Page last updated at 14:40 GMT, Tuesday, 11 August 2009 15:40 UK

Skywatchers set for meteor shower

Perseid (SPL)
The Perseids occur when the Earth passes through dusty cometary debris

Skygazers are preparing for the high point of the annual Perseid meteor shower.

The shower, which reaches its peak on Wednesday, occurs when the Earth passes through a stream of dusty debris from the comet Swift-Tuttle.

As this cometary "grit" strikes our atmosphere, it burns up, often creating streaks of light across the sky.

The meteors appear to come from a point called a "radiant" in the constellation of Perseus - hence the name Perseid.

"Earth passes through the densest part of the debris stream sometime on 12 August. Then, you could see dozens of meteors per hour," said Bill Cooke of Nasa's meteoroid environment office.

No special equipment is required to watch the sky show. Astronomers say binoculars might help, but will also restrict the view to a small part of the sky.

The Perseids can appear in any part of the sky, but their tails all point back to the radiant in the constellation Perseus.

Stargazers are advised to lie on a blanket or a reclining chair to get the best view.

Infographic (BBC)
The tails of the Perseids point back to a "radiant" in the constellation Perseus
They can appear anywhere in the sky
Composed of dusty debris from Comet Swift-Tuttle

Budding astronomers are being urged to take part in the first "Twitter Meteorwatch". Astronomers from around the world will be live-tweeting images of the meteors, as well as pictures of the Moon, Jupiter and other celestial objects.

The "48-hour Twitter marathon" will form part of the International Year of Astronomy 2009 (IYA2009).

Black Down in Sussex
Teign Valley in Devon
Penbryn Beach in Wales
Stonehenge Landscape in Wiltshire
Wicken Fen National Nature Reserve in Cambridgeshire
Mam Tor in Derbyshire
Friar's Crag in Cumbria

In the UK, the best times to see the Perseids are likely to be on the morning of 12 August before dawn and from late evening on the 12th through to the early hours of the 13 August.

This year, light from the last quarter Moon will interfere significantly with the view.

The UK's National Trust has published online guides to seven top Perseid viewing sites, including coastal spots, nature reserves and national parks.

Jo Burgon, head of access and recreation at the Trust, said: "Light pollution from our towns and cities has increased so much in recent years, but head out to the countryside for the perfect place to explore the beauty of the night sky, away from the intrusive glow."

The rock and dust fragments which cause the shower were left behind by Comet Swift-Tuttle when it last came near the Sun.

The comet orbits the Sun once every 130 years and last swept through the inner Solar System in 1992.

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