Total solar eclipses happen only because of a peculiar cosmic coincidence. For a total eclipse to occur the moon has to fit neatly over the disc of the sun, completely blocking it out.
By sheer chance this is possible because, although the sun is 400 times wider than the moon, it is also 400 times farther away from the Earth. From our viewpoint, therefore, the moon fits over the sun with uncanny precision.
Scientists predict that thousands of years from now there will no longer be any solar eclipses. The moon is very gradually increasing its distance from the Earth, so that eventually it will cease to cover the sun.
Every month the moon passes between the Earth and the sun, so why are total eclipses not much more common? The reason is that the moon's orbit is tilted, so that the shadow it casts usually misses the Earth altogether.
Sometimes the moon obscures the sun only partially, casting a "penumbra", the shady region just outside the area defined by a totally dark shadow. This is a "partial" eclipse.
Only when the Earth, moon and sun are correctly lined up is the sun completely obscured and the moon's full shadow, or "umbra" travels across the globe.
When today's eclipse occurs the umbra will race across the Earth from west to east at 2,000 m.p.h. Its path will follow a narrow corridor of "totality" just a few hundred miles wide.
The full glory of the eclipse can be seen only from points within this track, which cuts through Cornwall, south Devon, Alderney in the Channel Islands, northern France, Romania, Turkey and Iran until it reaches the Bay of Bengal.
The penumbra stretching out on either side of the line of totality covers a much wider area nearly 3,000 miles across, from where a partial eclipse will be visible.