Skygazers are setting their alarm clocks and arranging their deckchairs for the annual Perseid meteor shower.

Q: When?

A: It's expected to peak tomorrow night into Thursday morning. But there might be a dress rehearsal tonight when an extra surge of meteors muscles in on the act.

Q: Caused by?

A: A filament of dust newly drifting across Earth's orbit. It is expected to produce a show of mostly-faint meteors over Europe and Asia, possibly involving 200 meteors an hour.

Q: Where do they come from?

A: Comet Swift-Tuttle. It was discovered by

the Americans Lewis Swift and Horace

Tuttle in 1862. Three years later, Giovanni Schiaparelli realised it was the source of the Perseid meteors, so called because the meteors stream out of the constellation Perseus.

Q: What causes meteor showers?

A: They occur when earth passes through a trail of debris. The particles light up

briefly as they enter

the atmosphere and

burn up. They can be

as tiny as dust particles but they move at

thousands of miles

an hour. This is likely to be a good year for Perseids.

Q: Why?

A: Two reasons. First, the moon is new in mid-August, so moonlight won't spoil the show. Second, the extra surge will give skywatchers something more to look out for. It's relatively young and boiled off Swift-Tuttle in 1862. Other dust in the cloud is perhaps thousands of years old. It's more dispersed and responsible for the month-long shower that peaks tomorrow.

Q: Where can it be seen?

A: The best way to

watch it is to find an open view of the

north-east sky. Because of the way Swift-Tuttle's orbit is tilted, its dust falls on Earth's northern hemisphere.