The longest total solar eclipse to take place in the next 141 years will dramatically darken the skies July 11 over a narrow, 9,300-mile-long swath of the Earth.
Eagerly anticipated because of its unusually long duration of "totality" -- or time of complete shadow -- the eclipse has already generated a frenzy of advance raves and booked-solid accommodations in prime viewing areas.
Totality is expected to reach up to six minutes, 54 seconds near Mexico's Baja California peninsula.
"It's one of the most awe-inspiring events you'll ever see," said Baltimore astronomer Glenn Schneider, an admitted "eclipsoholic" who has traveled worldwide to view 14 total solar eclipses in the past 20 years.
But because the path of totality is so narrow, Maryland and its Northeast U.S. neighbors will barely notice the event.
The path of totality will cover less than 1 percent of the surface, measuring only 139 miles wide as the eclipse begins in the Pacific Ocean west of Hawaii just after sunrise on July 11.
Some of the best viewing is expected on the island of Hawaii. To the delight of astronomers, the total eclipse will occur -- at 7:30 a.m. local time, lasting four minutes, 10 seconds -- over the world's largest assemblage of observatories, atop the extinct Mauna Kea volcano.
For a total of three hours, 26 minutes, the shadow will travel southeast across the Pacific to the Gulf of California. It will reach Baja and an estimated 100,000 eclipse-watchers around noon, at a width of 160 miles, then continue down over Central America, Colombia and Brazil.
The world's second most populous urban area, Mexico City, lies beneath the eclipse's center line, but its chronic air pollution will deprive residents of a prime view.
An area 2,200 miles on either side of the shadow's path will see a partial solar eclipse. In Baltimore, a maximum 7 percent of the sun's disk will be obscured by the moon at 3:34 p.m. EDT.
"That's not enough to noticeably darken the sky," said Jim O'Leary, director of the Davis Planetarium at the Maryland Science Center. "Even a 90 percent partial eclipse is nowhere near as dark or dramatic as a total eclipse."
Other cities and the percent of the sun covered during the partial eclipse include New York, 2 percent; Pittsburgh, 8 percent; Chicago, 13 percent; Seattle, 20 percent; Denver, 37 percent; Dallas, 51 percent; Los Angeles, 69 percent; and Honolulu, 96 percent.
It's a bit of celestial magic that solar eclipses occur at all. The moon -- 400 times smaller than the sun -- just happens to be 400 times closer to the Earth, so they appear about the same size in the sky.
And if the right combination of positions occurs as the Earth and moon travel in their orbits, the moon will appear large enough to block the entire sun. Observers within the area of total eclipse, or umbra, get a rare view of the sun's outer atmosphere, the corona.
A solar eclipse happens somewhere on Earth two to five times a year, with a maximum of three total eclipses.
Historically, about 28 percent have been total, 35 percent partial and 37 percent "annular," when the moon is aligned for a total eclipse but its apparent size is not quite big enough to cover the whole disk.
Because the shadow path is so narrow, scientists estimate that any particular place on Earth will experience a total solar eclipse only once in 360 years. Since so much of the surface is ocean, many are visible only in inconvenient locations.
"The location of the [July 11] path puts more people in the moon's shadow than any other eclipse in history," said Alan Dyer, associate editor of Astronomy magazine. He predicts that more than 40 million people will see the "eclipse of the decade."
In fact, the coming total solar eclipse is the last for almost 20 years to be visible from anywhere in the United States and the fourth-longest in the 20th century. It will be a mere 15 seconds shy of the longest on June 20, 1955, which lasted seven minutes, 8 seconds.
The theoretical maximum for a total solar eclipse is seven minutes, 40 seconds. The closest any eclipse has come in the past 2,000 years was seven minutes, 24 seconds, on June 27, A.D. 363.
But the relative frequency of lengthy total solar eclipses will drop off during the next century, when not a single eclipse of seven minutes or longer will occur. The next event longer than the July 11, 1991, eclipse is slated for June 13, 2132.
The longer the eclipse, the more time scientists have to study the sun and its corona from the Earth, and the longer the "fix" for buffs such as Dr. Schneider.
"You really sort of feel the whole celestial dynamics in motion," he said. "Watching the approach of the shadow and the changes in sky color, feeling the temperature drop and the wind pick up, seeing the reaction of the animals. People break down and cry."
The stars appear, but not the familiar stars of the nighttime summer sky. They are the winter stars, normally invisible during the warm months because they ride up in the sky during daytime hours.
Connoisseurs look for "Bailey's Beads," which appear just before totality as the last shafts of light from the sun peep through the deepest valleys of the moon and look like a string of beads around the darkened disk.
And they relish the "diamond ring" effect: At the last instant before total eclipse, all but one of the beads disappear, leaving a glowing, diffuse "diamond" and ethereal ring of light around the dark body of the moon.
Eclipse veterans caution against looking directly at the sun to avoid eye damage from invisible ultraviolet and infrared radiation.
"The reason an eclipse is a hazard is that it may prompt people to stare at the sun, something they would not normally do," said Sky & Telescope editor Leif Robinson.
Direct viewing of the sun is advised only through special filters or the darkest welder's glass available. The best technique for amateur observers is to project the sun's image through a pinhole in paper or foil onto a piece of white paper.