Leonid shower. © Thomas Paulech & Juraj Toth, Bratislava, Slovakia Astronomers in Europe and the Americas were watching the sky on the morning of the 19 November 2003, observing the annual Leonid meteor shower, which at its peak may produce dozens of meteors per hour.

All the meteors will seem to come from the same point in the sky, or radiant, in the constellation of Leo, hence the name Leonid. Meteors associated with the Leonid shower were seen until 23 November, after 21.30, when the radiant rose above the horizon.

The Leonid meteors are associated with comet Temple-Tuttle, which most recently passed close to the Sun in February 1998. The Earth's orbit passes close to the comet’s orbit each year in November and during this time the Earth collides with particles of cometary debris that follow the comet's orbit.

During the 1998 passage of the Leonid meteors astronomers were surprised by many very bright fireballs, which came 24 hours before the expected maximum. This led David Asher (Armagh Observatory, UK) and Robert McNaught (Australia), to produce a new theory as to how the orbits of debris from successive passages of the comet change with time. Their theory was brilliantly proved in 1999 when they predicted a peak in the number of meteors accurate to within one minute. In 2000, 2001 and 2002 their models also performed strongly – their predictions were out by no more than 30 minutes.

The two peaks during 2003 were predicted to be favourable for UK observers – firstly on 19 November, after 06.30 GMT and secondly at 01.00 GMT on 20 November. These maxima are associated with streams of debris left behind by the comet in the years 1533 and 1333 respectively. The waning crescent Moon did not hamper visibility very much but observers were still advised to avoid the light from towns and cities for the best views.

A half-hour exposure of the sky in the early morning hours of 19 November 2001. The image shows more than 100 Leonid meteors, all of which appear to emanate from the radiant in the constellation of Leo. © Cheng Huang-Ming In recent months the gravitational field of the planet Jupiter has shifted the densest streams of material away from the Earth. No further storms are expected for the remainder of the 21st century although Leonid activity will stay high for some years with a particularly good display predicted for 2006.

With patience, meteors can be seen on any clear night of the year and most are caused by particles no bigger than grains of sand, which collide with the Earth's atmosphere at up to 70 km per second (157,000 mph) and burn up. Fireballs are caused by meteors a few centimetres in diameter and can leave tails that persist for several minutes.