Friday, August 01, 2008

Fact or Fiction: Eclipse Blindness

Today, August 1 2008, a total solar eclipse will be visible in parts of northern Canada, Greenland, Russia, China and Mongolia, with a partial eclipse visible in other parts of the the country.

I remember in elementary school, a big deal was made of not looking directly at a solar eclipse. A very big deal. Even a quick, unprotected glimpse could cause blindness. At the time, it didn't make sense to me - why would looking at an eclipse be any worse than looking at an uneclipsed sun? Was the seemingly heightened paranoia because it's an event that people are likely to want to look at? Or is there something else about an eclipse that warrants the extra attention? Even NASA acknowledges that eclipse warnings may be a bit hyperbolic:
In the days and weeks preceding a solar eclipse, there are often news stories and announcements in the media, warning about the dangers of looking at the eclipse. Unfortunately, despite the good intentions behind these messages, they frequently contain misinformation, and may be designed to scare people from seeing the eclipse at all.
The fact of the matter is yes, you can do serious damage to your eyesight looking directly at a partial solar eclipse. During the period of total eclipse - when the moon completely obscures the sun - it's safe to observe with the naked eye (see also NASA link above). However, this isn't without risk since the shadow will continue to pass and without proper timing you could find yourself looking directly at a partial eclipse. Even when 99% of the sun is obscured, the remaining 1% is intense enough to cause serious retinal damage.

Is looking at a partially-eclipsed sun more hazardous than an unobscured sun? In a way, yes. Because even a mostly blocked sun can be damaging and the surrounding illumination in these conditions can actually be quite dim. In this case the eyes natural defense - constriction of the pupil - doesn't kick in right away. The pupil is exposed to the sun in a dilated state, allowing more of the damaging light to enter the eye. (Of course an unobscured sun will be sending that much more light to your eye, so this isn't a proper comparison.)

The bottom line is that there's nothing magical about a solar eclipse, and nothing special about the light it gives off, but after every eclipse there's a spike in cases of retinal injury and eyesight damage. Take proper precautions when viewing an eclipse and don't stare directly at the sun - eclipsed or not.

[Image source: Wikipedia]


audupord said...

Thank you for making sense of this for me. As a kid I did think it had something to do with the intensity of the peripheral light, but never bothered to go back and learn the real deal.

Much peace,


Anonymous said...

OK, I thought the issue with an eclipse is that you are missing the reflex that prevents you from staring at a bright light source; however you are still effectively focusing an intense light source onto your retina. A series of retinal cells being blasted by a focused point of light representing a portion of the sun (eclipsed or not) don't really care whether or not the full image is focused across your retina. Now the lens might be more sensitive to damage based on the total amount of light, but your retina only cares about the intensity from the point source of light that is being focussed. Not sure if this is accurate, but that is the way I interpreted those eclipse warnings. I’ve never tried, but I thought it doesn’t hurt (cause pain) to look at an eclipse but it does cause damage. Sort of like a UV light, it doesn’t cause pain to look at it, but you are still frying your eyes! My two cents worth.

Kamel said...

That's pretty much the same thing. The 'reflex' that's missing (eg. contraction of the pupil) is because the general lighting conditions are still dim, but the small point of intense light is enough to cause damage. It's not the whole retina getting blasted, like you say, only the area with the intense light. In fact, eye doctors can often tell what phase an eclipse was in when the vision damage occured based on the shape of the damaged region of the retina.

As for not feeling pain, you don't feel pain because there are no pain receptors in the retina.

Anonymous said...

What do you mean?