PARIS: Total solar eclipses have struck awe or fear into hearts for millennia, but scientists are more interested in the unusual mathematics behind the gold-and-indigo lightshow.
A remarkable act of celestial geometry explains it all.
When the moon glides between Earth and the sun, it casts a cone-shaped shadow, called an umbra, that races from West to East.
The sun is 400 times wider than the moon, but it is also 400 times farther away. Because of the symmetry, the umbra, for those on the planetary surface, is exactly wide enough to cover the face of the Sun.
At an eclipse's height, a halo of gold, called a corona, flares around the darkened lunar disc, while the sky turns an eerie dark blue, disorienting birds and causing bats to emerge from their roosts in the belief that night has fallen.
Total solar eclipses are exceptional events, and the one that crosses Asia today is especially so.
If the clouds hold back, it could be the most-watched eclipse in history, for its path of totality traverses the world's two most populous countries, China and India.
People living outside totality, from Japan in the north to parts of Indonesia in the south, will be in the penumbra, or partial shadow, which means a "bite" seems to have been taken out of the Sun.
The lunar shadow will first strike the Gulf of Khambhat, off western India, at 00:53 GMT, taking 8 minutes to cross the center of the country before entering northern Bangladesh and the eastern tip of Nepal.
It then slices through some of China's biggest cities, including Chengdu, Chongqing and Wuhan, before arriving at Shanghai, a city of 20 million souls.
The eclipse may lead to the most people ever in the moon's shadow at once, said NASA eclipse experts Fred Espenak and University of Manitoba meteorologist Jay Anderson.
The umbra then flits across the western Pacific, where at one point the path of totality will be 258 kilometers (161 miles) wide, while the maximum duration of totality will be six minutes, 39 seconds.
By eclipse standards, this is "a monster," Espenak and Anderson estimate in the US magazine Sky & Telescope. Another eclipse of this duration will not occur until 2132. A total solar eclipse usually occurs every 18 months or so. Any given spot on Earth's surface will host a total eclipse on average once every 375 years.
Until now, the most-watched eclipse occurred on August 11, 1999, when the umbra raced from Britain, across Western Europe, part of the Middle East and India.
The last total solar eclipse was on Aug 1 2008, and also crossed China.
The next will be on July 11 2010, but will occur almost entirely over the South Pacific, where Easter Island - home of the legendary moai giant statues - will be one of the few landfalls.
That will be wonderful news to "eclipse junkies," an eclectic army that pursues total eclipses around the world, sometimes hiring seats on planes or ships to get the best view.