About 25 percent of eclipses are total eclipses, and there are about seven of these a decade, Espenak said. But at any given geographic location, a total eclipse will be visible an average of once in 375 years.
The last total solar eclipse visible from the United States was in 1979, and it was seen mostly in the Pacific Northwest.
When a total solar eclipse takes place, about half the daytime world doesn't see any of it, Espenak said. Another 49 percent of people see it as a partial eclipse.
Less than one percent of people see totality, which Espenak describes as "drop dead gorgeous."
"On a scale of one to ten, a partial eclipse is of some interest," he said. "A total eclipse on that scale is ten million. It can't be compared to anything else. It should be on everybody's life list."
Of course, the weather can throw a monkey wrench in any observation plans.
This year, conditions in China are likely to be most favorable for getting a good look at the full eclipse, according to weather data analyzed by Espenak and Jay Anderson from the University of Manitoba in Canada.
Their calculations show that the skies above China in August are cloudy around 35 percent of the time, compared with upward of 90 percent of the time in many other parts of the eclipse's path.
"It's always a crapshoot," Espenak said. "You try to stack the odds in your favor."
Pop Tarts and Bad Omens
Tom Burns, director of the Perkins Observatory in Delaware, Ohio, helped put together a viewing-safety Web site during the Christmas eclipse of 2000.
He was hearing too often, he recalled, of people trying to view the partial eclipse through sunglasses, compact discs, or—surprisingly—Pop Tart bags.
"The only time it's safe to observe an eclipse through a Pop Tart bag is if the Pop Tart is still in it," he said.
He and his colleagues, who frequently instruct groups of sky-gazers on safe sun-watching, instead recommend special eclipse glasses.
Eclipses haven't always been eagerly anticipated, NASA's Espenak added. In ancient times the events were often seen as bad omens.
A total solar eclipse that may be tied to a real event was described in Homer's epic poem The Odyssey as spreading "an evil mist" over the world while suitors courted Odysseus's wife.
And an eclipse viewed in England in A.D. 1133 was seen as portending the death of King Henry I.
Even today, in some places, fears persist.
"A lot of women believe viewing an eclipse will cause birth defects if they're pregnant," Espenak said. "I've seen this in Mexico, India, Indonesia, Bolivia."
For the growing population of sky-gazers who would love to see an eclipse and can't make it northward this year, it might be best to make reservations to visit southern Illinois, he added.
Weather permitting, people there will get to see total solar eclipses in 2017 and 2024.
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