A full solar eclipse will sweep across the Arctic and Siberia before ending in western China, where it will kick off the month in which Beijing hosts the Olympic Games.
The eclipse was due to begin in Canada, track across Greenland and eastern Russia and end around sunset on Friday (local time) to the east of Xi'an, China's ancient imperial capital.
Eclipses were dangerous omens for ancient Chinese astronomers, but this one comes exactly a week before the torch is lit in Beijing for the opening ceremony of Games designed to restore China's pride and showcase its achievements.
Planeloads of cheerful foreign eclipse chasers converged on Jiayuguan, in Gansu Province, and in the hot deserts of Xinjiang, to watch the sky go dark and a halo wreathe the hidden sun.
"I've come all the way from California for this. It's going to be my 11th eclipse, I try to see them all," Dave Balch, a cancer care advisor wearing an eclipse T-shirt, said.
Scientists studying the sun's surface prepared for a brief glimpse of the faint outer corona that is normally obscured by the sun's brightness.
"Nowadays, the equipment works well enough that we do have time to look up at the eclipse," Jay Pasachoff, a professor at Williams College who travelled to Novosibirsk, Russia for his 47th eclipse, said.
"It's very dramatic and awe-inspiring when the darkness suddenly comes. That's why thousands of tourists go to see."
Hundreds of millions of people will not have to go any further than their front doors on July 22, 2009, when the next solar eclipse will cross India and northern Bangladesh, then run along the Yangtze River from Chongqing to Shanghai in the most populated path ever.