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It's Perseid time again!

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!

A Perseid streaking through Cepheus and into Ursa Major. This particular meteor lasted for tens of seconds before fading away. M31 can be seen on the right. Credit: Nick James
Active between 17 July and 24 August, this year the peak favours watchers on Sunday night to Monday morning (11/12 August) and Monday night to Tuesday morning (12-13 August). Meteor activity picks up noticeably from around now, so it will be a good idea to observe on the nights of Friday/Saturday 9/10 and Saturday/Sunday 10/11, especially if you are blessed with dark skies or can easily travel and observe from one. The Moon is new on 6 August and reaches first quarter on 14 August, so it will set early to mid evening. 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. 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.

The radiant of the Perseid meteor shower. Credit: AN graphic by Greg Smye-Rumsby
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 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.