Why Can't You Look At A Solar Eclipse?
QUICK ANSWER
Looking directly at a partial or annular solar eclipse without proper protection can cause serious retinal damage and permanent vision loss. The Sun's intense light burns the retina even when most of it is blocked. Only during the brief totality of a total solar eclipse, when the Sun is fully covered, is naked-eye viewing safe.
Looking at a solar eclipse without proper protection is one of the most dangerous things you can do with your eyes. The Sun's light is so intense that even partial coverage by the Moon can cause permanent damage to the retina, often without any pain warning. This makes eye protection critical during all phases of all solar eclipses except for the brief totality of a total solar eclipse.
Why is the Sun dangerous to look at?
It can burn the retina. According to NASA, the Sun emits intense visible light, ultraviolet radiation, and infrared radiation. When focused by the eye's lens onto the retina, this energy can literally burn retinal tissue, a condition called solar retinopathy. The retina has no pain receptors, so damage can occur without any warning sensation. Even brief direct exposure to the unfiltered Sun (even with the Moon covering 99% of it during an annular or partial eclipse) can cause permanent vision loss. Damage may show up hours later as blind spots or distorted vision.
Why is an eclipse especially dangerous?
Pupil dilation in the dimmer light. During an eclipse, ambient light levels drop and pupils dilate to admit more light. If you then look at the partially-covered Sun, your dilated pupils let in much more solar radiation than they normally would. The reduced apparent brightness can also tempt people to stare longer than they would at the normal Sun. This combination makes eclipse viewing particularly risky for unprepared observers. Many cases of eclipse-related eye damage occur during partial eclipses or before and after totality.
What protection works?
Eclipse glasses meeting ISO 12312-2 standards. These special filters block 99.999% of sunlight, far more than normal sunglasses (which block only about 50% and are dangerous for solar viewing). Eclipse glasses can be purchased from astronomy retailers and reputable sources. Pinhole projectors, which project an image of the Sun onto a surface, are another safe option. Specialized solar telescopes with proper filters work too. Welder's glass of shade 14 is also adequate, though shade 12 or below is not safe.
When can you safely look directly?
Only during totality of a total solar eclipse, briefly. During the few minutes of totality, when the Moon completely covers the Sun's bright disk, it's safe to view with the naked eye and see the corona. The moment any part of the Sun's surface reappears (the diamond ring effect), eye protection must go back on immediately. Annular eclipses never reach this point and require protection throughout. Partial eclipses also require protection at all times.
Looking at a partial or annular solar eclipse without proper protection can cause permanent retinal damage and vision loss. The Sun's intense light burns the retina even when most is blocked by the Moon. Only during the brief totality of a total solar eclipse, when the Sun is fully covered, is direct viewing safe. ISO 12312-2 eclipse glasses, pinhole projectors, and specialized solar telescopes are all safe options. Regular sunglasses never are.
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