On the night of 17-18 November, the Earth will plough through a stream of dusty debris shed by a comet called Tempel-Tuttle. As each particle enters Earth's atmosphere, it will burn up in a streak of light - a meteor or "shooting star". Because of the effects of perspective, these particular meteors seem to spread out from the direction of the constellation Leo - hence the name "Leonid meteors".
The Earth intercepts this stream of debris every year, but usually we see only a few meteors. This year is different. Comet Tempel-Tuttle passed closest to the Sun last February, in its 33-year orbit, and near the comet the debris is clumped into particularly dense "ribbons" of dust. There's a very good chance the Earth will smash into one of these dense ribbons, and be deluged with dust particles burning up as a meteor storm.
But meteor showers are notoriously unpredictable, and the Leonids have a chequered history. On most of the comet's thrice-per-century returns, Earth has witnessed a meteor storm. But sometimes - as in 1899 and 1933 - the predicted cosmic bombardment has failed to materialise.
The earliest recorded Leonid outburst was in AD902, when Chinese astronomers reported that "the stars fell like rain". In 1799, the German explorer Alexander von Humboldt saw a spectacular shower from Venezuela, and Leonid storms in 1833 and 1866 astounded viewers in the US and Europe. After the disappointing no-shows of 1899 and 1933, Americans in 1966 were treated to a storm of more than a thousand meteors per minute.
Predictions for this year range from another storm like 1966, down to "only" half a dozen shooting stars per minute. What's much more definite is when and where the meteors will best be seen. The Earth rushes through the centre of the debris stream at 7.40pm (GMT) on 17 November. At this time, the constellation of Leo is below the horizon as seen from Britain. The best views will be had from Japan, China and Indonesia.
By the time the Earth turns enough for those in Britain to see any meteors, the best will be over. But we'll certainly be setting early alarms for the next morning, 18 November, to check whether the Earth meets a ribbon of meteors. Even on the most pessimistic assumptions, there should be more shooting stars about than on the average night. Wait until next year, though, and there's a much better chance that the UK may witness a Leonid storm in November.
WHAT'S UP: The brilliant "star" you can see high in the south is, in fact, Jupiter. With binoculars, held steadily, you can catch a glimpse of the planet's four biggest moons, changing position night after night. Unlike the twinkling stars, planets shine with a steady light. You can use this rule to identify the second planet of the night, Saturn, shining to the left of Jupiter. Binoculars reveal that Saturn looks strangely elongated; a small telescope is needed to show that the appearance is caused by its famous rings.
Mars is rising in the east at about 2am. It's still faint, but growing brighter month by month as the Earth approaches it.
4th 5.19am Full Moon
11th 0.29am Moon at first quarter; Mercury at greatest eastern elongation
17th-18th Maximum of Leonid meteor shower
19 4.27am New Moon
27 0.23am Moon at last quarter