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Mercury, January/February 1997 Table of Contents

Fred Espenak
NASA Goddard Space Flight Center

Most eclipse viewers crave every second of totality they can get. For them, there is only one place to view an eclipse: at the center line.

In 1970, while still in high school, I begged my parents for permission to drive the family car 600 miles into the path of totality. It was the first total eclipse of the Sun visible from the United States since 1963, and I was convinced it was my chance of a lifetime to witness this rare astronomical event. I'd been an avid amateur astronomer for several years, so I was well-versed on what to expect. But no photographs and no amount of reading could have prepared me for the first-person experience of witnessing totality.

The early stages of the partial phases proceeded at quite a leisurely pace. But as more and more of the Sun's disc was hidden behind the Moon, I noticed that time and events seemed to accelerate rapidly. Although the sky was crystal clear on that cold March day, the Sun's diminishing brilliance bathed the landscape in a pale and most unnatural light. Suddenly the ground was alive in rippling bands of shadow and light. Above, the Sun's image was a blazing, silver-white crescent that quickly zippered inward and broke into several dazzling beads while the corona flashed into view. As the last bead vanished, the world was plunged into an eerie twilight darkness. Venus shone brightly and sunset colors surrounded the entire horizon. High above me, the Moon's disc looked like a black hole ripped from the firmament, as the full glory of the solar corona radiated beyond it. Delicate rays crowned the Sun's poles while gossamer streamers stretched millions of miles and gradually faded into a cobalt blue sky. The corona was the most exquisite, most beautiful thing I had ever seen. I was completely overcome with a sense of awe at being a witness to this majestic clockwork of the universe.

It seemed like totality had only begun when a brilliant bead appeared along the opposite edge of the Moon and the corona quickly faded from view. Had three minutes passed already? How was this possible? But there was no mistake. Totality had begun and ended just as predicted. Only in my subjective recollections did the event last a mere 10 seconds. As the Moon's shadow swept towards the horizon, my heart raced along with it,; the hair on my neck stood on end. To say that the experience was staggering, awesome, electrifying would be to risk gross understatement. Upon regaining my composure, I was of one mind: I must see another total solar eclipse!

After more than a quarter of a century and 15 trips into the Moon's shadow, I can honestly attest that each eclipse is as inspiring, and as brief, as the first. Of course, I've learned to see far more during those fleeting seconds of totality, even while operating half a dozen cameras.

Although a camera can record a permanent image of the corona, the dynamic range of the human eye is far greater than that of photographic emulsion. The eye can simultaneously see the brilliant, crimson prominences as well as the subtle, low-contrast details in the faint outer corona. No single photograph can capture both. There is no substitute for witnessing it firsthand. And that explains the single-mindedness of eclipse chasers, who insist on traveling to the center line of each eclipse because totality and the views of the corona it affords last longest from the center of the path.

Extended views of Baily's beads, the chromosphere, and solar prominences can all be had near the edges of the eclipse path, but only at the expense of views of the corona, as the duration of totality drops precipitously near the shadow's limits. Of course, not all eclipse experiments focus on the corona. Measurements made near the path limits permit the determination, for example, of the Sun's diameter to high precision. This is important in investigations into the variability of the Sun's energy output, which, in turn, could have significant consequences for our understanding of solar physics.

Nevertheless, most observers, both amateur and professional, cluster near the center line. Fortunately, the duration of the eclipse varies little near the middle of the path, making it academic whether one is dead center or not. For example, halfway out from the center to the edge, one can still enjoy 87 percent of the maximum central duration. In contrast, seeing the prolonged beading effects requires traveling to within several kilometers of the path edge, where the duration of totality is one-quarter its central duration and dropping rapidly with distance. Small errors in geographic coordinates and uncertainties in the Moon's position and profile could shorten totality even more. Unless you have good maps and a GPS receiver for accurate positioning, you could even find yourself outside the path altogether.

For first-time eclipse observers ­ eclipse virgins ­ the choice is clear. Take no chances, head for the center line, and prepare to witness one of nature's most spectacular phenomena for as long as it is possible. Just don't be surprised if totality seems to last only 10 seconds!

FRED ESPENAK is a NASA astronomer who publishes detailed predictions for each solar eclipse. His eclipse-dedicated web site is located at http://planets.gsfc.nasa.gov/eclipse/eclipse.html. In March, Espenak will travel to Mongolia on his 16th eclipse expedition. His email address is u32fe@lepvax.gsfc.nasa.gov.