By Irene Klotz
CAPE CANAVERAL, Fla. (Reuters) - Two months before the first total solar eclipse to cross the continental United States in a century, NASA on Wednesday is expected to detail its plans to study and promote a celestial show that will darken skies from Oregon to South Carolina.
During the Aug. 21 eclipse, the moon will pass between the sun and Earth, blocking the face of the sun and leaving only its outer atmosphere, or corona, visible in the sky.
It is the first coast-to-coast total eclipse since 1918.
Weather permitting, astronomy enthusiasts can watch as the moon's 70-mile (113-km) wide shadow crosses the country, starting at 10:15 a.m. PDT (1715 GMT) around Lincoln Beach, Oregon, and ending at 2:49 p.m. EDT (1849 GMT) in McClellanville, South Carolina.
The U.S. National Aeronautics and Space Administration will discuss several solar physics and Earth science experiments to be conducted during the eclipse in a news conference on Wednesday afternoon. The agency also plans live broadcasts during the eclipse from dozens of locations along the path.
Total solar eclipses occur somewhere on Earth every year or so, but most cast their shadow over oceans or remote land. The last time a part of the contiguous U.S. saw a total eclipse was in 1979.
All of North America will experience a partial eclipse, though the difference between a full and partial eclipse is "literally the difference between night and day," said astronomer Rick Fienberg of the American Astronomical Society.
He noted that even a 99 percent eclipse will not reveal the sun's corona. And during a total eclipse, the temperature drops and the horizon is ringed by the colors of sunset.
"The sky gets deep twilight blue and bright stars and planets come out," Fienberg said. "Animals and birds behave strangely, like it's the end of the day."
More than 12 million Americans live in the path of the full eclipse, said astronomer Angela Speck, who heads the society's eclipse public outreach campaign.
The entire U.S. population, except for northern Maine, lives within 900 miles of the full eclipse's path.
Travel groups and many scientists will be heading to Oregon's northwest desert, which has the best odds of favorable weather for viewing, according to the website eclipsophile.com.
Experts caution that the only safe time to look at the sun without special eclipse glasses is during totality when the surface of the sun is completely blocked by the moon.
(Editing by Letitia Stein and Phil Berlowitz)