If skies are clear Thursday night, Marylanders will get their first good look at a total eclipse of the moon in nearly four years, and their last until May 2003.
By 11: 05 p.m., January's full moon -- known traditionally as the Old Moon -- is expected to take on an eerie, copper color as it slips into the darkest region of Earth's shadow.
The sky show will be visible anywhere in the Americas and Western Europe where clouds are absent, and no special equipment is needed. Observers watching under darker, rural skies will enjoy a bonus when the winter stars brighten as the moon's white glare is dimmed by the eclipse.
"It makes for great family memories. Everybody should get their kids up to see this," said Fred Espenak, an astronomer and eclipse expert at NASA's Goddard Space Flight Center in Greenbelt.
The Ramsey-Crosby Observatory, at the Maryland Science Center on Baltimore's Inner Harbor, will extend its regular "Stargazing Thursday" hours until 1: 30 a.m. to help visitors enjoy and understand the eclipse. (Call 410-545-2999 to check visibility.)
The last total eclipse of the moon visible anywhere in the world was in September 1996, but the event was hidden by clouds in Maryland. The last one anyone here could see was in April 1996, and that moon was already fully eclipsed when it rose at sunset.
The last time Marylanders had a complete look at a total lunar eclipse was in November 1993.
This time, the moon's left-hand edge will begin to enter the dark center of the Earth's shadow -- the umbra -- at 10: 01 p.m. EST. Over the next hour, the shadow will appear to creep slowly across the moon's bright disk. And at first, Espenak said, the shadowed portion of the moon won't display much color.
"However, at 11: 05 p.m. when the moon is totally immersed in the Earth's shadow, it will be very obvious with the naked eye that the moon is dramatically different," he said. "It will be a vivid red or orange, and it will be much dimmer than an hour before."
During totality, the Earth blocks all direct sunlight from reaching the moon. That red-orange color is created by indirect sunlight, filtered and bent by the fringes of the Earth's upper atmosphere, Espenak said.
If the Earth had no atmosphere at all, the moon would simply go dark and disappear while in the shadow. It's the same phenomenon that creates our red-orange sunrises and sunsets. In fact, if you were standing on the moon during the eclipse, you would see the dark, night side of the Earth, surrounded by a vivid red ring of refracted sunlight.
"You're seeing everyone's sunrise and sunset," said Yale University astrophysicist Bradley E. Schaefer.
Schaefer said the color of an eclipsed moon has long been used as an indirect measure of the clarity of the Earth's atmosphere.
Large volcanic eruptions, such as the 1991 eruption of Mount Pinatubo in the Philippines, can blow enough ash and sulfur compounds into the stratosphere to block nearly all the light from reaching the moon. The lunar eclipse in December 1992 was barely visible during totality, Schaefer said.
This time, scientists say the relatively slow pace of volcanic eruptions around the world has left the atmosphere relatively clear, promising a vivid, colorful eclipse.
Schaefer said anyone can go outside during a lunar eclipse and figure out both the shape of the Earth, and its size relative to the moon.
As the Earth's shadow creeps across the moon between 10 and 11 p.m., he said, "go out and look at what the shape of the Earth's shadow is." The shadow's edge is always gently curved, an observation that revealed to the ancients that the Earth was round.
And by extending that curve in your imagination to a full circle, you can even get a rough idea of how much larger the Earth is than the moon.
"It's a factor of 3 to 1, roughly," Schaefer said. "That's how the ancients knew what the size of the moon was."
A small telescope or binoculars can enhance the experience, but they're not needed, Espenak said. "The naked eye is fine for seeing the eclipse itself." And unlike a solar eclipse, no special safety precautions are needed, except for remembering to dress warmly.
It's not even important to watch from start to finish.
"I would suggest they go out maybe a couple times during the partial phase," between 10 p.m. and 11 p.m., he said. "Then go inside and drink some hot cocoa."
"Then come back out and watch at least the beginning of the total phase," he said. That's when the moon takes on its odd coloring. Totality begins at 11: 05 p.m. and lasts until 12: 22 a.m.
After that, the moon begins to re-emerge into direct sunlight. At 1: 25 a.m. Friday, it will be fully illuminated again, and everyone can go back to bed.
Information: http: //sunearth. gsfc.nasa.gov/eclipse/