Armagh Observatory, 2nd August 2016: The annual Perseid meteor shower is one of the best and most reliable meteor showers of the year. The number of shooting stars normally peaks on the night of the 12th/13th August, producing, under ideal conditions of a clear, dark sky with little or no moonlight or light pollution, approximately 100 meteors per hour. This year the number of visible meteors will be somewhat reduced owing to the waxing moon, which reaches first quarter on 10th August. However, astronomers are predicting that the normally rather broad meteor shower will be enhanced on the night of the 11th/12th August by an additional approximately 100 meteors per hour, that is, the shower will exhibit a rare meteor "outburst". The Armagh Observatory will be opening its doors that evening at 8pm for a free tour of the Observatory and Grounds, and later that night, weather permitting after approximately 10pm, there will be a chance to observe the Perseid meteor shower and the predicted meteor outburst.

Perseid meteors, or shooting stars, are produced by small fragments of material ejected during successive revolutions of a rather large short-period comet called 109P/Swift-Tuttle. It revolves around the Sun in a remarkably stable orbit on average once every 11 revolutions of Jupiter, that is, it completes one revolution, on average, every 130 years or so. The dirt-and-ice nucleus has a diameter of around 25km, making it one of the largest short-period comets known. Although the comet was only discovered in 1862, its most recent return in 1992 enabled earlier historic observations of the comet to be identified, in particular those in Chinese records dating back to 69 BC. These enabled the comet's orbit to be determined very precisely, and although the comet is on a potentially hazardous Earth-crossing orbit, there is no risk of it colliding with our planet in the foreseeable future.

Each time the comet enters the inner solar system, its icy nucleus is warmed by the Sun. This causes its surface ices to sublimate and leave the nucleus along with any entrained dust particles to produce the visible head, or coma, of the comet, and ultimately the two types of comet tail: a straight, highly structured gas tail, and a more diffuse, often curved dust tail. The larger dust particles form a dense trail within a rather broader stream of smaller meteoroids, and a meteor outburst is produced when the Earth passes close to or through one or more of the dense dust trails produced during a previous revolution of the comet around the Sun. This year's outburst, around midnight in the evening of 11th August, is caused by particles ejected by the comet around 1862, 1737 and 1479.

Such outbursts are of great interest to astronomers. They can help us to understand the activity of the comet centuries before the comet was discovered, and can also help us to test and improve our understanding of how dust particles are ejected from a comet nucleus, and how over many revolutions they become part of the broad, diffuse meteor stream that produces an annual shower.

Meteors, which are sometimes called "shooting stars" - or "fireballs" if very bright - are produced by small dust particles called meteoroids, with sizes ranging from millimetres up to centimetres or more, as they burn up in the Earth's atmosphere at speeds of typically several tens of kilometres a second. The regular annual meteor showers, such as the Perseids, are produced by streams of meteoroids in the inner solar system that happen to be moving on orbits that intersect the orbit of the Earth. The Perseid meteoroids run into the Earth's atmosphere at speeds of around 60 kilometres per second, roughly 135,000 mph, and burn up in the mesosphere at heights of around 80 to 100 km above the ground.

Those who wish to see a Perseid meteor should select a clear, dark observing site with as little moonlight or artificial light pollution as possible. In August, the constellation Perseus, from which the meteors appear to radiate, can be seen rising low in the northeast soon after dark. Although Perseid meteors will appear to come from this general direction, the visible meteor or shooting star can be seen in any part of the sky. It may be convenient, for example, to look at an angle of about 45 degrees away from the radiant, perhaps overhead or towards the north or east, whilst keeping the radiant near the edge of your field of view.

If possible, look away from any nearby light pollution and always allow time for your eyes to become accustomed to the dark. This means you may have to wait up to 20 or 30 minutes before seeing your first meteor. To avoid fatigue, wrap up warm and recline in a comfortable chair under a rug or sleeping bag to avoid the cold. Take your time, and enjoy the view of the dark, night sky, which this year includes the planets Venus, Mercury and Jupiter setting low in the west soon after sunset, and then Saturn and Mars low in the south-southwest after dark. On the night of the 11th, the Moon sets approximately half an hour after midnight.

FOR FURTHER INFORMATION PLEASE CONTACT: David Asher or Mark Bailey at the Armagh Observatory, College Hill, Armagh, BT61 9DG. Tel.: 028-3752-2928; FAX: 028-3752-7174; E-mail: dja@arm.ac.uk; meb@arm.ac.uk; URL: http://star.arm.ac.uk/ .

Last Revised: 2016 August 2nd