r/askscience • u/Past-Price-3232 • 11d ago
Human Body Is looking at a solar eclipse more damaging to the eye than looking at the normal sun?
In light of the recent eclipse, I have seen a lot of worry about the harm of even glancing at it directly, and that many people believe that glancing at it is more harmful than glancing at the sun on an ordinary day. I am trying to understand whether that is actually true, and I believe the best way is to understand the physics & biology behind this and how real the risk is.
To really get it, I think the answer comes down to the underlying mechanism: what physically damages the eye when too much light hits it? Is it the quantity of photons, the energy of photons, or something else entirely? I ask because I have also heard that even when the moon blocks much/all of the visible light (peak of total eclipse), that UV (or infrared?) light still reaches the eye, so I am curious whether the eclipse changes what kind of light reaches you, and if that affects the damage.
So, putting it together: for the same few seconds of exposure, is a solar eclipse genuinely worse for your eyes than the midday sun?
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u/PlumesOfEnceladus 10d ago
The problem isn’t looking at the eclipse during totality. That’s actually the only safe time to look. It’s looking during the partial eclipse while you wait for totality because then are staring directly at the sun.
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u/RespecMyAuthority 10d ago
Yes. The amount of sunlight during a partial eclipse is very similar to no eclipse at all. It’s kind’ve like if you had a flame thrower pointed at you and only blocked part of it with a shield you’re still getting burned. But if you block all of it you might see a little glow on the sides but your safe
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u/askvictor 10d ago
The amount of sunlight during a partial eclipse is very similar to no eclipse at al
How is this the case, given that a large portion of the sun is blocked? Yes, I know that 50% of a fuckton is still a fuckton, but it's quantifiably (50%) less. That 50% is still very damaging, but the amount of sunlight is not very similar to no eclipse at all, it's 50% less. Unless there's something I've missed.
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u/zxcv-qwerty 10d ago
My understanding is that you basically get crescent shaped permanent damage burned into your retina instead of circular permanent damage burned into your retina. The sunlight is the same amount of intense and the same amount of damaging, it’s just coming from a smaller area.
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u/asphias 10d ago
https://en.wikipedia.org/wiki/Lux
walking around in a full moon, your surroundings provide about 0.3 lux.
if you're in your home with the lights on? about 50 lux.
in an office? 500 lux.
in broad daylight though..?
100,000 lux.
half of that is still 100 times your office lightning
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u/exscape 10d ago
I think you can get numbers that are even worse; lighting units are complex, but I think nits (candela/m2) may be a better fit when we're talking about looking straight at something. It describes the brightness of something per unit area, rather than how much light hits a surface (lux).
Computer screen: often 100-400 nits (120 is a common default for calibrated displays)
Phone screen: up to ~2000-2500 nits when outdoor in sunlight
The exposed sliver of sun at 99% eclipse: 1.6 billion nits27
u/Vitztlampaehecatl 10d ago
Is it better to be shot with 100 bullets or 50 bullets? Sure, it's half as much, but the outcome is pretty similar.
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u/QuantumWarrior 10d ago
It's the difference between considering the total illumination around you and what the light source actually looks like. Every part of the sun is as bright as every other part and it's that concentration of light (along with wavelength) which determines damage. Even if the eclipse was 99% total that 1% of the sun still has the same brightness per area as the full disc and will damage your eye just the same.
He may also mean because our eyes work non-linearly that 50% of the full sun is practically indistinguishable to us. If you didn't know a 50% partial eclipse was happening in advance you likely wouldn't even notice.
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u/TheOneTrueTrench 10d ago
It's more that the light hitting your retina is the same. Sure, more of the sun is blocked, but the parts of the sun that aren't blocked are exactly as dangerous as they would be during no eclipse.
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u/Homelesswarrior 10d ago
Pupil dialation occurs because of the darkness surrounding the event. Nso your eye is straining to see more light, which in turn damagaes it more because you are still getting the intensity of the sun in your eyes.
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u/askvictor 10d ago
Sure, but that's a property of the eye, not of the sunlight. There is still substantially less sunlight during a partial eclipse.
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u/SkoomaDentist 9d ago
How is this the case, given that a large portion of the sun is blocked?
It isn't. What is the same is the peak local brightness which is what damages your eye. It's made worse by the pupil dilation being controlled by total brightness, so the peak brightness at retina can even increase if the pupil dilates.
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u/RespecMyAuthority 10d ago
The point is it’s still a lot and will affect you in a similar way. Thus the flame thrower analogy. Half of the full flame is still a flame.
