Eclipses of the Sun occur when the Moon passes in front of the Sun. It is an amazing cosmic coincidence that the two discs of the Moon and Sun appear the same size in the sky - but while the Sun is 400 times bigger than the Moon, it's also 400 times further away. If you are in exactly the right spot, you see the Moon overlap the Sun precisely. That's a total solar eclipse.
As the Moon speeds round its orbit, the eclipse - effectively, the shadow of the Moon - tracks across the surface at a speed faster than Concorde. Most people will have seen a partial eclipse, when a "slice" appears to have been taken out of the Sun. But nothing ever prepares you for the spectacle of totality. The familiar, glowing Sun is replaced by what looks like a Chinese lion-mask. The sky gets darker, a wind springs up, the temperature can drop by several degrees, and birds sing their twilight songs.
A lot of people are staying put and not planning to go south to see totality. But a partial eclipse - even one where the Sun is 99 per cent covered up - is as nothing compared to the real thing. Seeing a total eclipse is a truly primal experience. It's not surprising how much they terrified our ancestors.
Even today, there's a great deal of mythology about eclipses - that you should view them reflected in a bucket of water, or through smoked glass. Or that eclipses are dangerous. They're not - but take the right precautions. "Eclipse shades", made of mylar film (which blocks dangerous radiation) will be in plentiful supply as the big day approaches. ALWAYS view the Sun through these during the partial phases (and for the entire eclipse if you're in a place where it's not total). Make sure that children wear these shades, because they are especially sensitive to infrared radiation (heat) from the Sun. (Scores of people suffered permanent eye damage in the last total eclipse because they took insufficient care.)
Then, when totality strikes, whip them off! You'll know it's totality, because everyone around you will be whooping and yelling and clapping. This particular eclipse lasts just over two minutes on the "centre line", which passes close to Penzance and Falmouth - but from all other places, it will be shorter. Around the eclipsed Sun, you'll see the pearly halo of its glowing outer atmosphere, the corona. Closer in - near the Moon's edge - you'll notice tiny crimson "flames" sticking out. These are giant loops of magnetised gas. In the dark sky around, you will see the brilliant planet Venus, with fainter Mercury the other side of the Sun.
If it's your first total eclipse, don't bother taking photographs - just enjoy. It only seems to last seconds. The first you'll know about the finish is that the sky at the horizon starts to brighten, and you'll see the trailing edge of the Moon's shadow sweeping up from the west. Then - shades on again, now - a brilliant chink of sunlight will beam out through a gap in the mountains around the Moon's edge. This is the famous "diamond ring effect", and that's exactly what it looks like. And then it's all over. Full daylight returns in seconds, but you know you've had a once- in-a-lifetime experience (at every eclipse, there's always been someone who's said "it's better than sex").
Alas, there's a 50 per cent chance that in Britain and Europe it will be cloudy - in which case, we'll see nothing except darkness. These days, with dedicated solar satellites in orbit, eclipses of the Sun are not that scientifically important. But a total eclipse is something that everyone should try to see in their lifetime. The last one visible from Britain was in 1927; the next isn't for another 91 years. So catch it if you can!
WHAT'S UP After all the excitement of the eclipse, stay up late the following night (12-13 August) for an impressive display of shooting stars. That's the night the Perseid meteors reach their maximum intensity, though you'll see some shooting stars from this shower throughout the first three weeks of August. This is an excellent year for spotting Perseids, because the Moon won't be lighting up the sky. Look towards the north-east after midnight for the best view. On the planetary front, Mars is setting in the south-west just before midnight, as brilliant Jupiter rises in the east, closely followed by Saturn. In the morning sky, you may catch a glimpse of Mercury low in the morning twilight glow.
Heather Couper and Nigel Henbest