Why eclipses hurt eyes
Looking at the Sun is never safe, eclipse or not. Normally, its glare makes us look away within a fraction of a second. During a partial eclipse, the sky can dim enough that staring feels comfortable, yet the visible crescent of the Sun is just as bright per square inch as ever.
Focused sunlight can injure the retina, a condition called solar retinopathy. The retina has no pain receptors, so you feel nothing while the damage happens. The effects can include blurred or distorted central vision and a blind spot. Some damage heals over weeks or months; some is permanent.
The one rule
Never look at the Sun during the partial phases of any eclipse without proper protection. That includes:
- every partial solar eclipse, from start to finish;
- the partial phases before and after totality in a total eclipse;
- the entire course of an annular eclipse, including the ring phase, because the bright ring is still the Sun’s surface.
Use certified eclipse glasses
The safe way to look directly at a partially eclipsed Sun is through a dedicated solar filter. Eclipse glasses and handheld solar viewers should meet the ISO 12312-2 international standard for direct solar viewing. Through them, the ordinary daylit world looks completely black, and only the Sun shows up as an orange or white disk.
Check your glasses before each use:
- Look for the ISO 12312-2 reference and buy from a reputable supplier. The American Astronomical Society (AAS) keeps a list of vendors it has checked, because counterfeit glasses have been sold before major eclipses.
- Inspect the filters. If they are scratched, punctured, torn or coming away from the frame, throw them away.
- Put them on before looking up and look away before taking them off.
- Supervise children closely so they keep the glasses on.
If you wear prescription glasses, put the eclipse glasses on over them. A shade 12 or darker welding filter is also safe for direct viewing; lighter welding shades are not.
Never through optics with just eclipse glasses
Binoculars, telescopes and camera lenses concentrate sunlight. Looking through them while wearing eclipse glasses is dangerous: the concentrated beam can destroy the glasses’ filter and injure your eye instantly. Use optics only with a proper solar filter mounted on the front of the instrument, and get advice from an experienced observer if you have not done it before.
What is not safe
None of these protect your eyes:
- ordinary sunglasses, however dark, or several pairs stacked;
- smoked glass, exposed photographic film, CDs or DVDs;
- homemade filters or polarizing filters;
- looking at the Sun’s reflection in water or a mirror;
Indirect viewing
You do not have to look at the Sun at all. A pinhole projector casts a small image of the eclipsed Sun onto paper or the ground. Even the gaps between leaves, a kitchen colander or your crossed fingers will project dozens of tiny crescents. Keep your back to the Sun and look only at the projected image.
The exception: totality
During a total solar eclipse, and only then, there is a short window when it is safe to look with the naked eye. Once the Moon has completely covered the Sun’s bright surface and the last flash of light, the “diamond ring”, has vanished, you may remove your glasses and look at the corona.
As soon as the Sun’s edge starts to reappear, with a bright bead of light on one side, look away and put your glasses back on. If you are outside the path of totality, the Sun is never fully covered, so this exception never applies. Check where your location falls on the eclipses page, and read about the total solar eclipse to know what to expect.