A swath starting in India and crossing Shanghai to southern Japan will be plunged into darkness for about five minutes on Wednesday in the longest total solar eclipse that will happen this century.
Japan, which hasn't seen a total eclipse for 46 years, is celebrating with fireworks. An astrologer in Myanmar has warned the eclipse is a sign of impending chaos.
In India, some pregnant women have been told to stay indoors to follow a centuries-old tradition of avoiding the sun's invisible rays.
The eclipse will appear first at dawn in India's Gulf of Khambhat just north of the metropolis of Mumbai. It will move east across India, Nepal, Myanmar, Bangladesh, Bhutan and China before hitting the Pacific.
The eclipse will cross some southern Japanese islands and be last visible from land at Nikumaroro Island in the South Pacific nation of Kiribati. Elsewhere, a partial eclipse will be visible in much of Asia.
For astronomers, it will be a chance for a prolonged view of the sun's corona, a white ring 1 million kms from the sun's surface.
The previous total eclipse, in August 2008, was two minutes and 27 seconds. This one will last 6 minutes and 39 seconds at its maximum point.
Scientists are hoping data from the eclipse will help explain solar flares and other structures of the sun and why they erupt, said Alphonse C Sterling, a NASA astrophysicist who will be following the eclipse in China.
Man has been recording solar eclipses for 4,000 years, and even today they inspire a combination of fear, fascination and wonder.
One astrologer in Myanmar, also known as Burma, predicted in a magazine that the eclipse would trigger wars, instability and natural disasters for the next several months.
In India, hundreds of scientists have started arriving in the village of Taregna in Bihar state, where they hope to avoid the monsoon clouds hanging over much of the country.
Scientists plan to study atmospheric ionisation, geomagnetism, asteroids, animal and avian behavior and the impact on microorganisms.
A team led by Dr R K Sinha of Patna University will study birds. "The researchers will observe whether they suddenly move back to their nests, sound differently and behave in an unusual manner due to sudden darkness," he said.
Some families have advised pregnant relatives to confine themselves to curtained rooms, following long-held fears that the invisible rays would harm the fetus and the baby born with disfigurations, birthmarks or a congenital defect.
"I've been told to lie straight on the bed with my eyes open and to chant prayers and verses from the Hindu holy texts during the eclipse," said Sonya Chadha, a New Delhi accountant who is seven months pregnant and plans to take the day off.
"If even a tiny sliver of light falls on me, it could harm my child."
A travel agency in India is running a charter flight to watch the eclipse by air.
