Saturday, 2 July 2011

Has it gone over your head! (2)

Following on from my posting "Has it gone over your head" here is a little more on the subject of the sky at night.

There are other natural phenomena that you can observe in the night sky as you sit moored up in a remote area away from the town and city lights. Amongst the natural phenomena are meteors. We are not talking large objects now. But tiny fragments of material from the size of a particle of dust maybe up to something the size of a grain of rice. Travelling at very high speeds, (25,000 mph) the meteor creates friction within the upper atmosphere and disintegrate in a flash of light leaving behind an ionised trail.

Most meteors disintegrate soon after entering Earth's atmosphere. However, an estimated 500 meteorites ranging in size from a small pebble to a football in size do reach the surface each year. If the meteor survives and lands on the Earth surface, it then becomes a meteorite.

Meteors are crashing into the atmosphere all the time and are called "randoms" However, there are also regular showers of meteors that come around at certain times of the year.  A meteor shower is the result of the Earth, passing through the stream of debris left behind by a comet.

The next notable meteor shower is the Perseid's in mid-August which are associated with Comet Swift-Tuttle. This year the dates range from the 17th of July until the 24th of August with the peak being on the 13th of August. Typical maximum ZHR rate of around 100. Zenith Hourly Rate of a meteor shower is the number of meteors a single observer would see in one hour under a clear, dark sky.  The peak this year will coincide with a full moon that will spoil the show, but it will still be worth watching.

Where to look. Meteor showers are almost always named after the constellation from which the meteors appear to originate. For the Perseid's, you will need to look in the direction of Perseus a constellation, named after the Greek hero Perseus.

Look to the North East. On a moonless night, look for the constellation Cassiopeia, which is in the shape of a "W". Perseus will be near Cassiopeia and the left of the Great Square of Pegasus, a constellation that contains four stars in the shape of a giant square. The brilliant star, Capella in Auriga, will also help you find Perseus. It is between Cassiopeia and Capella. However, to see the meteor trails, you only need to look in the right direction and in this case, look to the North East.

Another one of my hobbies and interest is Amateur Radio. One of the more difficult methods of experimental communication between two radio amateurs, is to bounce a VHF radio signal off the ionisation trail left behind a meteor, burning up in the atmosphere. The communications path would last for typically one to three seconds. Some unusually strong ionisation trails might last as long as twenty seconds. Sometimes the meteors would come in large numbers and as one ionisation trail faded another one was created. The meteor trails were not usually visible as the trails were between 80 and 100 miles up and hundreds of miles away down range from where I was.

Sometimes, (quite rare unless you live in the far north) we might get to see the aurora borialis, or northern lights. The auroral curtain is made up of charged particles which will also reflect VHF radio signals. As we start to enter the next sunspot cycle (every 11 years) the chances of auroral activity will improve. I have seen the auroral manifestation from South Yorkshire about three times in the last twenty years. It is however, worth the journey north to see the real display.


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