Without a basic understanding of astronomy, a solar eclipse might be a pretty scary thing. Throughout history, many civilizations assumed solar eclipses where prophecies of terrible misfortunes to come.  A king may die. A plague may be unleashed.  All very dramatic (and terrifying) stuff.   A thousand years later, and armed with our knowledge of planetary motion, we have come to learn that while still very impressive, an eclipse isn't a demon eating the sun, but simply a geometric phenomenon that can be easily explained and predicted.  However, even with this knowledge, eclipses have actually been proven to rain-down punishment on a significant number of mortal humans who have dared to view them directly.  

Every 18 months, like a Medusa in the sky, a total solar eclipse is viewed from somewhere on earth, and every 18 months a massive increase in the number of patients presenting to local hospitals with eye problems is recorded. Yes, eclipses are one of nature's most impressive tricks, but also one of its (many) harmful ones.  Even though their frequency of once per 18 months makes them not uncommon, they seem to only pass over the same region of the planet once every 300 years, making them a truly once in a lifetime event.  Maybe this is why people go against everything they've been told in their lifetime up to that point, and decide that now is the time to start staring at the sun.  Or maybe its because we assume that since the sun is going to be 80% blocked by the moon (which is what Calgary's eclipse is going to bring) that the sun operating at 20% capacity won't really be that bright.  Well that is wrong.  That's like saying you're okay with only killing 80% of the bacteria on your hands before making dinner. 20% of a trillion microbes is still an astronomically large number, just like 20% of the sun is still unbelievably bright. 

 Light entering the eye is condensed to a highly energetic pinpoint on the retina (orange tissue).

Light entering the eye is condensed to a highly energetic pinpoint on the retina (orange tissue).

The sun is essentially a nuclear reactor the size of which none of us can comprehend burning right above our heads.  The byproduct of the reactions that go on in the sun produces light of all wavelengths, from radio waves to gamma rays.  The atmosphere and ozone layer block most of the lethal wavelengths from entering earth.  Of the wavelengths that do get to earth, humans can only perceive a very tiny band of them in the middle of the range, what we call the visible spectrum (ROYGBIV).  But even though we can't perceive them, we are also being inundated with infrared light and ultraviolet light.  And because an eye that is staring directly at the sun functions a lot like a magnifying glass being held up to the sun in an attempt to start a fire (ie condensing light to a pinpoint), all the wavelengths that do enter the eye are being focused to a small, extremely energetic pinpoint.  This focal point is on top of a layer of sensitive tissue known as the retina (which contains the sight enabling cells known as the rods and the cones).  The ultraviolet wavelengths (and the visible wavelengths to a lesser extent) can cause oxidative stress to these cells, initiating a cascade of changes in the cells that culminates with permanent damage to their ability to create sight (a leading cause of macular degeneration).  The infrared wavelengths, on the other hand, can actually pass through the photoreceptors only to get absorbed by the highly pigmented "backing" to the retina (termed the retinal pigment epithelium).  These highly pigmented cells easily absorb the infrared, but by doing so begin to heat up in the process, and essentially start cooking the retina in that spot.  Staring at the sun for as little as a few seconds can cause this devastating effect, and since there are no pain receptors in the retina, there is no "ouch" moment to remind you to look away.  

 Permanent damage to the retina (yellow lesion).

Permanent damage to the retina (yellow lesion).

This is why anybody viewing a solar eclipse needs to have a filter in front of their eyes that blocks 99.997% of all wavelengths, to be deemed safe.  Turns out, this is hard to achieve.   Most of the DIY solutions that people have used over the years to view eclipses fail to meet this standard (like squinting, shocking I know).  Although most (looking though polarized filters from 3D glasses or even using unexposed camera film) meet the standard when it comes to UV and visible light, they fail miserably when it comes to the infrared portion.  Even a shade 12 welders masks (which is too dark for most welders) still doesn't block enough of the infrared light to be deemed safe.  That is why the only two safe ways to view the solar eclipse are to use eclipse approved glasses that sport the stamp "ISO 12312-2" on the side, showing that they meet the proper safety standards, or by creating a pinhole device to project an image of the sun onto a piece of paper.  And in light of the numerous counterfeit eclipse glasses being sold online, I'd stick to the pinhole method, that is unless I want to walk around the rest of my life with a blindspot in the central 10 degrees of my vision (stick your thumb out at arms length, the size of your thumbnail equals the central 10 degrees most likely to get damaged by staring at the sun.  Doesn't seem so bad, until you remember that those 10 degrees are always exactly on top of whatever you're looking at).

Dr. Burke is an optometrist practicing at Calgary Vision Centre. Opinions above do not constitute medical advice, and readers should consult with their optometrist if they have questions or concerns about their eye health