AAP

THE world's most populous nations are gazing skywards as the longest total solar eclipse of the 21st century lays a carpet of darknessacross India and China, from Mumbai to Shanghai.

A partial solar eclipse has started in India, seen first in the eastern city of Gauhati.

But most other parts of the country remain under thick cloud cover on Wednesday to the disappointment of millions who have gathered outside to watch the longest solar eclipse of the 21st century.

Live television pictures showed a section of the sun being covered by the moon in Gauhati in eastern India at about 5.30am (1000 AEST) on Tuesday. But the sunrise was blotted in most other parts of the country.

The eclipse - visible only in Asia - will reach its peak in India at about 6.20am (1050 AEST).

It will move north and east to Nepal, Burma, Bangladesh, Bhutan and China. The total eclipse will last six minutes and 39 seconds.

The event is being hyped in the obscure world of eclipse-chasers as a once-in-a-lifetime opportunity which, due to its trajectory over some of the most densely inhabited areas on Earth, could end up being the most watched eclipse in history.

"This is a very important milestone. None of us will live long enough to see another one like it," said Federico Borgmeyer, the German-based manager of the specialist travel agency Eclipse City.

The American astrophysicist and acclaimed eclipse expert Fred Espenak has simply labelled the July 22 event "a monster".

Total solar eclipses occur when the moon comes between the Earth and the sun, completely obscuring the sun.

The excitement this time around is largely due to the unusually long duration of the instant of greatest eclipse, or "totality" - when the sun is wholly covered.

At its maximum, this will last six minutes and 39 seconds - a duration that will not be matched until the year 2132.

Is this the largest sunspot ever?

Find out more on the comprehensive NASA eclipse site.

The following links will provide live web coverage of the 2009 eclipse.

• Live Webcast (CHINA) - U. of North Dakota.
• LIVE! UNIVERSE, Japan Webcast (JAPAN)
• Taiwan Webcast Group Webcast (Mainland CHINA)