Princeton, New Jersey, May 15 (Bloomberg) -- Sky watchers across the U.S., Europe and southern Africa are preparing to watch a total lunar eclipse tonight as the moon moves in and out of Earth's shadow.

The phenomenon, which occurs every two to three years, will best be seen in the eastern U.S. at about 11:14 p.m. New York time and at around 9 p.m. Los Angeles time, weather permitting, said Fred Espenak, an astrophysicist for NASA's Goddard Space Flight Center in Maryland. It's the first time the lunar eclipse will be seen across the U.S. since Jan. 20, 2000, he said.

Eclipses happen in stages as the moon moves through different parts of Earth's shadow. The moon will darken and turn reddish- orange in color as the sunlight scatters around the edge of Earth and onto the moon, according to the Griffith Observatory in Los Angeles.

``A pair of binoculars is the only thing I'd recommend for a really good view,'' said Espenak, who said he's packed his car with equipment and cameras and is willing to drive 300 miles to catch the best view. ``Get away from lights, so that the colors will be much more vivid and easier to see.

Unlike with solar eclipses -- in which the moon blocks the view of the sun from Earth -- lunar eclipses can be safely viewed with the naked eye. Astronomers warn against viewing solar eclipses without eye protection because the crescent surrounding the moon during such an event is intensely bright and could result in permanent eye damage.

Northeast U.S. to Get Clear View

The U.S. Northeast, especially north of Boston, should have a clear view of tonight's eclipse, while some late-day clouds and rain may hamper views south of New York, Espenak said.

The eclipse will first be seen in Europe and southern Greenland, then over North America and most of Antarctica, Africa and the western Middle East, according to the U.S. Naval Observatory. It's expected to last about three hours from start to finish, with the best viewing time lasting about 53 minutes, astronomers said.

Sky watchers in South America can best view the eclipse around midnight, while those in Western Europe will catch the best views just before dawn on Friday, astronomers said.

Besides giving a dazzling show, lunar eclipses help astronomers study pollution, Espenak said.

``Lunar eclipses give us some information about the Earth's atmosphere,'' he said. ``Light that reaches the moon is sunlight passed through the Earth's atmosphere and processed. The color of that light or darkness tells us about how much dust and pollution is in the air. A redder, browner or dark gray means there's a lot of dust choking the atmosphere.''

Alternatively, yellow or bright orange colors created from a lunar eclipse signal a clearer atmosphere, he said.

The crew on the International Space Station will also be able to see the eclipse, but only at half-hour intervals as the station orbits Earth about every 100 minutes, Espenak said.

Astronomers are predicting the next lunar eclipse on Nov. 9, when it can best be viewed from the Americas, Europe, Africa and central Asia.

Last Updated: May 15, 2003 08:02 EDT