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Safety First When Observing a Solar Eclipse

by Howard L. Cohen, Associate Professor of Astronomy, Department of Astronomy
University of Florida, Gainesville, Florida 32611
June 13, 1991

A major total eclipse of the sun will occur on July 11th of this year. This solar eclipse promises to generate much public interest for at least two reasons. First, the eclipse path is relatively accessible to millions of people living in the Western Hemisphere. The path of totality includes the Big Island of Hawaii, Baja California Sur, Mexico including Mexico City, Central America, Colombia and Brazil. Second, the duration of totality is exceptionally long. Usually the sun stays completely hidden for only a few minutes. However, most regions in the path of totality on July 11th will have over four minutes of darkness. And in Mexico the moon will cover the sun for nearly seven minutes. This time is only about a half minute less than the maximum duration ever possible. Indeed, this is the longest duration of totality until the year 2132 A.D.

Although most of North or South America will not have a total eclipse, a partial eclipse of the sun will occur for nearly all of the continental United States. Only extreme northeastern New England will not have any eclipse. Many of the southern states, especially those in the San Francisco-Houston-Miami corridor, will experience a loss of 50 percent or more of the sun's disk. In fact, several hundred million people lie in the eclipse path. More people may witness this solar eclipse than for all other previous eclipses of the sun.

HOWEVER, UNLESS PEOPLE TAKE THE RIGHT PRECAUTIONS TO OBSERVE THIS ECLIPSE,
MANY WILL PERMANENTLY DAMAGE THEIR EYESIGHT.

The bright disk of the sun is very dangerous to look at without proper safeguards. A partially eclipsed sun is equally dangerous. Even a small silver of the sun's bright disk can severely damage the retina of an eye. These burns, called retinopathy or retinal scarring, are permanent. People who look at the sun with the naked eye, even for a few seconds, can suffer irrevocable damage to their eyesight. People may actually be unaware that the sun's rays are burning their retina because retinal burns can be painless. Also symptoms may not appear for several hours. And there is no treatment or cure for retinopathy.

Eight astronomical and health related societies have formed a coalition for the public good. This coalition includes the American Association of Physics Teachers, the American Astronomical Society, the American Optometric Association, the Astronomical Association of Northern California, the Astronomical League, the Astronomical Association of the Pacific, the National Safety Council, and the National Society to Prevent Blindness.

During March 1970 a solar eclipse occurred in the Southeastern United States including Florida. The Coalition states that 145 known cases of eye injury were reported: 40% of these victims mistakenly thought they were using safe direct viewing methods; the other 60% looked at the sun with no protection.

Hence, the Coalition warns that one should "never look directly at the sun." However, people can see the partial phases of the July 11th eclipse if they take the RIGHT PRECAUTIONS.

Because people are likely to misunderstand or misinterpret information. The Coalition does not recommend any direct methods for observing the luminous disk of the sun. Although safe, direct methods exist and are commonly used by astronomers, direct methods are therefore not advised for others. Viewing the sun through dark sunglasses (even if stacked), smoked glass, photographic filters, photographic films, welding goggles, a camera viewfinder, etc. can be EXTREMELY HAZARDOUS to eyesight. People should also not use eclipse glasses, which several companies are now marketing. Although the filters in these glasses may be safe, the method used to mount the filters make these glasses potentially dangerous to use. Finally, solar eyepiece filters, commonly packaged with inexpensive telescopes, are especially dangerous because the sun's intense radiation can crack the filter.

THE ONLY SAFE METHOD RECOMMENDED TO WATCH THE SUN,
BOTH OUT OF ECLIPSE OR IN ITS PARTIAL PHASES IS BY THE INDIRECT METHOD.

Fig. 1. Projection Box for Viewing the Sun

The INDIRECT METHOD requires that people view a PROJECTED IMAGE of the sun. Simple projection systems are easy to make. For example, take a large piece of paper and punch out a very small hole with a needle. Turn your back to the sun and use this sheet of paper to project a small image of the sun onto a second piece of paper held a foot away. You can also construct a more elaborate projection system using a cardboard box.
(See Figure 1).

Making A Projection Box

1. Get a large cardboard box (e.g. 12"xl2"x21" or larger).
2. Cut a 1/21" hole in the upper part of ONE END.
3. 3. Tape a piece of heavy aluminum foil over the hole.
4. Poke a hole with a pin in the aluminum foil with a needle. (The pinhole will act as a projection lens.) The hole should be as round as possible with no jagged edges.
5. Tape a piece of white paper on the inside of the box, opposite the pin hole. (The white paper will act as a projection screen.)
6. STAND WITH YOUR BACK TO THE SUN.
7. Hold the box over your head with the pin hole facing toward the Sun. (You can rest the pin hole end of the box on the top of your head for stability.)
8. A small image of the Sun will appear on the white paper. To get a larger (though fainter) image, use a longer box.

   (The above steps are similar to those recommended by the Coalition.)

THE ONE AND ONLY TIME it is safe to stare in the direction of the sun is during the TOTAL PHASE OF A TOTAL SOLAR ECLIPSE. First, the eclipse must become total in your location. Second, you can only directly look toward the sun when the moon totality hides the sun's luminous disk.

Since the July 11, 1991 eclipse will not be total anywhere in the continental United States or Canada, the sun will not be safe to look at during any stage of this eclipse. Consequently, one should not look directly at the sun either before, during, or after the eclipse from any of the 48 contiguous states and Canada including Alaska and Puerto Rico. Use an indirect method similar to those described above.

In Florida, for example, the Moon will cover about 40-60% of the sun's diameter at mid-eclipse depending on your location. This will occur roughly between 3:30 and 3:50 p.m. EDT. Obscuration of the sun will also produce a drop in sunlight from 30-50% at maximum eclipse. Since the July 11th eclipse never becomes total in Florida, people should use only the indirect method to view the sun.

For instance, in Gainesville, Florida, the eclipse begins at 2:35 pm EDT. Maximum eclipse occurs at 3:41 pm EDT. And the eclipse ends at 4:41 pm EDT. At maximum eclipse the moon will cover about 47% of the solar diameter. This will result in a loss of 36% of the area of the sun. Nevertheless, since the moon never totally hides the sun, people should use an indirect method to view the progress of the eclipse.

Although partial eclipses of the sun do not produce any of the spectacular phenomena of total eclipses, they are still fascinating to watch. See the July 11th eclipse but do it safely.

Notes:

1. The magnitude of the eclipse is the percent of the sun’s diameter hidden by the moon at maximum eclipse.
2. The obscuration of the eclipse is the percent of the sun’s area hidden by the Moon at maximum eclipse.

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