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Aug 1 (Reuters) - The moon blocked the sun across a swathe of Russia, Mongolia and northwestern China just before sunset on Aug. 1, launching a momentous month for China as it hosts the Olympic Games in Beijing.

- 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 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 millenium 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 surface of the sun is relatively quiet at the moment, with fewer sunspots than expected.

- The next solar eclipse will occur on July 22, 2009, and could be viewed by hundreds of millions of people as it crosses straight through India and northern Bangladesh, then runs along the Yangtze River from Chongqing to Shanghai.

Sources: Dr F Richard Stephenson, Durham University; Dr Jay Pasachoff, Williams College (Reporting by Lucy Hornby; Editing by Catherine Evans)

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