Perseid meteor shower set to light up the skies

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The Independent Online

A display of celestial fireworks is expected tonight and in the early hours of tomorrow as showers of "shooting stars" light up the night sky.

Astronomers are predicting that between 80 and 100 vibrant streaks of light an hour should be visible from the ground, even for observers who do not have binoculars or telescopes.

The light show, known as the Perseids, are not caused by real stars but by tiny meteors, objects the size of grains of sand, which burn up in spectacular fashion as they slam into the Earth's upper atmosphere. The meteors are travelling so fast - about 37 miles per second or 60 times the speed of a bullet - that they vaporise instantly as they hit the relatively thick atmospheric barrier that protects the Earth from the vacuum of space.

Claire Gilby, a spokeswoman for the Royal Observatory at Greenwich, said meteors could appear anywhere in the sky but the best view in Britain should be towards the north-east where the sky will be the darkest, away from the glare of a waning gibbous Moon.

There is no need for binoculars or telescopes because the naked eye provides a wider angle of view. And shooting stars are best seen from out of the corner of your eye due to the way that the eye's most light-sensitive nerve cells are arranged away from the central focal point of the retina.

"Weather permitting, the sensitivity and wide field of view of the human eye are perfect for watching the Perseids. So, to see the Perseids, all you need to do is sit back and watch the night sky," Ms Gilby said.

The Perseids appear every July and August as the Earth's orbit around the Sun crosses the dusty tail of the comet Swift-Turtle, whose centre or nucleus is many millions of miles away. It is these dust particles that burn up to produce the effect of shooting stars.

Meteors are travelling at up to 157,000 mph when they hit the atmosphere at a height of about 60 miles. At that speed they compress the air in front of them to such an extent that it causes temperatures to rise to 1,650C - enough to vaporise the material instantly in a flash of light.

Meteors can range in size from something the size of grit to objects as big as a pea or a marble. Bigger meteors can sometimes survive the encounter with the atmosphere to reach the ground, when they are called meteorites.

The comet, named after the two American astronomers, Lewis Swift and Horace Turtle, who discovered it in 1862, is the largest object known to make repeated passes near to Earth.

In fact in the early 1990s one astronomer caused some alarm by predicting that the comet could one day pass close enough to Earth to actually hit us - a prediction that was quickly downgraded to a near miss, which will occur sometime in the year 3044.

When the Perseids appear they seem to fly out of the constellation Perseus - hence the name - but in fact this is an optical illusion. It is the same illusion that causes snowflakes to come straight at you from a single point on the road when you drive through a snow storm.

The early hours are usually the best time to see the Perseids because this is when that point of the Earth begins to face the direction in which the meteors are coming from - the constellation Perseus.

As the Earth slowly rotates, the side facing the direction of its orbit around the Sun runs into more grains from the cometary tail. This direction is right overhead at dawn, which is why about twice as many shooting stars are seen before dawn than at sunset.