Solar eclipses are quite common and there will be between 2 and 5 each year somewhere in the world. What is rare, and quite spectacular, is when you happen to be right in the Moon's shadow during a total solar eclipse, something that you only get to see once in e lifetime perhaps.

A solar eclipse occurs when the Moon comes between the Earth and the Sun. However, if the Moon only covers part of the Sun (even up to 95%) it is likely to pass unnoticed, unless someone has previously drawn your attention to it and you have some darkened filters to look through. The Moon needs to cover the Sun totally for the full and dramatic effects to be seen with the naked eye.

The Moon varies in apparent size because its orbit is elliptical. At times when it is furthest from the Earth the Moon appears 11% smaller than the Sun and cannot block out the Sun totally from anywhere on Earth. However, when the Moon is closest to the Earth it is over 6% larger than the Sun and hence can totally cover the Sun, provided you are situated exactly at the right location on the Earth. This location is precise because the Moon's shadow on the Earth can be no larger than 170 miles across. In Cornwall in 1999 its shadow will be 65 miles across.

You have to be somewhere in this tightly defined 65 mile corridor - even 100 yards outside will ruin the view. Staying in London, Teignmouth, Launceston or Exeter is pointless, get on your bike for heaven's sake - you may not get another chance until 2090. Having said that, it is not important to be in the centre of the shadow, as long as you are within it you will have a similar awe-inspiring sight as those on the centre line, albeit shorter. Even on the centre line you will only get 2 minutes of darkness. Fortunately the duration drops off very slowly away from the centre line, and even when you are only 1 mile from the edge of the shadow, you will still see 30 seconds of totality.

Unfortunately the statistics show that on 11th August there is only a 40% chance of the Sun being visible. The probability does not alter significantly anywhere on the Cornish peninsular. Chances are very slightly better on the Isles of Scilly and Alderney (the only Channel Island where it is visible), but not so much higher as to make it worth the journey. If you want 95% certainty you'll have to travel to the desert in Turkey, Iraq or Iran I'm afraid.

Once you have chosen your location - stay put. Come eclipse morning you won't be able to travel. In all likelihood the roads will be closed or jammed with traffic. The Civil Aviation Authority will almost certainly have banned all light aircraft too. If you see a hole in the clouds 1 mile away, by the time you got there it would have moved, or a hole appeared just where you've left!

One major problem is that total eclipses actually generate clouds. Within the Moon's huge dark shadow it is many degrees cooler than outside, and as the shadow rushes at 1,000 mph across the globe the weather can change before your very eyes - clouds can form in seconds in a cloudless sky.

Unfortunately you do need a cloud free view of the Sun to see the full splendour of the eclipse. If the sky is overcast everything will go as dark as a summer midnight, but you'll not witness your life-giving Sun swallowed into a pitch black hole, surrounded by eerie, wispy, glowing streamers.