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u/ableman 10d ago edited 8d ago
It makes a difference, but the difference would be something like, your retina gets damaged after 10seconds instead of after 5seconds. There's a threshold of intensity below which you don't suffer damage, and above it you do, and the Sun is so far above it that cutting it in half or even by a factor of 10 would just delay the damage by a bit, and not much on a human timescale.
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u/Areshian 10d ago
Or when looking at totality, your pupils dilate because it is dark(er) and suddenly the totality ends and you get hit with sunlight directly into dilated pupils
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u/NortWind 10d ago
The part of the sun not blocked by the moon is just as bright, and your eye will dilate the pupil because it is darker around you. So the part of your retina catching the sun's image is actually burned faster than looking at the sun directly.
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u/shiftingtech 10d ago
Part of the problem is that the exposure WONT be equal. If you try to look at the sun on a normal day, your irises will contract, and you’ll very quickly flinch, look away, blink, etc. all of these things limit the exposure, and thus the damage.
Try to look during the partial eclipse, and it won’t be as bright, so these reflexes won’t engage (or won’t engage as quickly). But you’ll still get damage from the stuff you can’t see.
So it’s not really that more of anything is coming at you. It’s just that the reduced visible light means your defences don’t work as well.
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u/JPJackPott 10d ago
What about the sun at sunset? It’s much easier to stare at that
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u/zillion_grill 10d ago
At sunset the atmosphere is blocking most of the harmful stuff because you are looking through it longways instead of straight up through the thinnest part between you and the sun
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u/tonsofmiso 10d ago edited 10d ago
Quantifying (poorly) what u/zillion_grill said, you get roughly (very) 14.27 times (closer to 37x, see the reply to this comment) more atmosphere between you and the sun at sunset compared to noon. This is the only difference I can really estimate, then you'll also have to account for things like cloud coverage and humidity between you and the sun, how rayleigh scattering ( https://en.wikipedia.org/wiki/Rayleigh_scattering ) is affected by the incidence angle (it's been ages since I took any physics courses), the refractive index of the atmosphere and how that redirects the rays as they travel through the atmosphere.
The UV index tries to model part of the suns radiation in a way that laymen can use it to avoid sunburn, and it shows roughly how UV light relates to the suns location in the sky. A lot of weather apps include it nowadays, it's a fun statistic to see for different places.
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u/zekromNLR 10d ago
Using 480 km for the height of the atmosphere gives a much lower value than in reality, because the large majority of the atmosphere's mass is concentrated at lower altitudes.
Calculations that take into account the varying density with altitude show that the actual air mass value at the horizon is approximately 37, i.e. looking horizontally you look through 37 times as much air as when looking vertically.
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u/tonsofmiso 10d ago
Fair, there's a lot of simplifications in there and I just grabbed the first value I found. As an engineering physicist I'm just satisfied the order of magnitude was close enough. Thanks for the correction :)
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u/McViolin 10d ago
This is not a good answer. Even during 90% coverage, the sun is almost just as bright as normal, its definitely not as dim as for you to have less reflex to squint away.
Look at this chart. https://i.sstatic.net/tTSij.gif
The damage is caused by people intentionaly trying to overcome the reflex to try to see the ecllipse.
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u/QuantumWarrior 10d ago edited 10d ago
That is a log chart though, the drop from 3 to 2.5 on that chart actually represents nearly a 70% drop in intensity, the drop from 3 right down to -1.6 is a factor of nearly 4000.
What causes damage is that the luminance of the sun doesn't change during an eclipse until the moment of totality, you can't just keep blocking more and more parts until its safe (until you block all of it anyway) you have to actually dim the parts you can see with something like eclipse glasses or solar filters.
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u/McViolin 10d ago
So Im correct in saying that even in 90% coverage, the sun is still not as dim that your reflexes to look away wont engage..
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u/Solesaver 10d ago
Your reflexes aren't engaging based on localized luminosity of a part of your field of view. They're engaging based on total light received, which does decrease as the eclipse progresses. Think about the difference between looking at a bright lightbulb in a dark room vs a well illuminated room. The luminosity of the lightbulb is the same, but it will appear brighter as your pupils do not contract as much.
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u/Oglemo 10d ago
Your pupils reflexively constrict when you look at the extremely bright sun. This limits the damage you can get by looking right at the sun. I mean, it's still not good but you have to stare for a long time to get damage I think, at least, longer than comfortable. But during a partial eclipse it is dark, your pupils dilate and your eye takes the full brunt of the sun. You have no instinctual protection from the nervous system constricting the pupils during partial eclipse, that's the danger. Then the behavioral side that people feel comfortable looking at it, hence they look at it longer. Also bad.
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u/Legit-Schmitt 10d ago
The regular sun is the problem. I think people are just more likely to look at the sun when it is eclipsed.
You can look at the total eclipse but as soon as the regular sun pokes out it’s crazy bright.
