If the skies aren't overcast tonight, backyard astronomers and insomniacs across Maryland will be able to watch more than two-thirds of the full moon vanish in the first of this year's two lunar eclipses.
The moon's bright disk will enter the darkest part of the Earth's shadow beginning at 11:27 p.m. EDT. A maximum of 69 percent of the moon will be in darkness at the midpoint in the eclipse, just before 1 a.m. By 2:27 a.m., the moon will have returned to its full brightness.
It's safe to watch a lunar eclipse with the naked eye, but binoculars or a small telescope can make the experience more interesting.
Simply find a comfortable spot with a clear view of the full moon, about halfway up in the southern sky.
The first part of the moon to pass into the Earth's shadow, at 11:27 p.m., will be in the upper left quadrant. The shadow will then appear to spread slowly across the upper two-thirds of the moon's disk, before retreating off to the upper right.
The whole show will last about three hours. If skies are clear, everyone in most of the eastern half of North America, and all of South America, will get a chance to watch it from start to finish.
Fred Davis, chief National Weather Service meteorologist at Baltimore-Washington International Airport, said there was a 60 percent chance of skies clear enough to make the eclipse visible in Maryland.
"We're right on the borderline between a cloud cover and a fairly decent night," he said.
Jim O'Leary, director of the Davis Planetarium at the Maryland Science Center, said tonight's eclipse -- if clouds don't interfere -- may be especially dark because of dust and gases pumped into the Earth's upper atmosphere by the June 1991 eruption of Mount Pinatubo in the Philippines.
Normally, he said, sunlight passing around the Earth through the halo of its atmosphere is "bent into the shadow, so it's not completely black."
Depending on conditions in the upper atmosphere, this scattered light can make the portion of the moon in shadow appear gray, brown, a sort of orange or coppery red.
But eclipses since Mount Pinatubo's eruption have been unusually dark because more of the light passing through the Earth's upper atmosphere is being absorbed by the volcano's debris, Mr. O'Leary said.
Tonight's eclipse is only "the appetizer" for an even more impressive total lunar eclipse around dinnertime Dec. 9.
On that night, the full moon will move into the Earth's shadow beginning shortly after sunset, and will remain in full shadow from 6:06 p.m. until 7:21 p.m.
For Baltimore-area residents, one of the best spots to watch tonight's eclipse may be outside the science center at the Inner Harbor.
Experts from the Davis Planetarium and amateur astronomers from the Baltimore Astronomical Society will be on hand from 11 p.m. until 1:30 a.m. to help visitors understand the eclipse and other objects visible in the night sky.
Several small telescopes will be set up to provide a closer look.
Additional security will be on hand during the late-night observations.
In case of cloudy skies, the event will be canceled.