Williams College – Hopkins Observatory
Williamstown, Massachusetts 01267, USA

How to Observe the Solar Eclipse

Professor Jay M. Pasachoff described how and when to observe the partial eclipse, and how to do so safely while avoiding eye damage. Preparing his 26th solar eclipse expedition, Dr. Pasachoff is Field Memorial Professor of Astronomy and Director of the Hopkins Observatory at Williams College, Williamstown, Massachusetts, USA. He is also Chair of the Working Group on Eclipses of the International Astronomical Union and Chair of a Committee on Public Information and Eclipses of the Teaching Commission of the International Astronomical Union. He will observe the February 26, 1998, total solar eclipse from Aruba.

“A solar eclipse occurs when the moon comes between the sun and the earth,” explained Pasachoff. “On February 26, the tip of the moonUs shadow will sweep out a path about 150 kilometers (100miles) wide, making the total eclipse,” he said. “During the total phase of the eclipse, the sky turns dark in daytime, and the faint outer layers of the sun that are normally hidden behind the blue sky become visible. During the few minutes of totality–between about 3 minutes and 4 minutes on the boundary between Panama and Colombia and on some islands in the Caribbean–scientists can make observations that cannot be carried out in any other way. These observations can be put together with observations made from special spacecraft studying the sun in order to understand in detail how and why the sun shines and how its energy gets to us on Earth.

“To the sides of the band of totality,” Pasachoff explained, “the eclipse is still visible but we see the moon only partially covering the sun. It will look as though a bite has been taken out of the sunUs surface. The size of the bite depends on how far you are from the path of totality. The partial phases will last an hour or two. Even people in the band of totality will see a partial eclipse before and after the total eclipse.”

Partial phases of the eclipse will be seen southeast of a curved line extending from San Diego to Chicago and Newfoundland. Chicago will have only a 5% eclipse; Boston, New York City, and Washington, D.C., a 22% eclipse; and Miami a 50% eclipse.

“Though the sun will be partly covered,” said Pasachoff, “it is still not safe to look at directly. Each bit of the surface of the sun is so bright that it can burn your retina. So if you want to follow the progress of the eclipse, you must take precautions.” Pasachoff stressed that the precautions “are the same ones you would take to look at the sun on any ordinary day. The sun does not give off any extra rays during an eclipse. We astronomers give out special warnings when eclipses approach because it is much more tempting to look at the sun when an eclipse is going on.

“A pinhole camera is the safest way to observe the partial eclipse,” said Pasachoff. “To make one, simply punch a hole perhaps 5 millimeters across in a piece of cardboard, and hold the cardboard up to the sun so that the shadow of the cardboard falls onto a piece of white paper. Look down at the paper, and not up through the hole in the cardboard. In the middle of the white paper, you will see the image of the sun, which will look like a circle with a bite out of it. Often, without making your own pinhole camera, you will find such pinhole images on the ground or on a wall under a tree, since the interstices between the leaves or branches act as natural pinholes.

“If you want to look up toward the sun, you must have a special filter. Ordinary sunglasses are not safe, since they do not absorb anywhere nearly enough sunlight. The filter must block out all but about one part in a million of the light that hits it, and must do so all across the spectrum, even in the infrared. So neutral density filters sold for cameras are not safe either. Special solar filters that block out enough light are commercially available. They usually are made of deposits of a film of metals on a plastic or glass backing. Or one can make a solar filter by fogging and completely developing black-and-white film to make it black. Color film does not work for this process, since it does not contain silver, and so does not absorb the infrared rays, and even some new black-and-white films don’t use silver and so aren’t safe. Even with fogged and developed black-and-white film, try two layers at first, for safety, and then judge if the image is too faint with two layers. Further,” Pasachoff stressed, “never stare at the sun. Even with a solar filter, just glance through it for a second or two. All you want to do is see how much of the sun is covered. Even through the filter, it is sufficient to take a one-second glance every five minutes.”

The following total solar eclipse will occur on August 11, 1999, and is widely anticipated because it will cross Europe. Over two minutes of total eclipse will be visible, weather permitting, from a band passing over Cornwall, England, and the Channel island of Alderney; northern France; Luxembourg; southern Germany, including Stuttgart and Munich; Austria; southwestern Hungary, including Lake Balaton; Romania, including Bucharest; middle Turkey east of Ankara; northern Iraq; southern Iran; southern Pakistan, and middle India. The path ranges up to about 110 km (70 miles) wide. Partial phases will be visible from all of Europe; the eastern halves of Russia and China, and Asia west of India; the northern half of Africa; and even, at sunrise, the northeast United States and Canada. The sun will rise partially eclipsed by 14% in Washington, D.C.; 46% in New York City, and 73% in Boston. The most widely used calculations of eclipse coverage are made by Fred Espenak of NASA, a member of the IAU Working Group on Eclipses.

Between these two total eclipses, an annular eclipse, in which the moon appears too small to completely cover the sun, will be visible from Southeast Asia on August 22, 1998. The annular phase will be principally visible from parts of Indonesia and Malaysia, and the partial phases will be widely visible in Asia.

Information on eclipses and scientific research at them is summarized on the World Wide Web at:
http://www.astro.williams.edu/IAU_eclipses.

References: Donald H. Menzel and Jay M. Pasachoff, Peterson Field Guide to the Stars and Planets (Houghton Mifflin, updated 1997).
Jay M. Pasachoff and Michael Covington, Cambridge Guide to Eclipse Photography (Cambridge University Press, 1993).
Fred Espenak and Jay Anderson, NASA Reference Publications 1383 for the 1998 eclipse and 1398 for the 1999 eclipse.
Leon Golub and Jay M. Pasachoff, The Solar Corona (Cambridge University Press, 1997).