What's so special about a total eclipse of the Sun?

Here's a brief overview of what the TV images cannot show you. The partial eclipse begins about an hour before the total eclipse. During that period, the silhouette of the Moon starts to pass in front of the disc of the Sun. If you look toward the bright Sun through a protective filter, you can see by eye that the black outline of the Moon's disc is creeping across the Sun's disc very gradually. All around you, the air takes on a strange hue and an eeriness that is not seen under any other conditions. As the eclipse advances, all of nature senses the change. Birds come to roost, cocks will crow, and all manner of animals and plants revert to their nighttime posture even in the middle of the day. So you can already see that the phenomenon is considerably more influential to nature than, say, the Sun going behind a cloud. Clouds block the light of the Sun, but eclipses block all wavelengths of solar radiation.

The landscape gets progressively darker, and temperatures start to drop. Only those standing under the approaching shadow sense these effects. If one looks at the ground under a tree with leaves, or anywhere light from the Sun passes through small spaces, each such hole will project an image of the crescent Sun blocked by the advancing Moon on the ground. Hundreds of crescent images may be visible all over the ground.

If one has a view toward the west, the advancing shadow of the total eclipse may be visible even before it arrives. It is especially noticeable if a cloud bank or bright landscape is in the direction where the blackness will cover it before it reaches your own position. The shadow advances at an average rate of 50 km (30 miles) per minute.

In the final minutes before the eclipse becomes total, many more things happen. Shadow bands may suddenly appear and shimmer over all ground objects. The Sun's atmosphere, called its corona, starts to become visible as a bright ring around the black lunar disk. The disappearing crescent shrinks to a brilliant gem on the edge of the brightening corona. This effect is called the "diamond ring". Mountains along the lunar limb break up the last of the crescent into "Baily's beads". The inner solar atmosphere, called the chromosphere, makes its reddish presence known -- called the "flash spectrum". Brighter stars and planets appear. Solar prominences, gigantic explosions along the Sun's limb, may be seen with the aid of binoculars. A total solar eclipse is a multifaceted experience, and the impression it leaves on the viewer is one of awe.

For two hundred years, astronomers predicted and observed total eclipses of the Sun, and simply assumed that the best place to be was in the middle of the path. It was obvious that the eclipse was longest on the centerline of the path; and that the rare view of the Sun's beautiful white outer atmosphere, the corona, which can be seen only during eclipses, was also longest on the centerline. So without much further thought to the matter, astronomers assumed that all interesting total eclipse phenomena were best seen from the centerline.

What an embarrassing blunder! To show just a single example of how wrong that naive notion was, consider the most colorful part of the Sun's atmosphere -- the innermost past above the Sun's visible disc, called the chromosphere. It received that name because of its ruby red color. But astronomers have tried their best to study the composition of the chromosphere for two hundred years, limited to brief moments during total eclipses when they could see it. On the eclipse centerline, the chromosphere is visible only for a few seconds between the disappearance of the brilliant photosphere of the Sun's disc, and the moment when the chromosphere itself disappears behind the Moon's black obscuring disc.

Graph showing duration of phenomena during a total solar eclipse at the path edge. (Click on thumbnail image to see the full-sized graphic.)

During those precious few seconds, astronomers tried to take photos of the spectrum of the chromosphere to learn its composition. So brief was the appearance of this most intriguing part of the solar atmosphere that successful results were referred to as the "flash spectrum". In just moments, the opportunity was gone. How foolish those astronomers would have felt if they had realized that, near the edges of the path of totality, where the disc of the Moon slides by the disc of the Sun for a prolonged period and the chromosphere remains visible throughout, they might have prolonged their view of the chromosphere by a factor of ten or more. Indeed, from our Eclipse Edge Expedition site for the 1998 eclipse, we may expect about 90 seconds of continuous chromosphere visibility. We will see more of this rare phenomenon than most astronomers who have chased eclipses all their lives all over the world. Moreover, eclipse photos taken from our site will have more color in them than corresponding photos from the centerline because of the chromosphere's red color.

But it is not just the chromosphere view that is prolonged near the path edges. Most total eclipse phenomena, the ones that make the eclipse spectacular, are also prolonged near the path edges. Moreover, their probability of being seen at all is enhanced. Consider a few more examples.

The mysterious "shadow bands" are far more likely to be seen, both before and after the total eclipse, from sites near the path edges; and they will usually last 2-5 times longer than on the centerline. The diamond ring effect will usually last ten times longer near the path edges. Baily's beads are also prolonged in duration by an order of magnitude. Moreover, while just a few beads may form and stay in place for centerline viewers, near the path edges numerous beads are forming and dissolving constantly and seeming to travel along the limb as the two discs glide past one another. Each eclipse has its own personality, especially in regard to bead formation and corona appearance.

Finally, prominences (giant eruptions on the Sun's limb) may be seen far longer from the path edges. And historically unusual events, such as expanding rings of light and other rare occurrences, are usually seen only from near the path edges.

Several considerations favor centerline viewing. The solar corona is visible for the greatest time from the centerline. Faint stars near the Sun that might show up the Einstein light-bending effect on sensitive photos can be seen longer from the center line. There is more time to view other sky objects such as planets during totality from the centerline. And amateur astronomers who compete to have the greatest number of minutes of totality accumulated in a lifetime must view from near the centerline to improve their personal statistics.

The following considerations apply to most total solar eclipses: (1) As one moves off the centerline by as much as 20% of the distance to the edge, the average duration of totality drops merely 2%. 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% zone. So precise positioning exactly on the centerline has no real value. (2) The position where the duration of totality falls to about half that on the centerline is about 90% of the way to the edge of the path. (3) The best overall views of the eclipse and related phenomena are generally seen at about 95% of the way to the true (lunar-limb-corrected) edge of the path, where the duration of totality is one-third of the centerline duration.

For the most part, people who view from the centerline do so for historical reasons, or because the relative merits of the choice to be made have never been presented to them. Such people may not realize that they may have missed the most spectacular view. Astronomers who have in recent years gone both to the centerline and to near the path edges seldom go back to the centerline. The consensus opinion of people who have tried both is that one misses out on too many enhanced edge phenomena from the centerline to make the extra overall eclipse length a good tradeoff.