A total solar eclipse is one of nature's grandest spectacles but it only occurs because of a cosmic coincidence.

From Earth, the apparent size of our Moon is almost exactly the same as the apparent size of the Sun.

This is not the case for any other planet in the Solar System, so only Earth witnesses such close-fitting solar eclipses.

The Sun is, of course, much larger than the Moon. It is 400 times bigger in fact, but it is also 400 times further away. So when they coincide in the sky, the Moon exactly blocks out the Sun.

It is a cosmic coincidence that the Sun appears to be the same size in the sky as the Moon

This throws the Earth into darkness and allows the normally invisible outer atmosphere of the Sun, the corona, to be seen.

If the Sun and the Moon were aligned perfectly, there would be an eclipse every month at the time of the new moon. But the Moon's orbit around the Earth is inclined by five degrees compared to the Earth's orbit around the Sun. So a total solar eclipse happens much less frequently.

Five eclipses a year

In every calendar year, somewhere on Earth there will be at least two eclipses and there may be as many as five. But from any one point on Earth, most people are fortunate to see one in a lifetime.

Partial eclipses are more common, when the Moon partly obscures the view of the Sun.

The reason each total eclipse is only visible over a small part of the globe is because the Moon's shadow is relatively small when it falls on the Earth.

The "path of totality" is the band over the Earth from where watchers can view a particular total eclipse. It is never more than 272 kilometres (169 miles) wide and is usually less.

The time for which the Sun is completely hidden by the Moon ranges from just a few seconds to as much as 7 minutes and 31 seconds, depending on the particular orbital positions.

Another factor determining the type of eclipse is the elliptical shape of the Moon's orbit. This means that when the Moon is further away than average, it does not completely cover the Sun. This leaves a ring, or annulus of the solar disc exposed, hence the name annular eclipse.

Moving away

However, spectacular eclipses will not grace the skies of the Earth forever.

Due to tidal friction, the Moon is slowly drawing away from the Earth at a rate of four centimetres (2.5 inches) a year. So, in time, it will be too far away to just fit over the Sun's disc and all eclipses will be of the annular type.

Our distant descendants, if they are still around in a billion years time, will be the poorer for not having total eclipses to admire.

This page was prepared for the 1999 total solar eclipse on 11 August. The next total eclipse can be seen across southern Africa on 21 June, 2001.