Bad Astronomy: Misconceptions and Misuses Revealed, from ...

122 SKIES AT NIGHT ARE BIG AND BRIGHT
protecting us. Even if the atmosphere were not here, the corona is
also simply too faint to hurt our eyes. And remember: whether the
Sun is eclipsed or not, the corona is still there; it’s just too faint to
see. So if it could hurt your eyes during an eclipse, it could do so
at any random time. In reality, the corona can’t hurt you.
PPP
There are several ways to enjoy an eclipse without risking your
eyes. You can use a telescope or binoculars to project the image of
the Sun onto a piece of paper or a wall. You can wear very dark
goggles, like welders wear; make sure they are rated as #14 so that
they are dark enough to be comfortable.
You can also use a solar filter on a telescope or binoculars, but
only the kind that mounts in front of the main lens or mirror. This
stops most of the light from entering the optics in the first place.
Some companies sell filters that go on the eyepieces, which block
the amount of light leaving the optics. However, the optics focus
all that sunlight right onto that filter, which can heat up a lot. Filters
like this have been known to melt or crack. I heard one story
of a solar filter that actually exploded! That’s bad enough, but
then your eyes are flooded with all that sunlight concentrated by
the optics. The lesson: stay clear of such devices.
Also, despite some advice I have seen, do not use unexposed
film to block the light. Even as lofty a source as the CNN web site
once claimed that it was safe to view an eclipse this way. This is
actually a very dangerous way to do it; it lets through less visible
light, so your pupils widen. However, it does not block the dangerous
wavelengths of light, so that even more damaging light
floods your eye. I and several hundred other people flooded CNN
with e-mail, and the web site was hastily fixed.
Solar eclipses do not happen very often, and they usually occur
over scattered parts of the planet. I have never seen a total solar
eclipse, though I’ve seen a dozen or so partial ones. Someday I
hope to see a total eclipse myself, but when I do, I’ll be careful.
And I’d better hurry. As is discussed in chapter 7, “The Gravity
of the Situation,” the Moon is slowly receding from the Earth. It’s
only moving away about 4 centimeters (2 inches) a year, but over