Things to look for in 2015:
The Perseid Meteor shower 2015
What are they?
It has been known since the 19th century that meteor showers are associated with comets. In fact they are caused by the Earth passing through the remains of a comet’s tail. As a comet orbit around the Sun they follows some kind of elliptically shape path. As they gets closer to the Sun the heat evaporates from the surface and the characteristic tail starts to form. The tail consists of various gases and particles and these are left floating in space. It may happen that the path crosses the orbital path of the Earth. When the Earth passes through this trail the small particles enter the atmosphere at very high speed – around 50 kilometres per second. The friction causes them to burn up at high altitude (50 to 80 km, up in the mesosphere). The Earth will pass through this remaining patch of dust once a year and we see a meteor shower. There are a number of such showers each year and the Perseids are probably the best known.
The Perseid meteors have been known to mankind for at least 2000 years, they are mentioned in ancient Chinese records for example. However in more modern times Adolphe Quetelet, the Belgian astronomer is usually credited with recognizing them as returning annually. It is now know that the Swift–Tuttle comet is responsible for the Perseid meteors; the cloud consisting of particles ejected by the comet as it traveled on its 133-year orbit.
Observing the Perseid meteors
It is a property of all meteor showers that the meteors in a given shower all seem to emanate from the same point in the sky. In the case of the Perseids they appear come from a point close to the constellation of Perseus – hence the name. This is an effect of perspective – in the same way that a pair of railway lines appear to converge as they get further away – although we know they are really parallel. The meteors all come from the same direction so appear to converge. It is a common misconception that it is necessary to look in the direction of the constellation. In fact the meteors can appear anywhere in the sky, they just appear to emanate from Perseus.
The shower lasts for much of early August but is usually at its best around the 12th, 13th and 14th. You should see some a week or so either side of these dates. This year the maximum is in the early hours of the 13th. The best time of night to see them is just before dawn. This is because of the way the Earth is oriented to the shower. Although meteors before midnight are less common they can be more dramatic as they tend to skim the atmosphere at lower angles.
To observe the shower find a nice dark spot away from any artificial light and preferably away from towns where there will be light pollution. Make yourself comfortable on some kind of reclining garden furniture so that it is easy to look up at the sky for some time. It does not really matter which direction you look in because, as described above they can appear anywhere. Be sure to allow your eyes a minimum of 10 to 15 minutes to adjust to the dark, longer if you have been watching television or using a computer screen, which are bright and have a lot of blue in them.
The meteors vary greatly in brightness depending on the size of the particles. Some are no more than tiny sparks but you may be lucky enough to catch some impressive fireballs. I have personally observed a few very spectacular Perseids from my home at the edge of Abergavenny with glowing trails that persisted for up to 15 seconds.
You should expect to see 1 or 2 every 5 minutes or so but it can be hard to predict. Sometimes two or three come in quick succession then you may wait 15 minutes before seeing another. Last year, before midnight I was observing around 12 per hour.
This year (2015) the Moon is out of the way and if it is clear on the 12th or 13th we should get a good show. At this time of year the sky at night is spectacular in any event, the Milky Way is at it brightest and the wonderful constellations of Cygnus, Aquila, Lyra, Delphinus and Hercules are on full show, with a wonderful array of deep sky object to observe. So even if you only spot a few meteors you should have had a great time perusing our galaxy.
The figure below shows the sky as it will appear looking south east at around 2am on the 13 August 2015. I have shown a representation of the way the meteors would be expected to appear radiating out from Perseus.