A solar eclipse occurs somewhere in the world every time the Moon is lined up perfectly between the Earth and the Sun. On these rare occasions the Sun appears to be 'eclipsed', blocked from our view for a brief moment by the Moon. A darkness then descends on that part of the world that is in the Moon's shadow.
Those parts which can see no part of the Sun at all, are said to be in total eclipse. Those parts of the Earth which can see only a part of the Sun are said to be in partial eclipse.
Solar eclipses occur when the Moon comes between the Earth and the Sun. Since we have a
New Moon every 29 1/2 days, you might think we would get a solar eclipse about once a month.
Unfortunately we don't because the Moon's orbit around Earth is tilted about 5 degrees relative
to the plane of Earth's orbit about the Sun (as in the diagram above). As a result, the Moon's shadow usually
misses the Earth as it passes above or below our planet at the time of a New Moon.
Every so often, however, the New Moon coincides with the time when
the Moon is passing through the ecliptic (the plane of the Earth's orbit around the Sun). When this happens, there is a total solar eclipse. Roughly speaking, therefore, there is a solar eclipse somewhere
on Earth about twice a year.
Eclipses of the Sun and Moon occur in series of similar eclipses known as Saros series. Successive eclipses within any particular series are 6585.32 days apart (just over 18 years), a fact that has been known since ancient times. This period is called the 'Saros cycle', a term first used by Edmund Halley which he took from the Babylonian language. Each series on average comprises 73 eclipses and therefore lasts over 1300 years. There are 42 Saros series running at any one time, therefore solar eclipses of one sort or another occur 18/42 years apart, ie just over 2 per year, though only about 30% are total eclipses. Each total eclipse on average is experienced by only 0.5% of the world's surface, so every location experiencing totality this 22nd July 2009 will have to wait, on average, over 250 years before the next occasion.
Many total solar eclipses take place in isolated places - e.g. oceans or polar regions - where few people get to experience them. However, what will make the solar eclipse on July 22nd 2009 so special is that much of its path of totality lies across inhabited regions of South and East Asia, with partial eclipse coverage throughout Eastern Asia.