Science: Dirty snowball brings a summer storm of shooting stars: Dying meteors should make August a month to remember. Watch for the Glorious Twelfth, say Heather Couper and Nigel Henbest

IF YOU have a clear night on 11-12 August, don't mistrust your eyes if you look up and discover that the heavens seem to be descending all around you. It's not rain that's falling down on you - it's shooting stars.

Every second week of August we get to see more shooting stars (meteors) than usual. This is because the Earth ploughs into a cloud of dusty debris in space, and the tiny dust grains - which weigh only fractions of a gram - plummet down to Earth. Because they are travelling at speeds of up to 60km per second (more than 200,000kph), they burn up by friction high in the atmosphere. The streak of light that we see in the sky represents the demise of a meteor: a column of hot, glowing air tracing the meteor's fall to oblivion.

The August meteors are called the Perseids, because they emanate from the constellation of Perseus (low in the North-east this month). The meteors are not at all associated with the distant stars, and only appear to fan from the direction of Perseus because of perspective.

August is not the only time of the year we see meteor showers, but it is one of the best. Earth encounters dust clouds quite often during its annual tramp around the Sun, made up of debris deposited by comets. These 'dirty snowballs' shed gas and dust in huge quantities when they are close to the Sun and heated by its rays. The meteors we see see in late October, for instance (the Orionids), are debris from Halley's Comet.

The Perseids come from a comet called Comet Swift-Tuttle, so named because it was discovered in 1862 by the American astronomers Lewis Swift and Horace Tuttle. Although astronomers then calculated that the comet should return to the vicinity of the Earth about 120 years later, in 1982 it was a no-show. Brian Marsden, a British orbit-expert, recalculated a new orbit, based on a premise that a comet seen in 1737 was also Swift-Tuttle, and predicted that it should reappear in late 1992. On 27 September 1992, the Japanese amateur astronomer Tsuruhiko Kiuchi rediscovered the comet heading towards us on precisely the predicted track.

Although the dust that causes the Perseid meteors is spread along the whole orbit of Comet Swift-Tuttle, the concentration is greatest near the comet itself. Earth is now in a position to encounter the main dust belt located just behind the comet next month. The International Meteor Organisation (IMO) is predicting a very strong meteor shower as a result.

In fact, there has been increased meteor activity from the Perseid stream over the past two years, tied in with the proximity of the parent comet. Last year, observers in Asia and Eastern Europe briefly saw bursts of hundreds of meteors an hour - a great increase on the usual rate of one meteor a minute.

The IMO cannot predict how intense the activity will be, but says 'there is even the possibility that a real storm of shooting stars may take place'. Meteor storms, while very rare, are the most spectacular sky-sight possible. Meteors shoot in at the rate of several per second, although a bombardment like this has only happened on a handful of occasions since the mid-19th century. One storm - the Leonids - took place in November 1933, and witnesses described the event as like a blizzard.

Although it is impossible to make exact forecasts - you would have to know the precise distribution of dust in the comet's wake to do that - the IMO's calculations suggest that the peak of the meteor activity will occur at 2am on the morning of 12 August. This time is especially favourable for people viewing from Europe. Because of the uncertainty involved, the IMO recommends that you watch any time over the night of 11-12 August. They also add that you might see higher than normal activity over the whole period from 9 to 13 August.

With moonlight causing few problems, and relatively balmy temperatures, there could not be a better time to go meteor-spotting. You need no equipment except your eyes - in fact, binoculars or a telescope will limit your view of the sky, which, although favouring the North-east, should be as wide as possible. Sit or lie on a sun-lounger and watch for a couple of hours. The chances are that you will see the most impressive display of shooting stars over Europe for many years.

Moon and planets

SHORTLY after sunset, you may just glimpse Jupiter and Mars (more difficult to spot) going down in the West. The crescent Moon is just below Jupiter in the twilight of 20 August.

This leaves Saturn as the only planet visible in the night, gaining prominence week by week as it and the Earth are carried towards one another in their orbits about the Sun. On 19 August it is at 'opposition', standing in line with the Earth and the Sun, and as close as it is possible for Saturn to be (about 1.3 billion km) It looks like a bright yellowish star in a rather barren area of the sky. A small telescope will reveal its glorious rings.

Venus still dominates the morning skies, rising more than three hours before the Sun. Mercury, too is a morning star, visible to the lower left of Venus, and can be seen rising 1 1/2 hours before the Sun during the first week of August.

The stars

LOW in the South this month are two of the most spectacular constellations in the night sky, Scorpius and Sagittarius.

Unfortunately for those who live in Britain, however, they never rise high enough to put on a good show. But seen from Mediterranean latitudes, or the southern United States, they dominate the sky. Scorpius, in particular, is a huge constellation that looks just like its namesake - a scorpion with a blood-

red star (Antares) marking its heart and a curving line of stars strung along its stinging tail. The main stars of Sagittarius, which lies to Scorpius's left, look less like an archer and more like a teapot.

Both constellations lie in the direction of the centre of our star-city, the Galaxy. Our line of sight to the galactic centre crosses some of the most fertile territory in the Milky Way, for we are looking straight along its active spiral arms.

If you are stuck in Britain but still have a good southern horizon, a sweep of these two constellations with binoculars will reveal a host of nebulae (star birth sites) and young star clusters. Charles Messier, the 18th- century French astronomer who listed 'fuzzy objects' like these in the sky that could be mistaken for comets, found 15 in Sagittarius - a record for any constellation.

August diary (all times BST)

2 1.10pm full Moon

4 Mercury at greatest western elongation

10 4.20pm Moon at last quarter

11-12 maximum of Perseid meteors

17 8.29pm new Moon

19 Saturn at opposition

24 10.57am Moon at first quarter

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