Come on, skywatchers, it's your turn to shine

On the night of 12 November 1833, a cotton planter in South Carolina was awakened by screams of distress: 100 slaves lay prostrate on the ground, crying for mercy. Looking up to the sky, the planter saw shooting stars streaming down by the bucketload. "The scene was truly awful: for never did rain fall much thicker than the meteors fell towards the Earth; east, west, north and south."

Across the United States people leant out of windows or dashed into the streets as the sky was lit up by the brilliant meteors, speeding outwards from a point in the constellation Leo (the lion). Many believed the Day of Judgement was at hand, and this shower helped to provoke a spread of new sects in the country over the next few years.

A professor at Yale University took a more down-to-earth view of the Leonid meteors. Searching through the historical catalogues of meteor showers, Hubert Anson Newton found several "storms" of shooting stars in November. They had occurred in the years 902, 934, 967, 1037, 1202, 1366, 1533 and 1799 - all around the turn of a century or roughly 33 or 66 years into the century. Newton realised the shower was coming round every 33-and-a-quarter years, on average. Sometimes when it was due there was a storm; at other times there was nothing of note.

Newton predicted the next big display of Leonids in 1866. Indeed, that November the sky was once again filled with shooting stars. And shortly before, a comet had been discovered following almost the same orbit. Found by the French astronomer Guillaume Tempel and the American Horace Tuttle, the faint comet Tempel-Tuttle orbits the Sun in a long oval that takes 33 years for one revolution, most of it spent in the realm of the outer planets.

Tempel-Tuttle, like all comets, is littering the solar system, shedding tiny specks of cosmic dust. Over the millennia, a broad band of dust has spread all along the comet's orbit. Every November the Earth crosses the comet's path and some of these particles burn up in the atmosphere as meteors, seeming to fly away from Leo. Near the comet itself, however, freshly shed dust is clumped into narrow but dense filaments near the centre of the broad band.

Every 33 years, when the comet returns our way, the Earth may run into one of these dense filaments, full of comet debris. This is the recipe for a Leonid storm. Victorian astronomers alerted the world to another cosmic extravaganza in November 1899, but nature failed to oblige. In 1933 the Leonids were dismal once more. Astronomers concluded that the comet's orbit had shifted, so that the Earth was missing the dense filaments of fresh dust.

Astronomers are now looking forward with some excitement to 1999. But the Leonids have sometimes been a few years ahead or behind, depending on exactly where the dense filaments of debris lie. So this month marks the start of a campaign to watch the Leonids year by year, to observe how they build up to a peak by the end of the decade - and to prevent them taking us unawares if they come early.

This year the Leonids are predicted to reach their maximum on the night of 17-18 November. The constellation Leo does not rise until after midnight, so you will need to stay up late. Look towards the east where Leo will be rising. More details of the campaign are given below. It is not just for astronomers: the more people who can observe and report the better, especially in a country such as Britain where many astronomers are likely to be clouded out on a November night.

What's up

Saturn is the only planet you will see easily this month, shining with a steady light in the south-west until it sets around midnight. It is prominent because it lies in a region devoid of bright stars, but Saturn is little more than half its normal brightness because we are seeing its rings almost edge-on. In the middle of November something even odder happens: we get to see the side of Saturn's rings that is not lit by sunlight. Through a large telescope, the rings will look like a negative of their true selves. The densest regions (normally the brightest) will look dark as they block most of the Sun's light and the narrow gaps in the rings will appear bright, because a tenuous haze will be backlit by sunlight, like dust on a car windscreen.

Look above Saturn for a large square of faint stars, which the ancient Greeks saw as Pegasus, the flying horse. He is actually upside-down in the sky, with his head at the right pointing upwards. The star at the end is appropriately called Enif, meaning "nose". Where you would expect to find the horse's tail is a line of three stars that marks Andromeda. Below Pegasus are the dim constellations of Capricornus, Aquarius, Pisces and Aries.

Apart from Saturn, the other planets are lurking in the glow of sunrise or sunset. At the beginning of November, Mercury is a morning star, but it quickly moves towards the Sun and is invisible by mid-month. Meanwhile, brilliant Jupiter is slipping down into the evening twilight - and the even brighter planet Venus is drawing away from the Sun to meet it. The two planets pass on 19 November, with fainter Mars nearby.

Diary

7 7.21am full moon

15 11.41am moon at last quarter

18 4am maximum of Leonid shower

19 conjunction of Venus and Jupiter

22 3.43pm new moon

23 Mercury at superior conjunction (behind sun)

29 6.28am moon at first quarter

If you would like to take part in a national survey to record this year's Leonid shower, send a large SAE to Leonid Watch 95, PO Box 7, London W5 2GQ. Please mark the envelope "A" if you are an experienced sky watcher.