The Total Solar Eclipse Experience

A total solar eclipse is a wonder of the astronomical world. We have the seven wonders of the natural world, the seven wonders of the ancient world, and the seven man-made wonders of the world. There are three View-Master reels, twenty-one wonders of the world to be seen in their three-dimensional glory. Maybe the astronomers should catalogue seven wonders of the celestial world. I suspect astronomers would pick more subtle phenomena than I would, but the most dramatic heavenly events are eclipses, where the sun or the moon goes away in the sky.

There are many wonderful eclipse web sites. I rely on Fred Espenak's supported by NASA. You can always use Google to search for the string total solar eclipse. There are some nice web sites out there, well worth looking at.

The astronomy behind eclipses is simple enough. The earth and moon orbit each other and, every once in a while, the earth, moon, and sun line up in a nearly-perfect row. In the science-textbook picture of the sun, earth, and moon orbits, it looks like this should happen twice each month, but the real orbits of sun, earth, and moon are at an angle so the three bodies only line up a few times each year. When the earth comes directly between the sun and moon, the entire moon is plunged into darkness for about an hour. Seen from the earth (where most of us live), a curved shadow moves across the face of the full moon which retains a ghostly red glow until the bright moon reappears on the other side. Anybody on earth who can see the moon will see this eclipse because the darkness is actually on the surface of the moon. The challenge in seeing a lunar eclipse is staying up late enough, remembering to go outside, and waiting for it to happen. If you miss one because it happens during the day, then you can hang on and wait until the next one. Lunar eclipses are frequent enough that they're seldom worth traveling for.

When the moon comes between the earth and the sun, the greater spectacle of a total solar eclipse can happen. Seen from the earth, the sun and moon are almost exactly the same size, half a degree of arc, so the moon almost exactly covers the disk of the sun. That almost-exact size match is both the rub and the glory.

It is the rub because the eclipse is only total on a tiny area of the earth, the part in the darkest part of the moon's shadow, called the umbra. An area about twice the size of the entire moon, the penumbra, can see a partial eclipse. Sometimes the moon's orbital path is far enough away from the earth that the umbra does not reach the earth's surface at all. Instead there is a zone where earth viewers see a ring of sun around a dark shadow of moon, an annular eclipse. (The word "annulus" is Latin for "ring.")

It is the glory because the near match in size means the moon covers the disk of the sun almost perfectly. That would not be terribly spectacular except that the sun's terribly bright photosphere is surrounded by the corona, a beautiful part of the sun normally overwhelmed by the blue sky, photosphere light scattered by earth's atmosphere. (I presume that, out in space, one can view the sun's corona by holding a small object, perhaps a thumb, over the disk of the photosphere.) The moon-sun size-match is so close that solar flares, called promenances, are often visible on the surface of the sun. This is our chance, as ground-bound earthlings, to see parts of our own sun that are otherwise obscured by the brilliance of the photosphere. And, as luck would have it, this event is only visible from a small part of the earth.

The stages of an eclipse:

First, some smart people with some computational savvy figure out the motions of the sun, earth, and moon to calculate the exact time and location of an eclipse. Digital computers help the process, but astronomers knew eclipse schedules centuries ago when they had nothing but pen, paper, and their wits. A bunch of entrepeneurs plan out long, expensive trips and a bunch of enthusiasts sign up for these trips. (As a non-eclipse-fan might see it, a bunch of idiots pay a lot of good money to travel to places that nobody would ever want to go just to experience a couple minutes of darkness.)

The planning process is made more involved by long-term weather forecasts. For example, on 2002 December 4, the eclipse path cut through Africa in the morning and South Australia in the evening. The African eclipse path promised about a minute and a half of totality but had a 70 percent chance of cloud obscuration as December starts the rainy season (and the malaria season) in southern Africa. The Australian path had only a 30 percent chance of clouds but the total eclipse time would be less than half a minute. Further inland in Australia promised better weather probabilities and shorter total eclipse times closer to sunset.

Second, the eclipse enthusiasts gather at their appointed place waiting for the Big Event. They are often armed with binoculars, telescopes, and all kinds of fancy cameras. The people who wander all over the globe to see eclipses tend to be astronomy buffs, so there is often a "star party" where a bunch of folks stay up late and together view objects in the heavens. Eclipses in the Southern Hemisphere elicit more of this kind of attention as most eclipse viewers are Northern Hemisphere residents who seldom get to see the southern sky. The Southern Cross dominates the June winter sky and the December summer sky has Orion's sword pointing up instead of down.

