for National Geographic News
A total solar eclipse passing over some of Earth's most densely populated regions on Wednesday, July 22, 2009, may become the most viewed eclipse ever.
People across central India and in parts of Nepal, Bangladesh, Bhutan, and Myanmar will briefly find themselves in daytime darkness before the solar eclipse proceeds into China.
Most of the best viewing opportunities are in China, where some 30 million people will be able to witness the solar eclipse in the coastal cities of Shanghai and Hangzhou alone, according to veteran eclipse scientist Jay Pasachoff of Williams College in Massachusetts.
(Related pictures: "Solar Eclipse 'Ring' Seen Over Indonesia.")
The eclipse will then continue east, passing over Japan's Ryukyu Islands before reaching its maximum duration point over the Pacific Ocean, where the sun will be completely blocked by the moon for 6 minutes and 39 seconds, according to NASA scientist Fred Espenak.
Thousands of overseas tourists and potentially millions of Chinese are flocking to areas along the eclipse path, where hotels are charging higher rates, according to Chinese media reports.
The July 2009 total solar eclipse is expected to have the longest duration of totality in the 21st century, experts say, and should give Pasachoff plenty of data to keep him and his team busy for months.
Pasachoff will see only about five and half minutes of totality from a site in eastern China, but "once you have five minutes-plus of totality, the extra minute that we could have [seen] is not significant," he added.
Pasachoff and his team will observe the solar eclipse from a remote hotel at an altitude of about 3,000 feet (900 meters) on Tianhuangping, a mountain outside the Chinese city of Hangzhou. The location sits above pollution that could obstruct a full view of the eclipse.
He chose the site years in advance so he could witness the longest totality from the Asian mainland. Teams of astronomers from around the world have already joined him at Tianhuangping.
Pasachoff, a National Geographic Society Committee for Research and Exploration grantee, will witness his 49th solar eclipse and 29th total eclipse since he began chasing the sun on October 2, 1959. (The National Geographic Society owns National Geographic News.)
"We brought about half a ton of equipment and picked up an equal amount borrowed here from our Chinese colleagues, so there is a lot to get ready," he added.
Pasachoff wants to understand why the sun's corona—gas that extends millions of miles out from the sun—is millions of degrees hotter than the sun. The sun is just about 6,000 degrees Fahrenheit (3,300 degrees Celsius).
"Somehow energy has been put up into the corona from lower down, heating the gas, and we'd like to see how that happens," he said.
Scientists believe the coronal phenomenon has to do with the sun's magnetic field, and Pasachoff is looking to identify vibrating magnetic waves that move from the sun out into the corona.
Scientists can't usually see the corona from Earth because its light is fainter than the blue sky created by our atmosphere.
Furthermore, instruments attached to space satellites can't isolate all areas of the corona because the sun and the light it scatters are too bright.
The only time certain observations are possible is when the moon blocks out the sun, creating a darker sky, which highlights the coronal light around the sun.
Although the sun is about 400 times bigger than the moon, it's also about 400 times more distant. So from the ground, the moon appears to be just a little bigger than the sun.
The sun's disappearing act attracts so-called eclipse tourists, who travel the world to watch solar eclipses which happen between two and five times a year, though total solar eclipses are less frequent.
(See solar eclipse pictures.)
Rollie Anderson, a retired actuary from St. Louis, Missouri, is in China now to watch his 14th eclipse.
"The cosmic coincidence that the sun and moon both appear in the sky as the same size, and then, on top of that, they line up every now and again. Just the very idea of that is pretty mind-blowing," he said.
"As you get to the last several minutes before totality, that's when your eyes actually start noticing things getting dark around you, and you can feel the air cooling," he said. "It gets really dark and totality appears, and that's when it gets most spectacular."
"You see a black hole in the sky where the sun used be, and if there are birds around, they may stop chirping, because they think it's night."
Chasing eclipses has also allowed Anderson and his wife to see the world.
"It's kind of an excuse to see whatever the part of the world the eclipse happens to be in."
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