Amateur and professional astronomers from around the world will congregate in parts of Brazil, Africa and western Asia to view a total eclipse of the sun on Wednesday. Without a doubt, a total eclipse of the sun is one of the most spectacular natural sights that one can witness.
Only during totality can one observe the pearly white solar corona, as well as the ruddy chromosphere and prominences — sights that are normally hidden from our view by the brilliant light of the sun itself. In addition, darkness similar to 20 or 30 minutes after sundown suddenly falls over the surrounding landscape, allowing the brighter stars and planets to appear while strange and exotic colors rim the horizon.
Contrary to popular belief, a total eclipse of the sun is not a rare or unusual spectacle.
In fact, over the past 25 years there have been no fewer than 16 total solar eclipses, an average of one roughly every 18 months. The regions from which the spectacular sight of a totally eclipsed sun can be seen, however, are strictly confined to a narrow track; the path that the dark central shadow of the moon (called the “umbra”) traces out over the Earth’s surface. That track may run for thousands of miles, yet may average less than a hundred miles in width.
While the dark lunar shadow might sweep over Earth twice over a span of just three years, for a specific geographical location, the odds of lying directly in the path of that shadow is very small.
So if you intend to wait for this, the greatest of celestial road shows to come to your hometown, your wait is likely to be (on average) about 400 years. That is why many dedicated eclipse watchers — sometimes referred to as “umbraphiles” — will literally chase total solar eclipses around the globe. All for the privilege of “basking in the moon’s shadow” for a few precious minutes.
The last time skywatchers had an opportunity to see the sun in total eclipse was last April, when the moon’s umbra briefly touched Earth over the South Pacific Ocean for an unusual “hybrid” eclipse. Besides being accessible only to shipboard observers, the maximum length of totality lasted only about 42 seconds.
In contrast, this week's eclipse will be far more accessible and totality will last much longer: just over 4 minutes in the Libyan Desert.
Only six other times over the past quarter century have there been total eclipses that have lasted as long as this; this will be the longest total solar eclipse that occurs between June 2001 and July 2009.
Region of visibility
The path of totality for this eclipse begins over easternmost Brazil, where the coastal city of Natal and adjacent communities will be treated to the spectacle of a total solar eclipse shortly after the sun has come over the horizon. But this is a very humid climate and is prone to low clouds and coastal fog, which could very well eclipse the eclipse. Nonetheless, many South Americans will likely congregate here hoping against hope for a break in the overcast to afford them a glimpse of the sun’s corona.
Quickly moving offshore and in a northeastward direction, the dark umbral shadow of the moon will then take 36 minutes to sweep over the open waters of the North Atlantic.
It finally makes landfall again in western Africa, moving inland over Ghana and plunging its capital city of Accra into a late-morning darkness for three minutes and four seconds. Continuing on a northeasterly trajectory, the totality path slices through Togo and Benin, and into northwestern Nigeria. The cities of Gusau and Katsina, fortuitously positioned near the center line of the eclipse track, will each experience just over three minutes and 50 seconds of total eclipse. But typically, early spring is the rainy season for all of these regions, so the odds are not very good for getting a good view of the sun.
The totality path will then cut across central Niger, northern Chad and central and eastern Libya (just barely missing the city of Tobruk). The greatest eclipse, where totality will last for four minutes and 6.7 seconds, occurs along the border of Chad and Libya. Weather prospects in these areas improve dramatically, as this is basically a desert regime. But while there’ll probably be a paucity of clouds, local haze and dust might be a problem.
Passing over the Mediterranean Sea, directly between Crete and Cyprus, the eclipse track sweeps through central Turkey, where the towns of Kayseri and Sivas will experience more than two minutes of totality. Unfortunately, this is also near an active storm track, which usually brings a 40 to 70 percent chance of clouds.
After passing over the Black Sea, totality will strike the northwest corner of Georgia, then move across Kazakhstan before leaving Earth at sunset along Mongolia’s northern border.
The partial eclipse
A partial eclipse of varying extent will be widely visible across all but the southernmost part of Africa, as well as all of Europe, the Middle East and western sections of Asia. The closer you are to the totality path, the larger the eclipse.
Across much of central and eastern Europe, the eclipse will be a midmorning to early-afternoon event, with the moon appearing to cover at least half of the sun’s diameter at maximum. For parts of Greece, Turkey, Bulgaria, Romania and Ukraine, more than 80 percent coverage will result in an eerie “counterfeit twilight” effect at the peak of the eclipse.
Full prediction details for many cities are available from NASA.
In addition, NASA astronomer Fred Espenak has a Web site dedicated to the eclipse, which contains maps, tables and additional prediction details.
Once again it bears repeating: To gaze at the partially eclipsed sun without proper eye protection is dangerous.
By far, the safest way to view a solar eclipse is to construct a “pinhole camera.” A pinhole or small opening is used to form an image of the sun on a screen placed about 3 feet (1 meter) behind the opening. Binoculars or a small telescope mounted on a tripod can also be used to project a magnified image of the sun onto a white card. Just be sure not to look through the binoculars or telescope when they are pointed toward the sun!
A variation on the pinhole theme is the “pinhole mirror.” Cover a pocket-mirror with a piece of paper that has a quarter-inch (6mm) hole punched in it. Open a sun-facing window and place the covered mirror on the sunlit sill so it reflects a disk of light onto the far wall inside. The disk of light is an image of the sun’s face. The farther away from the wall, the better: The image will be only 1 inch (2.5 centimeters) across for every 9 feet (2.75 meters) from the mirror. Of course, don’t let anyone look at the sun in the mirror.
Acceptable filters for unaided visual solar observations include aluminized Mylar. Some astronomy dealers carry Mylar filter material specially designed for solar observing. Also acceptable is shade 14 arc-welder’s glass, available for just a few dollars at welding supply shops.
Unacceptable filters include sunglasses, color film negatives, black-and-white film that contains no silver, photographic neutral-density filters, and polarizing filters. Although these materials have very low visible-light transmittance levels, they transmit an unacceptably high level of near-infrared radiation that can cause a thermal retinal burn. The fact that the sun appears dim, or that you feel no discomfort when looking at the sun through the filter, is no guarantee that your eyes are safe.
Remember: Only if you are in the path of totality, and only during those moments when the sun is totally eclipsed, is it safe to gaze directly at the sun.
On Sept. 22, a spectacular annular solar eclipse will be visible soon after sunrise from parts of the South American nations of Guyana, Suriname, French Guiana and Brazil. Since the moon will be too small to completely cover the sun, the sun will appear as a fiery ring of light.
In 2007, there will be two partial solar eclipses, on March 19 (mostly over northern Asia) and Sept. 11 (over the southern half of South America).
The next total solar eclipse will occur on August 1, 2008, and will be visible from northernmost Canada, Russia, western Mongolia and parts of central China.