Nature's most awe-inspiring display is a scientific 'fluke'

A trick of the light can change the course of history, writes Steve Connor

A total solar eclipse - when the disc of the Moon completely covers that of the Sun - occurs because of a fluke of nature. The apparent size of the two discs in the sky are almost identical, which makes it possible for the Moon to totally blot out the Sun.

The true diameter of the Sun is some 400 times greater than that of the Moon, but because the Sun is about 400 times further away, they both appear to be the same size when viewed from Earth, which creates the "black-out" effect, which is what matters in a total solar eclipse.

Jacqueline Mitton of the Royal Astronomical Society said that total solar eclipses, such as the one that swept yesterday across a large swathe of south-east Asia, from India to Indonesia, are an ''awe-inspiring'' event.

''Gradually, the Moon edges further over the face of the Sun. For just a few minutes, the Sun's yellow disk is completely covered, leaving only the ghostly light of the Sun's corona. Darkness falls as if it were night.''

Such is the drama of a total solar eclipse that, in 585BC, an eclipse ended the five-year war between King Alyattes of the Lydians and King Cyaxares of the Medes. Another in 413BC led to the downfall of thethe Athenians, who became so terrified that they were easily slaughtered by the Syracusians. And in AD840, a total eclipse caused Louis of Bavaria to die of fright.

The apparent sizes of the Sun and Moon can vary slightly because the distance between the Moon and the Earth also changes.

This means that during some solar eclipses the Moon's disc is too small for a total eclipse, causing an ''annular'' eclipse, where the bright outside rim of the Sun remains visible.

In 1919, British astronomers used a total eclipse to demonstrate that light rays from distant stars are bent by the gravity of the Sun, helping to prove Einstein's theory of relatively, according to Yvonne Elsworth, lecturer in physics at Birmingham University.

Dr Elsworth added that recording the times and dates of solar eclipses has also enabled scientists to make precise measurements of the orbital track of the Moon and the planets in the solar system. A total solar eclipse also allows astronomers to study the mountains of the Moon, using the shadows thrown onto the surface.

Another subject of research assisted by a solar eclipse is the Sun's outer corona, or upper atmosphere, which at one million degrees Celsius, is much hotter than the surface.

A total lunar eclipse occurs when the Earth moves between the Sun and the Moon, causing the Earth's shadow to fall on the moon and cover it completely. Christopher Columbus used his knowledge of lunar eclipses to predict such an eclipse on the night of 29 February 1504, which enabled him to trick the natives of Jamaica into giving him supplies for fear that that God would be angry with them, unless they gave him supplies for his ship - which they did.

Meanwhile among the modern day amateur astronomers, a popular destination seems set to be the far less hospitable climes of Outer Mongolia.

The next total solar eclipse will take place there in two years' time, even though temperatures in Ulan Bator can fall as low as -35C (-95F). Interest in astronomical holidays has grown since 1986 when thousands converged on Australia to see Halley's Comet.