Your guide to an unusual solar eclipse

Caution: Don't look at the sun
To look at the sun without proper eye protection is dangerous.

Unlike a total eclipse of the sun, concentrating its excitement into a few fleeting minutes, a partial eclipse can be watched in a relaxed manner from wherever one happens to be. If proper protection is employed, observations can be made with or without telescopes or binoculars. However, looking directly at the sun is harmful to your eyes at anytime, partial eclipse or no.

Most people are under the mistaken impression that when a solar eclipse is in progress that there is something especially insidious about the sun’s light. But the true danger that an eclipse poses is simply that it may induce people to stare at the sun, something they wouldn’t normally do. The result can be "eclipse blindness," a serious eye injury that has been recognized at least since the early 1900s. About half of the reported victims of eclipse blindness recover their precious quality of eyesight after a few days or weeks. The other half carries a permanent blurry or blind spot at the center of their vision for the rest of their lives.

Public warnings by news media have vastly reduced solar eye injuries at eclipses in the last few decades. After the solar eclipse that crossed the United States on March 7, 1970, no fewer than 245 cases of retinal injury were reported. Of these people, 55 percent suffered permanent impairment of vision. In contrast, after the solar eclipse of May 30, 1984, Sky & Telescope magazine was able to locate only three cases of eclipse blindness in the entire United States. During any direct observation of the eclipse, your eye or must be protected by dense filters from the intense light and heat of the focused solar rays.

By far, the safest way to view a solar eclipse is to construct a "pinhole camera." A pinhole or small opening is used to form an image of the sun on a screen placed about three feet behind the opening. Binoculars or a small telescope mounted on a tripod can also be used to project a magnified image of the sun onto a white card. Just be sure not to look through the binoculars or telescope when they are pointed toward the sun!

A variation on the pinhole theme is the "pinhole mirror." Cover a pocket-mirror with a piece of paper that has a quarter-inch hole punched in it. Open a sun-facing window and place the covered mirror on the sunlit sill so it reflects a disk of light onto the far wall inside. The disk of light is an image of the sun’s face. The farther away from the wall is the better; the image will be only 1 inch across for every 9 feet from the mirror. Modeling clay works well to hold the mirror in place. Experiment with different-sized holes in the paper. Again, a large hole makes the image bright, but fuzzy, and a small one makes it dim but sharp. Darken the room as much as possible. Be sure to try this out beforehand to make sure the mirror’s optical quality is good enough to project a clean, round image. Of course, don’t let anyone look at the sun in the mirror.

• Acceptable filters for unaided visual solar observations include aluminized Mylar. Some astronomy dealers carry Mylar filter material specially designed for solar observing. Also acceptable is shade 14 arc-welder’s glass, available for just a few of dollars at welding supply shops. It used to be widely advertised that two layers of fully exposed and developed black-and-white negative film was safe. This is still true but only if the film contains an emulsion of silver particles. But beware: some black-and-white films now use black dye, which is no longer safe. It is always a good idea to test your filters and/or observing techniques before eclipse day.
• Unacceptable filters include sunglasses, color film negatives, black-and-white film that contains no silver, photographic neutral-density filters and polarizing filters. Although these materials have very low visible-light transmittance levels, they transmit an unacceptably high level of near-infrared radiation that can cause a thermal retinal burn. The fact that the sun appears dim, or that you feel no discomfort when looking at the sun through the filter, is no guarantee that your eyes are safe.

What's ahead
The next solar eclipse will occur Oct. 3 this year. It will be an annular solar eclipse with a maximum duration of just over 4½ minutes that will sweep across the Iberian Peninsula and stretches across the African continent. Madrid, Spain, finds itself directly in the center of the annular eclipse track and will see the midmorning sun turn into a blazing ring of fire for over four minutes.

The next solar eclipse visible over a large swath of North America won't come until May 20, 2012, when the path of an annular solar eclipse passes across portions of eight southwestern states.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York. This is an updated version of a report originally published April 1.

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