The last solar eclipse to cross the continental United States until 2012 will cast its shadow over Maryland Tuesday.
The celestial spectacular -- called an "annular," or "ring," eclipse -- has a handful of scientists mobilizing for a battery of observations, while vision specialists are warning the public against trying to watch it directly.
In Baltimore, the eclipse will be partial. At 1:28 p.m., more than 80 percent of the sun will be obscured by the moon, creating an odd sort of midday twilight if skies are clear.
"It is a subtle change to the character of the light. If you're just driving down the road, you might think it's just getting cloudy," said Jim O'Leary, director of the Maryland Science Center's Davis Planetarium.
To our north and west, however, along a 145-mile-wide band from West Texas to the coast of Maine, observers with good weather will see the full show.
For them, the dark moon will appear to pass directly across the face of the sun, as it would for a total eclipse. But, because the moon will be near the most distant point in its orbit, it will appear slightly smaller than the sun's disk -- like a dark penny atop a shiny nickel.
Instead of completely blocking the sun's direct light, the moon will leave a brilliant ring, or "annulus," of sun light visible for up to six minutes along the shadow path.
About 37 percent of solar eclipses are annular, but they rob the event of much of its interest for astronomers and eclipse chasers. Most prefer a total eclipse that can fully block the sun and reveal its shimmering outer atmosphere, or corona.
Vision specialists fear that some of the millions of Americans in the path of this eclipse might damage their vision by trying to watch directly as it nears its maximum.
It is never safe to look at the sun, which, even partly eclipsed, can cause burns to the retina in less than a second, doctors say. Looking through a telescope or binoculars only multiplies the hazard.
"When you look at the sun for any period of time, the light can sear the center of the retina, with which we do our central viewing. It's our television viewing or reading vision," said Dr. Neil Richard Miller, head of the neural ophthalmology service at Johns Hopkins Hospital's Wilmer Eye Institute.
"There is no treatment," he said. The injury leaves a permanent dark circle at the central focus of vision.
"We saw a lot of this in young recruits who looked at the sun to PTC keep from going to Vietnam," Dr. Miller said. "Unfortunately, the victims did not recover."
An eclipse of the sun occurs when the moon passes between the sun and the Earth, sweeping its shadow across the Earth's surface. They occur two to five times each year, but since two-thirds of the planet is covered by ocean, most have few observers.
Lunar eclipse May 24
An eclipse of the moon occurs when the moon is darkened passing through the Earth's shadow. Weather permitting, Marylanders will be able to see a partial eclipse of the moon on May 24 beginning at 10:37 p.m.
Tuesday's solar eclipse begins at sunrise in the Pacific and ends at sunset in Morocco. It will be visible, at least partially, throughout North and Central America, and parts of Europe and Africa.
The chance of clear skies for this eclipse range from near 70 percent in El Paso, Texas, to less than 30 percent in upstate New York and New England.
Also in the shadow path are Amarillo, Texas; Springfield, Ill.; Fort Wayne, Ind.; Detroit; Cleveland; Erie, Pa.; Rochester, N.Y.; Concord, N.H.; and Bangor, Maine.
In Baltimore, the moon will begin creeping across the sun at 11:41 a.m. At its maximum at 1:28 p.m., the eclipse will leave just 20 percent of the sun shining in a brilliant crescent.
Leafy trees, like pinhole cameras, will project tiny crescents of sunlight over the ground.
The full sun will shine again at 3:15 p.m.
At the University of Maryland College Park, the astronomy department will set up several telescopes on the main campus mall at the head of the reflecting pool. Some glasses with protective coatings will also be available.
Science Center event
Baltimoreans can get a safe view of the eclipse at the Maryland Science Center at the Inner Harbor.
"We will have telescopes outside and members of the Science Center staff out there to tell people what's going on," Mr. O'Leary said.
The center's telescopes will be equipped with filters or will be used to project the sun's light onto screens to prevent eye injury.
"The person in history who was most famous for [retinal burns] was Galileo," Mr. O'Leary said. The 16th century Italian astronomer "was one of the first to notice sunspots, but he lost most of his eyesight later in life."
Tuesday's solar eclipse will be the first visible in Maryland since July 11, 1991. That eclipse was total in parts of Hawaii and Mexico but obscured only 7 percent of the sun's disk as seen from Baltimore.