A solar eclipse is awe-inspiring, and the temptation to look at the source of the spectacle can be overwhelming. Solar astronomers and ophthalmologists warn that that's a mistake that can render you permanently blind. It's a warning to take seriously, but there is one time when it doesn't apply. You probably won't hurt your eyes if you peek at the precise moment of totality and look away as soon as the sun reappears.
The sun is basically a single, large, continuous thermonuclear explosion, and people have a natural aversion to looking directly at it. There's a good reason for that aversion, besides the headaches and temporary vision distortion that visible sunlight produces. Ultraviolet sunlight can cause a number of eye disorders, according to the Cleveland Clinic, including macular degeneration, solar retinitis and corneal dystrophies. Moreover, the effects are cumulative, so looking at the sun twice is twice as bad for your eyes as looking at it once, even if you look on different days.
Looking at the sun during an eclipse is more dangerous than looking at full sun. The darkness that accompanies an eclipse can override the natural tendency to squint and avert the eyes, increasing the amount of ultraviolet radiation landing on the retina and making it more likely that you'll sustain eye damage. Your eyes can sustain damage even if only a small sliver of the sun is visible. The cornea focuses sunlight on the retina and scorches it, and because the retina has no pain receptors, you don't know the damage has been done until it's too late.
Your eyes need the protection of an effective UV-blocking filter if you want to look at an eclipse. That protection isn't provided by conventional sunglasses, nor is it provided by smoky or colored glass. You need the industrial-strength protection of No. 14 welder's goggles or, even better, eclipse glasses that are specially made for viewing eclipses. A simple projector consisting of two pieces of cardboard -- one with a pinhole -- also allows you to view the eclipse safely, if not in great detail. The pinhole projects the sun's image onto the other piece of cardboard.
The one time when it is safe to fleetingly glance at the sun during an eclipse, according to solar astronomer Mitzi Adams, is at the moment of totality, when its disk is completely hidden by the moon. You won't have time to study the corona in detail, however, because you must look away at the first sign of light. A photographic lens or telescope focuses and intensifies sunlight, and you should never look through either without proper filters. Unfiltered telescopes or binoculars are dangerous, even if no one looks through them, because the focused sunlight they project can quickly start fires.
