The Leonid meteors
One of the most prolific meteor showers is the Leonids. The radiant is in the constellation Leo and meteors from this shower can be seen over a period of about 2 days centred on approximately November 17.
This meteor stream is associated with Comet Tempel-Tuttle. As the meteors in the stream are thought to be dust particles released from the comet it is thought likely that there is a dense stream of such particles in the path of the comet and located close to it. The Earth's orbit passes close to the comet’s orbit each year in November and the Earth collides with these particles of cometary debris following the comet's orbit.
The Leonids have been seen to have very brief periods when hundreds of meteors can be seen. This does not happen every year but seems to come to a peak every 33 years, which is the period of Comet Tempel-Tuttle. These storms of shooting stars were seen in 1799, 1833, 1866 and 1966. The expected storms in 1899 and 1933 were very disappointing.
The 1998 Leonids were predicted to peak at around 1943 UT on 17 November. However, observers were surprised by a burst of bright fireballs which were visible over Europe about 15 hours ahead of the predicted peak time. David Asher and Mark Bailey of Armagh Observatory, in collaboration with Vacheslav Emel'yanenko of South Ural University, Chelyabinsk, Russia, have explained this by calculating the orbits of large dust grains ejected by Comet Tempel-Tuttle. Their mathetical model calculates the braided structure of dust trails left by the comet on previous orbits of the Sun, allowing them to predict meteor activity to an unprecedented accuracy of five minutes.
David Asher at Armagh Observatory and Robert McNaught from Australia used a similar mathematical model to successfully predict the exact time of the peak of the storm in November 1999, when the Earth grazed the edge of the trail left in 1899. European and Middle Eastern observers saw the meteor shower arrive at 0208 UT on the morning of 18 November with a peak of between 2000 and 6000 meteors per hour.
The shooting stars should appear to come from within the 'sickle' of the constellation Leo which rises around 23.00 local time. Leonids are typically extremely swift meteors and a notable portion leave a persistent train, some of which are visible for up to 30 minutes.
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.
Photography
For photography, the BAA recommend using a wide-angle lens at f/2.8 or faster, with a fast film such as Ilford HP5plus, pushed to 800 ISO. Mount the camera on a tripod and aim it in the direction of Ursa Major or Taurus. Leave the shutter open for 10-15 minutes from a dark location. Bear in mind that only the brightest meteor tracks will be recorded but these should be seen against star trails resulting from the Earth's rotation. Remember to record the start and end times of your exposures and the sky conditions at the time.
