PARIS, July 28, 2008 (AFP) - Facts about total solar eclipses:

- Total solar eclipses occur when the Moon comes between the Earth and the Sun, completely obscuring the solar disk for a few minutes along a narrow path. The eclipse follows a West-to-East track that can last several hours until this perfect alignment ends.

- The width of the path of the eclipse at totality is at most 269 kilometers (167 miles). The record duration of a totality is seven and a half minutes.

- The totality path is caused by the dark, cone-shaped umbra (the Latin word for 'shadow') of the Moon. On either side of the umbra is the penumbra ('almost shadow'), which is around 8,000 kilometres (5,000 miles) wide. In the penumbra, people see a partial eclipse.

- Total eclipses happen about once every 18 months, but there are also periods of relative infrequency. Partial eclipses are much more frequent, and may occur several times a year.

- Most eclipses occur at sea or in sparsely populated areas. The great total eclipse of August 11, 1999 was exceptional because it traversed such a populous area, from western Europe to India. The July 22, 2009 eclipse could be the most-viewed in human history, visible in a swathe from central India to central China.

- The shadow of an eclipse travels at 1,770 kilometers (1,100 miles) per hour at the Equator, and up to 8,046 kilometers (5,000 miles) per hour at the poles. Any given spot on Earth's surface will host a total solar eclipse on average once every 375 years.

- Before the advent of modern atomic clocks, studies of ancient records of solar eclipses allowed astronomers to detect a 0.001 second-per-century slowing down in Earth's rotation.

- Eventually, total solar eclipses will come to an end. The Moon is slowly moving away from Earth, at the rate of 3.8 centimetres (1.5 inches) per year. Ultimately, the Moon will be too far away to completely cover the Sun. But it will take another 600 million years for this to happen.

- Safety first: Eclipses, even partial ones, should NOT be viewed with the naked eye or through binoculars, a telescope, beer bottle or photographic film as this can permanently damage the retina. Observers should use proper optical filters such as welding-goggle glass, eclipse spectacles or a solar projection kit for their telescope. The safest way to view is on television or the Internet.

- FURTHER INFORMATION:

. Background: (http://sunearthday.gsfc.nasa.gov/2008/index.php); http://www.space.com/scienceastronomy/080715-st-solar-eclipse.html

. Eclipse photos:(http://www.mreclipse.com)

. Webcast: (http://www.exploratorium.edu/eclipse/2008/)