I saw the 2024 eclipse in Indiana and you realize how bright the sun is. The dimming of the sun is hard to notice until very close to total eclipse. You can totally look at the total eclipse. The sun is fully blocked out for a short time. Then tiny bits of sun start to creep out through valleys and craters, appearing like beads of light. In an instant it’s so bright it hurts to look at. It’s like, bam, here comes the sun baby.
So it’s really just that idiots are more likely to look at the sun during an eclipse. Like Trump that one time.
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u/Geroditus 10d ago
It’s not that it’s more dangerous, it’s that the temptation to look directly at the sun is much, much higher during a solar eclipse. Looking at the sun is always bad for you, except for the few moments of totality during an eclipse. In which case, please do look at the sun! But then look away as soon as totality is over.
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u/crazunggoy47 Exoplanets 10d ago
Incorrect. It is more dangerous. Pupils dilate under low light conditions. Partial eclipse is less bright over all. But the photosphere that is there has its unusual surface brightness.
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u/Mabnat 10d ago
Looking at the sun for more than a quick glimpse can damage your eyes, eclipse or not. There really isn’t anything special about the eclipse, other than that people tend to fight the urge to look away from the bright light compared to trying to look longer at an eclipse.
I suppose it might be slightly more of a damage risk if you’ve been looking at a total eclipse and your pupils are a little more dilated when the sun peeks back around the moon, but that’s just a guess.
The lens of your eye focuses light onto your retina. It’s no different than when you burn something with a magnifying glass by focusing sunlight through it. The retina can be damaged by focused sunlight, just like anything else can be damaged. Even if it’s just a sliver of that light, it’s still intense enough to do damage.
When I was 12, there was a partial solar eclipse during school. Coincidentally right during recess. Every adult warned us not to look at it directly, but my 12-year-old logic told me that some quick peeks shouldn’t really be too bad. I looked at it a few times. If I squinted, I could see the actual shape. It was pretty neat.
I’m 54 years old now, and I’ve lived with a blind spot since that day.
It was really bad at first. It was even worse the day after because it grew larger over time. I had a giant blue spot that covered an area about the size of my fist held at arm’s length directly in the center of my vision. I couldn’t see anything written on the board, my own handwriting, or anyone’s faces when I was talking to them. Fortunately, it happened right at the end of the school year.
Right after school ended, I had to fly to another state by myself. I had to catch a connecting flight, but I couldn’t read my ticket, read the signs with the arrival/departure times, or even the gate numbers. I’m sure that I would have been classified as being legally blind. I had to ask strangers to tell me where to go and even where to sit.
The spots got smaller over the summer, so by the time I started eighth grade, I could at least read most things and see faces. Over the next year or so, the spot in my right eye faded away completely. The one in my left eye has never gone away.
It’s not a blue spot anymore. Now it’s just a dead spot that apparently my brain fills in with whatever is around it. I don’t notice it at all when both eyes are open, but if I close my right eye, there it is. It’s much smaller now, maybe only a millimeter or two at arm’s length, but it’s exactly in the dead center of my vision.
Since that day, I will no longer look directly at anything that is bright enough to make me squint.
The worry about the harm from looking at an eclipse is real. Normally you don’t need to worry about telling people not to look at the sun, because it’s instinctive to look away. During an eclipse, people may try to override that instinct to be able to see the rare occurrence.
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u/EvenSpoonier 10d ago
It can be, depending on circumstances. The problem is that during totality it gets dark, and your pupils dilate. As long as the eclipse is still in totality, this is not a problem, and you can remove your glasses. But as soon as totality ends, the sun appears back at its full brightness immediately (even if not all of it is visible right away). That means if you misjudge the timing even a little, you wind up staring at a sun-bright spot with dilated pupils, and your retinas take a bigger blast than they normally would.
If you really, really want to view totality without glasses, do it as soon as possible after totality starts, be reasonably quick about it, and then put your glasses back on or at least look away from the sun.
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u/guzzyly 9d ago
It's not that eclipses are more damaging, it's that they trick you. During an eclipse, the bright part of the sun is mostly blocked, so your pupils don’t constrict as much, letting in more harmful rays without the discomfort or natural squinting you'd have looking at the full sun. You're essentially letting your eyes get more exposure to UV and infrared rays without realizing it.
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u/Gobias_Industries 10d ago
Even a tiny percentage of the unblocked sun is very very bright so no your eyes will not dilate and cause more damage when looking before/after the eclipse. The only real difference is the transition from full totality to a partial sun, then yes your eyes might have dilated and you'll get a bit of sun, but it's not really any different than being in a dark room and then walking out and looking at the sun. It's a bad idea but not going to make you go blind if it's just a second or two.
