Solar eclipses happen because of a weird coincidence: the Sun and Moon appear almost exactly the same size in the sky. In fact, the Sun is 400 times larger than the Moon - but it is also 400 times further away. Twice a year, the Moon crosses the disc of the Sun. If you are in precisely the right spot on Earth (to within a few kilometres), you will see the Moon overlap the Sun. That is when you get a total eclipse.
It is a cliche, but nothing prepares you for the experience of totality. We have witnessed two total eclipses: from a small island in Indonesia, and from the top of the Big Island of Hawaii. The first thing you notice as the Moon moves inexorably across the Sun's disc is that the quality of light changes. It takes on a flat, artificial appearance, a bit like a film set. Minutes before the eclipse, it starts to get colder, and seconds before, you see the shadow of the Moon racing across the landscape in your direction. Then the whole world changes. Replacing the bright, dependable Sun in the sky is a pitchblack intruder - like a Chinese death- mask. All around, frozen crimson flames, great gas-arches called prominences, lick at its edges. And surrounding the whole is the Sun's outer atmosphere, the exquisite pearly corona, which fans away to invisibility against the darkened sky.
The scene lasts just minutes. As the Moon moves away, sunlight bursts through, creating the glorious "diamond ring effect". Night turns into day - and you wonder if you imagined it all.
Although the 12 October eclipse does not come into this category - it will not be total anywhere on Earth - the Moon will still bite a respectable chunk out of the Sun. In London, 61 per cent of the Sun will be obscured; in Edinburgh, 64 per cent.
Because the eclipse is not total, you must be careful how you watch it. With nearly 40 per cent of sunlight still getting through, the Sun will still be dangerous to look at directly. You can observe the eclipse safely indoors by allowing a chink of sunlight through a hole in a blind, which forms a miniature "pinhole camera" image of the Sun. It's also safe to view through a special eclipse filter - but DO NOT use exposed photographic film (a favourite method in past years) because modern colour emulsions allow some of the Sun's harmful radiation through.
The last eclipse to rival this one was in 1961, when Londoners saw 91 per cent of the Sun covered up (86 per cent in Edinburgh). The most recent total eclipse in the UK was on 29 June 1927, lasting a mere 24 seconds over North Wales and the north of England. The weather was appalling that day, and the eclipse was a wash-out. Only a few people got to see it.
Those keen to see the next total eclipse visible from these shores have but a few years to wait. In the morning of 11 August 1999, a total eclipse lasting between one and two minutes (depending where you are) will sweep over the Scilly Isles, south Devon, Cornwall, and the island of Alderney. After that, it takes off for Europe; but the best place to be is between Falmouth and Penzance, where the eclipse will last for 122 seconds.
Hotels in the area are already reporting heavy bookings. So, if you want to see the celestial spectacle of a lifetime, get in there quickly - or you will have to wait until 2081 for the next British total eclipse.
Those interested in British eclipses will be fascinated by UK Solar Eclipses from Year 1, by Sheridan Williams (pounds 11.95 plus pounds 1 p&p, from Clock Tower Press, PO Box 5010, Leighton Buzzard, Beds, LU7 0ZZ).
Brilliant Jupiter now hangs in the south-west during the early evening, setting by 10pm. As its glory days fade for this year, all the attention is focusing on Saturn. It is visible all night long, and lies high in the south at midnight. A telescope will reveal the famous rings, and at least one moon. Above Saturn, a large square of stars marks the body of Pegasus, the winged horse. Rather confusingly, the left-hand star of the square is officially part of the neighbouring constellation, Andromeda.
October diary (all times BST)
3 Mercury at greatest western elongation
4 1.05pm Moon at last quarter
12 3.15pm New Moon
Partial eclipse of the Sun (exact times depend on position within the UK: Edinburgh 1.49 to 4.21pm, London 1.59 to 4.31pm)
19 7.10pm Moon at first quarter
21 Maximum of Orionid meteor shower
26 3.12pm Full Moon
27 2am British Summer Time endsReuse content