The longest eclipse of this century was passing through the world's two most populous nations, India and China, on Wednesday.

The total eclipse was crossing straight through India and northern Bangladesh, then along the Yangtze River in China from Chongqing to Shanghai. NASA said it would last a maximum of 6 minutes, 39 seconds.

Here are some facts about a solar eclipse:

- A total solar eclipse occurs when the moon moves between the sun and the earth, blocking out the sun from the areas in the moon's shadow. Without the sun's light, the sky darkens enough for stars to be seen and the sun's corona makes a spectacular halo around the moon.

- The first datable record of a solar eclipse was in 753 BC, in Assyria, but earlier notations, among them Chinese diviners' queries on oracle bones from 1,300-1,100 BC, clearly refer to eclipses.

- From 720-480 BC, astronomers in the state of Lu, now China's Shandong Province, recorded eclipses that can be reliably dated. By the first millennium AD, Chinese imperial astronomers could predict eclipses with an accuracy of within 15 minutes.

- Ancient Chinese eclipse records can be used to calculate the slowing of the earth's rotation, due to the braking action of the moon.

- A solar eclipse in 1919 helped confirm Einstein's theory of general relativity.

- Eclipses are also scientifically interesting because they allow a rare glimpse of the cooler corona, glowing gases near the sun's surface and solar flares, which are normally not visible due to the brightness of the sun.

- The next annular eclipse, in which the moon crosses the center of the sun but does not fully cover it, will be visible in central Africa, India, Bangladesh, Myanmar and China on January 15, 2010.

The next total eclipse, on July 11, 2010, will cross the South Pacific, making landfall only on the Cook Islands and Easter Island.

Sources: Dr F Richard Stephenson, Durham University; Dr Jay Pasachoff, Williams College; National Aeronautics and Space Administration (NASA)

(Reporting by Lucy Hornby; Editing by Sugita Katyal)