Brighter Perseids should shine through the Moonlight
BY MARK ARMSTRONG
Posted: August 11, 2014
It's time for the Perseids, most observers' favourite meteor shower and usually one of the summer's premier observing events. The shower's peak is forecast for 1am BST on the night of Tuesday/Wednesday 12-13 August. The bad news is that the Moon will be just past full so observations of the fainter Perseids will be hampered to a large extent.
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.
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.
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 on 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 have an unobstructed view to the east and south. With the Moon in the south in the small hours observed rates will most likely be restricted to 10-20 per hour of the brighter Perseids that can punch through the moonlight.
Meteor observing requires no expensive equipment, just the naked-eye, a clear sky and a comfortable garden recliner or deck-chair. Warm summer nights are another reason the Perseids are so popular, but it's prudent to have some warm clothing.
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. However, this year the bright gibbous moon lies just below Pegasus and Astronomy Now's Tony Markham recommends looking for Perseids with your back to the Moon, which means observing the northern sky. The best advice being to concentrate your viewing around the area of the Pole Star.
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 are the ever-present sporadic meteors. But Perseids are readily identifiable by projecting their path backwards towards the radiant on the Perseus/Cassiopeia border.
If you have a camera then have a go at imaging some meteors; try a wide angle lens at ISO 800-1600 and two minute exposures at most at a dark site and only 10 to 15 seconds from typical urban sites. The pointing directions are the same as for visual observers.
However you choose to observe the shooting stars, remember to have fun and let's hope for clear skies.