Third, the eclipse enthusiasts are all lined up with their gadgets on a sunny day waiting for the Big Event. At least we hope it's a sunny day. We have some equipment for viewing the sun because the photosphere disk itself is very bright. Dark eclipse glasses are popular and some are officially sanctioned by people who know what they're talking about. Another approach is to use a pinhole and a viewing screen, usually a piece of paper, a meter or two away. The sun's disk is half a degree of arc, about one centimeter for each meter of distance. Another approach is to use a pair of binoculars as a projector to get a larger image of the sun's disk.

Fourth, a chip appears in the side of the sun, easily visible to the unaided eye through eclipse glasses. (I say "unaided" rather than "naked" because of the eclipse glasses.) That chip gets bigger and bigger until the sun forms an obviously shrinking crescent in the sky. For the first fifty minutes or so, the sun's area is shrinking and people are getting more excited, but nothing else is happening.

As children, we have all been told not to look directly at the sun because it is too bright. This danger is greater at a solar eclipse for two reasons. Before local eclipses, both the English and Australian governments have warned people not to look directly at the eclipse at all to prevent eye damage, and, instead, to watch it on television. I would caution people to avoid watching television to avoid brain damage. I know I did not travel thousands of miles to watch an eclipse on TV or on a web page.

The first reason an eclipse is dangerous is the plain and simple reason that it is tempting to watch it. Normally, I have no reason to gawk at the sun in the sky. I might glance that way for a second or two and find myself seeing a green-blob after-image for a few minutes. That's about as harmful as the sun gets in everyday life. But a partial eclipse is something to look at, something interesting, and people are tempted to stare at it. Solar brightness is unlikely to damage a human eye in a few seconds but minutes of continuous viewing can do serious eye damage.

The second reason an eclipse is dangerous is that the overall ambient light level is dwindling but the remaining sun in the sky is just as bright as the full daylight sun. So your eyes are adapting to twilight darkness while the sun that you're just dying to stare at is at its full brightness. While eye experts will tell me my actual numbers are full of shit (and they're right), the reasoning is sound, so bear with me. Consider the time when the sun is ninety-percent covered by the moon. The ambient light level is one-tenth of normal daytime and the eye has adapted by becoming ten times as sensitive to light. (In fact, the actual change in pupil size is less than ten-to-one, but bear with me.) When you look at the solar crescent with your dark-adapted eye, ten times as much daylight is reaching your retina as it would during a normal sunny day. Another way to look at it is that the total daylight is concentrated into one-tenth as much sky. Whichever way you look at it, as the sun's area shrinks in the sky, our dark-adapted eyes are in increasing danger of damage from direct solar viewing. So be careful!

Fifth, after almost an hour of sitting around peeking at the shrinking sun through eclipse glasses and gawking at crescent shaped sparkles amid shadows, the excitement begins a few minutes before totality and things start happening fast. It is now getting noticeably darker and colder. Any animals in the area are getting quiet as if it were twilight, so there is a hush of anticipation in the air. The sun is now a tiny curved slit in the sky.

The ground may have lighter and darker patches moving like the pattern on the bottom of a swimming pool. These shadow bands are caused by the same atmospheric variations that make stars twinkle. The sun's presence in the sky is small enough to "twinkle," but it is so bright that its twinkling manifests itself in these shadow bands. I have only seen shadow bands in one of my four total eclipses.

Sixth, as the sun is winking out, it is reduced to a tiny area of disconnected sun-bright points in the sky called Baily's beads. The gaps are mountains on the moon's surface and the total eclipse is just a few seconds away as these beads disappear.

Seventh, it is dark, there is a black hole in the sky where the sun used to be, and the corona is visible in all its glory. Each time the corona is different, blown into changing shapes by various solar winds of one sort or another.

We use exponential units to define ratios of brightness, and there are several different choices of measurement. A decibel (dB) is a ratio of about 1.26 so 10 dB is a factor of ten. We use decibels in audio and radio. Astronomers use magnitudes and each magnitude is 4 dB so five magnitudes is a factor of one hundred. Photographers use f-stops where each f-stop is 3 dB, a factor of two. So a factor of one thousand is 30 dB, 7.5 magnitudes, or ten f-stops.

The best analogue films can render a range of about seven f-stops (20 dB, 5 magnitudes, a factor of one hundred) and no photographic reproducing medium comes close to that kind of range. Typical "high-resolution" digital displays have a theoretical maximum range of 255-to-1, 24 dB, but I don't think any monitor or paper realistically reproduces that kind of contrast in illumination or albedo. We have the same issue in audio reproduction where a musical event has interesting stuff happening over a 60 dB range and even the best hifi equipment cannot resolve detail over that kind of range.

