It's Perseid time again!
BY MARK ARMSTRONG
Posted: 8 August 2013
It's August, so it's time for most observers' favourite meteor shower, and without doubt one of the summer's premier observing events. Meteor enthusiasts have the Perseid maximum circled on the calendar well in advance and thousands over the length and breadth of the country will gather in groups to observe the shower's peak, forecast for 7pm on Monday 12 August. The great news is that the blight of moonlight will not be a problem!
The Perseids is a very rich shower with many bright meteors, often coloured yellow-white and comparable to Vega (mag. +0) or even Jupiter in brightness. They can leave lingering trains or trails as a result of ionisation of the atmosphere around them and occasionally even more spectacular fireballs. Perseids are fast meteors with entry speeds into the atmosphere around 60 kilometres per second. The millimetre-sized particles are the debris from the shower's parent comet 109P Swift-Tuttle, and every year the Earth ploughs into the cosmic dust storm left behind by the comet in its 133-year journey around the Sun. If you are observing with the radiant well-up from a reasonably dark site well aways from artificial light pollution, then observed rates can reach a meteor per minute if you also have an unobstructed view to the east and south. The fickle UK weather can effect or entirely ruin observing plans, but the preliminary forecast for the weekend and early next week is certainly not a complete write-off.
It's fun to just relax and just enjoy the celestial show,but simple counts can be made of the number of Perseids seen; often this is even more fun, especially if watching in groups. Remember not all meteors will be Perseids, as there is the ever-present sporadic meteors. But Perseids are readily identifiable by projecting their path backwards towards the radiant on the Perseus/Cassiopeia border. More experienced observers will be able to make notes for individual meteors, including time of appearance to the nearest minute and magnitude, with suitable guide stars for estimating the magnitude including Vega (mag. +0), Altair (+0.8), Deneb (+1.2), delta Cygni (+2.9), Albireo (beta Cygni, +3.1) and nu Cygni (+3.9). To make reports really useful, then an estimate of the sky transparency and darkness is very important; the best way to do this is by establishing the faintest star you can see (the stellar limiting magnitude).
If you have a camera, then have a crack at imaging some meteors; Try a wide angle lens at ISO 800-1600 and two minute exposures at most at dark site and only 10 to 15 seconds from typical urban sites. The pointing directions hold as for visual observers. However you choose to observe the shooting stars, remember to have fun and let's pray for clear skies.