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Blazing a trail with the Perseids

Posted: 8 August 2012

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Perseid meteors Where to see the Perseid meteor shower. AN Graphic: Greg Smye-Rumsby.

The Perseid meteor shower has come to be one of the most eagerly anticipated events of the astronomical year and one of the most reliable to put on a good show. Active between 17 July and 24 August, activity peaks on this coming weekend 11-12 August (the actual peak is predicted to be around midday UT on Sunday 12 August). It will be a good idea to observe on the nights of Friday/Saturday 10/11, Saturday/Sunday 11/12 and Sunday/Monday 12/13, as it's always possible the maximum could arrive early or late. In fact it's well worth observing in the week either side of the predicted maximum. The radiant (the point in the sky from our perspective where the meteors appear to emanate) is high enough in the north-eastern sky from about 10pm and is above the horizon for the duration of the night; by around 2am it is upwards of 50 degrees high.

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. Observing with the radiant well-up from a reasonably dark site, observed rates can reach a meteor per minute if you also have an unobstructed view to the east and south. Unfortunately we can do nothing about moonlight, and this year there is the waning crescent moon rising around midnight among the stars of Taurus close to Jupiter, that will be a bit of a distraction but at least it will rise later each day and its effect diminish night on night.

Meteor observing requires no expensive equipment, just the naked-eye, a clear sky and a comfortable garden recliner or deck-chair. Another reason the Perseids are so popular is the real chance of warm summer nights, but it's prudent to have some warm clothing with you if you are planning to observe away from home. The best advice to maximise your chance of seeing the most meteors is not to target your gaze right at the radiant but perhaps 40 degrees to one side of the radiant and at an elevation of 50 degrees from the horizon. Our late colleague Neil Bone, a great meteor observer, recommended looking at Cygnus in the late evening and Pegasus in the early morning hours.

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 perhaps five minute exposures, depending on how dark your sky is. The pointing directions hold as for visual observers.

However you decide to observe the shooting stars this weekend, have fun and pray for clear skies! The preliminary forecast looks pretty favourable, with it a ridge of high pressure moving in and temperatures rising.

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