The usual photographic tactic for total solar eclipses is to concentrate on just one part of the picture, a short exposure for the inner corona only and a longer exposure for the outer corona with the inner corona overexposed. An even shorter exposure renders the solar flare activity. Some digital photographers take a bunch of pictures and mix them into a single image that shows some detail for every part of the picture, a high-tech version of the after-eclipse sketch. My own strategy is to point my automatic Olympus camera on a tripod in the direction of the sun and to press the shutter button a few times during the eclipse. The results are sometimes pretty good.

The black hole of the moon isn't competely dark, either. After all, the moon is seeing a full earth (with a small, dark dimple where the moon casts its shadow) and lunar features can be seen in an appropriately overexposed eclipse photograph. As bright as the dark side of the moon may be, it is overwhelmed by the inner corona.

A total solar eclipse has an astounding range of brightness. The inner corona is about 30 dB brighter than the outer corona. The darkness surrounding the eclipse is not quite night, dark enough to see planets but not always dark enough to see stars. The horizon is not dark because it is still sunlit. There is a penumbral glow from the edge of the umbra of the moon's shadow.

A viewer with a pair of decent binoculars can see red flares, enormous storms on the surface of the sun. These prominences are the side-view of sunspots, to put it loosely. A three-minute eclipse is a busy time to gaze at the corona, to use binoculars to look for solar flares, and to take pictures.

Eighth, there is a brightening of one side of the black disk. Sometimes a single point of light appears, a tiny piece of the sun's disk. While we have been gazing at an inner corona 30 dB brighter than the outer corona, the photosphere itself is about 40 dB brighter still, and now it is returning to view. When that tiny point of light is appearing, the entire corona is still visible and the effect is called the diamond ring. The total range of illumination is 70 dB, far beyond the range of any image reproduction. The diamond ring lasts two or three seconds and has to be seen, live, to be appreciated.

Every total eclipse has some kind of diamond ring effect at the end, but sometimes the moon is positioned so there is a large canyon at the edge, so the diamond ring is especially beautiful.

Photographs and images of the diamond ring cannot hope to render the single point of brightness that makes the ring so magnificant, so they create some pattern of light, a blob or some rays at maximum saturation, to use quantity where intensity is not available in the medium.

I liken the entire eclipse photographic process (or Handel's "Messiah" on a hifi) to taking pictures of the Grand Canyon. If you have seen the real thing, then you don't need the pictures. If you have not seen the real thing, then the pictures will not begin to communicate why somebody else went to all the trouble to experience it. I still went through a bunch of rolls of film in my two Grand Canyon visits (driving a car and flying a light airplane) because the pictures remind me of what I saw. That is why I'm always so anxious to get pictures of each eclipse. But showing pictures will not convince the unconverted to become eclipse chasers like us.

Now we're back to eclipse glasses and pinhole projections as the photosphere is back in the sky, a very thin crescent that grows back into the full disk over an hour. Then the eclipse is over.

Why do eclipses vary so much? The moon and sun vary in size in the sky, so the amount of time can be a few seconds or nearly seven minutes. The moon moves about 3500 Km/hour in its orbit and the rotating earth is moving the same direction almost half that fast at the equator, so eclipses at the equator have the double advantage of being 7000 Km closer and having a slower relative motion of moon and viewer. The sun is farthest from the earth in June and closest in December. June eclipses can be longer, especially if the moon is obliging enough to be at a near point on its own orbit, for a smaller sun and larger moon allow a longer blackout period. Shorter eclipses allow more features near the surface to be seen, so they have their advantages.

People always seem to see a lot during eclipses. In Woomera, South Australia, on 2002 December 4, I enjoyed the corona and saw no prominences with my naked eyes. Since the total eclipse was only twenty-six seconds, I decided not to try to use my binoculars. I took a quick look to notice the sky was brighter north and south than it was east and west because the eclipse was near sunset and the moon's umbral shadow was oblong as a result. I saw the most beautiful diamond ring I have ever seen. When the partial sun was setting I saw no green flash from either horn of the crescent. However, I did see the Loch Ness monster the next day from the bus.

Why do eclipses seem to happen in remote places? It isn't Ripley's Believe It or Not where amazing thing always seem to happen someplace amazingly far away. The fact is that a 50 Km-wide line drawn across the earth's surface at some arbitrary place is not always going to reach places where a lot of people live. Sometimes there is a populated path like the 1999 August 11 eclipse that ran through the heart of Europe. And sometimes there is an unpopulated path like the 2002 December 4 eclipse that ran through sparse areas of Africa and Australia with most of its path in the Indian Ocean. The 2003 November 23 eclipse in Antarctica is even more remote with Mirnyy (the Russian base) being the closest thing to a population center on its path.