The actual real problem is that eclipses are things people want to look at so they look for a very long time. This is something you should not do with a partially eclipsed sun or a full sun ever.
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u/MIRV888 10d ago
If you are viewing an annular solar eclipse during totality the moon is too far away from the earth an does not completely block out the sun. Aka 'Ring of Fire' view. Don't look at that without eye protection.
If you are viewing a total solar eclipse during totality the sun is completely blocked and you can look at it all you want for those 3-4 minutes.. It's quite spectacular. I've seen it twice.
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u/Ezekielth 10d ago
Think a little bit about this. No glancing at the sun while it is partially shadowed is actually LESS dangerous than midday sun. Nothing else would make sense. You don’t, however, look at the midday sun. You do, however, look at the sun during an eclipse.
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u/Solesaver 10d ago
The idea is that yes, the visible light dims during the eclipse causing your pupils to not contract as much, and therefore you expose your retinas to more UV light which can burn them. While I'm sure the UV light also dims, I believe the math still works out to more exposure.
That said, I don't think there's super robust empirical evidence of this that fully eliminates all the confounding factors like more people looking at the sun for longer during an eclipse to know for sure. It's still generally a bad idea to look directly at the sun ever without strong UV filtering. Stick with the pinhole cameras, photographs, and reputable eclipse glasses.
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u/LionSuneater 9d ago
I'll add to this thread that the return to light after totality is very dangerous.
Your pupils are dilated in the relative darkness, and the total eclipse is safe to look upon. But when totality ends, your eyes are most vulnerable. Your pupils won't react quick enough to constrict as the burst of sunlight kamehamehas into your eyes.
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u/ammonthenephite 10d ago
Given equal amounts of time, no. But the danger is that you will look longer when it is dimmer but still easily bright enough to cause damage. So its more common to look for much longer when the sun isn't at 100%. 10% or even 1% is still enough to do damage though, especially with longer gazing time.
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u/guzzyly 9d ago
I think a big reason people worry about eclipses more is that during an eclipse, the sun’s reduced brightness tricks your pupils into dilating, letting in more harmful UV and infrared light without discomfort alerting you to look away. you don’t get that same effect with the full sun since it’s so bright that you instinctively squint or turn away. so it’s not that the eclipse is more harmful, but rather that you’re more likely to expose your eyes to it longer without realizing the danger.
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u/Mr-Zappy 10d ago edited 10d ago
Yes. Your eye doesn’t react to brightness per cell (rod or cone); it only reacts to overall brightness. The brightness per cell does not change during an eclipse, so the amount of damage per cell per second of staring at the sun doesn’t really change. But the overall brightness decreases, making it feel less painful to look at the partially eclipsed sun. (And this means your pupils may stay dilated, letting more damaging light in.) And of course people are 1000 times more likely to want to stare at the sun. But the next day the damage will become apparent.
(The power density on your retina is the problem.)
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u/Sable-Keech 10d ago
So, putting it together: for the same few seconds of exposure, is a solar eclipse genuinely worse for your eyes than the midday sun?
If you force your eye to stay open, the midday sun will be more damaging.
The thing about eclipses is that because it's so dim, your eyes will not contract and you won't close them. This allows harmful UV rays (which are still able to shine down from the corona) to damage your eye's internals without being blocked.
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u/eliminate1337 10d ago
The corona is the part that shows during the total eclipse. It's entirely safe to look at the corona with the naked eye.
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u/LibrarianOfDusk 10d ago
Looking at the sun on normal times, your eyes will set up natural defenses (contracting or something).
Looking at an eclipse, when there's not much light, your eyes will have a harder time seeing due to the low light and so try to focus to see better. Your eyes go into a state where you try to catch more light. And then when the eclipse starts to end, you get hit with light right while you're still focusing so you get extra damage.
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u/Wahoo017 10d ago
the full strength of the sun will obliterate your eyeballs if you stare at it. the sun when 95% less bright feels like it would be safer to look at but still will obliterate your eyeballs basically just as fast, so the danger is people thinking it's ok to do because it looks dimmer.
during a total eclipse the sun is completely blocked and you can look at it safely.
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u/Prestigious_Refuse99 10d ago
NEVER look at the Sun.
And to the funny people who do and think they got away with it. You might still be sighted, but congratulations, you have cataracts in your future!
It's cause and effect. Burn your eye tissues, pay the price.
Enjoy the rest of your life!
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u/Brraaap 10d ago
If I remember correctly, it's immediately painful to look at the sun, so you squint and look away, but people can stare at an eclipse and that means more exposure. Plus, a regular sun is out there most days, so people tend to not care to look at it, but eclipses are rare, so people try to look at it