August 2, 2014. Copyright 2014, Graphic News. All rights reserved

BACKGROUNDER: Perseid meteor shower

LONDON, August 2, Graphic News: The Comet Swift-Tuttle was discovered independently in 1862 by Lewis Swift and Horace Parnell Tuttle. ItÕs nucleus is 26km in diameter and it is the largest Solar System object that makes repeated close passes to the earth.

Orbital calculations predict there is absolutely no threat of its colliding with the Earth in the next two thousand years or so. However it has a velocity relative to the Earth of 60km/s, and should it collide, the impact energy has been calculated to be about 27 times that of the event believed to have wiped out the dinosaurs. As such it has been described as Òthe single most dangerous object known to humanityÓ.  

Comets have been described as Òdirty snowballsÓ because the solid, core (nucleus) is composed of rock, dust, water ice and frozen gasses. The stream of dust and gas released from the comet is called the ÒcomaÓ and the force exerted on the coma by the SunÕs radiation pressure and solar wind cause the ÒtailÓ to point away from the sun. 

Particles ejected from the Comet Swift-Tuttle as it travels on its 133-year orbit, produce the Perseid meteor shower, so called because it appears to radiate out of the constellation Perseus. The particles are generally smaller than a metre wide. When they enter the earthÕs atmosphere they become visible -- so-called meteors or shooting stars. If they survive the earthÕs atmosphere and hit the earth they are called meteorites.

The Perseid shower is visible each year high in the Northern Hemisphere in the north-east sky, with a peak this year between August 11-13. On years when the sky is particularly dark a watcher might expect to see as many as 90-100 meteors per hour. How dark the sky is depends on atmospheric conditions, local light pollution and how bright the moon is. The brightness of the moon depends on its phase and how close it is to the Earth on its elliptical orbit. This year a nearly full moon -- called a ÒSupermoonÓ when the moon is closest to Earth -- will be rising at about the same time as the Perseid shower. So although stargazers are expecting to see fewer meteors this year, they can enjoy the brightest and biggest full moon of the year. 

UTC Time: In astronomy time is given in UTC (Universal Time Coordinated). UTC is determined using highly precise atomic clocks and is used to co-ordinate time across computer networks. It is often casually interchanged with Greenwich Mean Time (GMT) but there are key differences, particularly when considering fractions of a second.

/ENDS