV. The fact that that’s still uncomfortable (and that anything more transmissive is injurious) should tell you something about how damn bright the sun is.
I saw a crescent eclipse by accident, once. I hadn’t been keeping up with the news at all, but I regularly walked under trees while going to class. I looked down and there were hundreds of little crescents on the sidewalk. The leaves of the trees were making hundreds of pinhole cameras.
After that, did your eyes adjust fast enough for you to get a good view of the corona?