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Mercury, January/February 1997 Table of Contents

Tom Van Flandern
Meta Research

Trade-off. As viewers venture away from the center line of the eclipse path, the eclipse lasts for a shorter time (smooth line), but edge effects such as Baily's beads persist for longer (spiked line). The duration of edge effects is longest when viewers are about 95 percent of the way from the center line to the edge of the eclipse path. From that vantage point, the eclipse lasts a third as long as it does at the center line. Diagram by Tom Van Flandern.

Traditionally, eclipse observers have flocked to the very center of the eclipse path, where the Moon obscures the Sun for the longest time. Yet eclipses are much more spectacular when viewed from near the edge of the path.

In 1925, the shadow of a total solar eclipse passed over New York City. Ernest Brown, an expert on lunar motion, planned on observing the eclipse from near the center line with the many other astronomers who did not wish to miss a single second of totality. Brown also put out an appeal to the residents of New York to climb to the roofs of their apartment buildings and report on whether the eclipse was total or not, so that he could determine the exact location of the path edge. Many who saw the eclipse from near the path edges, even those slightly outside the shadow of totality, were thrilled by the experience. But another half-century would pass before astronomers realized that Brown had sacrificed the best view of the eclipse to those citizen volunteers.

For two centuries, astronomers have been able to predict the location of the narrow track across the surface of the Earth made roughly once a year by the Moon's shadow. They have then traveled into the path of the shadow, where they could observe a total eclipse. But until fairly recently, astronomers simply assumed that the best place to be was in the middle of the path. The eclipse is longest on the center line of the path; the rare view of the Sun's beautiful white outer atmosphere, the corona, which can be seen only during eclipses, is also longest there. So without much further thought, astronomers concluded that all interesting phenomena were best seen from the center line.

That remained the situation until the 1960s, when astronomer David Dunham popularized observations of grazing occultations of stars by the Moon ­ in effect, short eclipses of stars by the edge of the Moon. Grazes in general, and edge proximity in particular, proved to have advantages for observation and data collection. In March 1970, Dunham, Joan Bixby-Dunham, and I tested this idea on a solar eclipse for the first time.

For just a single example of how wrong the old notion was, consider the most colorful part of the Sun's atmosphere: the innermost part above the Sun's visible disc, the chromosphere. On the eclipse center line, the chromosphere is visible only for a few seconds between the disappearance of the Sun's visible disc and the moment when the chromosphere itself disappears behind the Moon. During those precious few seconds, astronomers have tried to take spectra of the chromosphere in order to learn its composition. So brief is the appearance that successful results are referred to as the flash spectrum [see "Observing the Sun During Eclipses," July/August 1981, p. 108].

How foolish those astronomers might have felt if they had realized they could have been watching from near the edges of the path. There, the disc of the Moon slides by the disc of the Sun for a prolonged period ­ and the chromosphere remains visible for 90 seconds or more. Because of the chromosphere's rich redness, eclipse photos taken from near the path edges have more color in them than do photos from the center line.

It is not just the chromosphere view that is prolonged near the path edges (see diagram above). The mysterious "shadow bands" are far more likely to be seen from sites near the edges, and they usually last two to five times longer than on the center line. The diamond ring and Baily's beads will usually last up to 10 times longer. Moreover, while just a few beads may form and stay in place for center-line viewers, edge viewers can see numerous beads form and dissolve constantly; they seem to travel along the limb as the two discs glide past one another. Finally, prominences ­ giant eruptions from the Sun's surface ­ can be seen for longer.

For most total solar eclipses:

• As one moves off the center line by as much as 20 percent of the distance to the edge, the average duration of totality drops merely 2 percent. The true duration of totality is governed by irregularities (mountains, craters, valleys) at the Moon's limb, so the longest eclipse may occur almost anywhere within this central 20-percent zone. Precise positioning on the center line has no real value.

• The best overall views of the eclipse and related phenomena are generally seen at 95 percent of the way to the edge of the path, where the duration of totality is one-third of the center-line duration.

• Usually, people who still view from the center line do so for historical reasons, or because the relative merits of the choice have never been presented to them. They may not realize they may have missed the most spectacular view. Astronomers who in recent years have gone both to the center line and to the path edges seldom go back to the center line.

• Perhaps Guy Ottewell, veteran eclipse viewer and author of the annual Astronomical Calendar, captured it best: "In 1991 I found [viewing from the edge] to work dramatically well. I watched shadow-bands unmistakably for the first time; the Diamond Ring was overpowering; and the red chromosphere, which I had never before glimpsed, lingered throughout totality."

TOM VAN FLANDERN is an astronomer at Meta Research in Washington, D.C. He directed the Celestial Mechanics Branch of the United States Naval Observatory from 1978 to 1983. He is author of the 1993 book, Dark Matter, Missing Planets and New Comets. Van Flandern will lead expeditions, open to the public, to the edge of the February 1998 (Galapagos Islands) and August 1999 (Europe) eclipses. His email address is metares@well.com.