ABSOLUTE BEGINNERS

This article was updated in the November 2006 Beginners Magazine

METEORS

On any dark, clear night, if you sit back and look up into the night sky, every few minutes you will more than likely see a streak of light speed across the sky, this will be a METEOR or shooting star. It is not a star at all, just a small speck of dust (known as a 'METEOROID') entering the top of the Earth's atmosphere at very high speed. Just as the space shuttle or other space craft become very hot as they re-enter the atmosphere, at about 20 thousand miles per hour, these dust particles get even hotter at their re-entry speed of up to 150 thousand miles per hour. At this speed the dust is vaporised by the heat and the surrounding air is also heated and glows much like a florescent light which we see as a METEOR.

There are two types of Meteor, the first are thought to originate from the large lumps of rock left over when the planets formed, known as ASTEROIDS which orbit the Sun between Mars and Jupiter. Very rarely two Asteroids collide but when they do, chips of Rock and Iron are thrown off and occasionally head towards Earth. These may be a few millimetres across or up to tens or even hundreds of metres across. They are quite rare and are seen as individual 'fireballs' sometimes impacting the ground as METEORITES and if big enough may even cause craters. The second type originate from comets and are much more common. As a comet, which can be thought of as a giant dirty snowball, approaches the Sun, the frozen gases and water boil off and are blown away by the radiation from the Sun. Dust particles released by the melt are heavier and therefore continue more or less on the same orbit. These particles spread out along the orbit path and may eventually form a complete ring around the orbit. Once a year the earth may pass through this stream of particles which enter the atmosphere as Meteors. Travelling at between 11 and 76 kilometres per second they burn up in the thin atmosphere at a height of about 100 kilometres.

Different particle streams may be inclined at different angles to the Earth's orbit therefore meteors can enter the atmosphere at almost any angle but each stream always appears to radiate from the same area of the sky each year. The shower on the end of October will appear to radiate from the constellation of Orion that is why it is called the Orionids. See the chart below. Orionid meteors are associated with Haley's Comet. Meteor showers occur at the same time each year and appear to radiate from the same point in the sky each year so each shower is named after the constellation in which the radiant point is located. No equipment is needed to observe meteors just sit in a reclining chair or lay on the ground and look up. On any night there will be a few stray meteors called sporadic meteors but during showers there may be one or more every minute. The shower usually lasts from a few days to a couple of weeks, often with an extra peak in the middle of the period. The following table lists the main meteor showers and the date of the peak.

THE MAIN METEOR SHOWERS

PERIOD                            SHOWER NAME                    MAXIMUM

Jan  1 - 4                               Quadrantids                                    Jan 3

April 10 - 22                          Lyrids                                            April 2

May 1 - 8                              Eta Aquarids                                   May 4

June 17 - 26                          Ophiuchids                                     June 19

July 15 - Aug 15                    Delta Aquarids                                July 27

July 15 - Aug 25                    Capricornids                                  Aug 2

Oct 15 - 25                           Orionids                                         Oct 21

Oct 26 - Nov 16                    Taurids                                           Nov 3

Nov 15 - 19                          Leonids                                         Nov 18

Dec 9 - 14                             Geminids                                        Dec13

Dec 17 - 24                           Ursids                                             Dec 23

 

OBSERVING METEORS

Before rushing out into the garden to look for meteors, there are a couple of things to consider for your own comfort, the first a most important is clothing. The nights at the end of October can be cold so it is essential to dress in warm clothes. A number of layers of clothes is often better than one overcoat so a vest or tee shirt, a second long sleeved shirt, one or two jumpers, perhaps a body warmer and then an outer jacket. Wax jackets are cheap and are good for keeping the damp out if you will be observing from a position on grass. Two or three layers on the legs are also necessary. Long leg thermal pants are excellent for men or women although tights will do but it must be said, tights look better on women. Track suit bottoms are also good as an under garment. Then jeans or a thick pair of trousers should be worn over the top. Water proof trousers are also good for keeping the damp and wind out. Two pairs of socks are a good idea and warm shoes, most trainers these days are padded and are quite warm. A woolly or padded hat is essential because up to 20% of the heat of the body is lost from the head but peaked hats should be avoided. Finally a pair of gloves must be worn. It is always best to start warm and stay warm because once the cold has set in it is very difficult to get warm again.

A garden chair is an excellent piece of equipment especially the type that can be reclined into a near laying position. This will help avoid neck and back ache when looking up. When sitting in a garden chair a blanket or old quilt can be used for additional warmth. A sky chart will be useful to locate the part of the sky where the meteor has been seen. To read the star chart in a dark garden will require a torch but a bright white light should be avoided. A red lamp such as a rear cycle lamp is better, to avoid ruining the dark adaptation of the eyes. Even this may prove too bright so adhesive tape and card can be used to shutter off some of the light, a 10mm hole in a card black out should provide enough light.

You may wish to take notes of what you have seen or even mark the positions of the meteors on your star chart so a pencil should be taken out to the observing spot. If you intend to have a long observing session, especially for a meteor watch, then a hot drink in a flask would avoid missing some of the show and avoid losing dark adaptation by going indoors to make a hot drink.

Most of all, ensure that you are comfortable before you start observing and have everything you need to hand. It is very irritating to have to keep getting up and try to find something you have forgotten, especially if you need to go indoors to get it. Once you are comfortable and settled, with everything you need, then you are ready to start the meteor watch session.

Now on to where to position yourself. Use the patio or path if possible, they are more comfortable and less prone to dampness from dew. Obviously try to set up away from trees or buildings but this may not be possible so set up in the best place to view your intended target, you can always move to another position later. Make sure you have everything to hand, a small table or box by your side, will provide a convenient place to put your chart, torch, spectacles or even a hot drink and will save fumbling around on the ground for things.

To start viewing allow about five minutes for your eyes to get used to the dark. This period can be used to familiarise yourself with the sky and work out where everything is. Try to turn off all lights around you. If there is a street light bothering you, it may be possible to erect a screen around yourself using garden canes, step ladders, washing poles, string and old sheets, curtains, towels or even news papers. Even lights which appeared dim, when you first began your session, seem to get very bright when your eyes are fully adjusted to the dark.

It is useful if you can observe from your own back garden because you can quickly get used to the positions of stars from one night to the next. It is not always possible to use your own garden due to dazzling effect of street lights or perhaps trees or buildings blocking the view. It may be necessary therefore to go to a darker area away from lights. If this is the case it is much better to go with a friend, if possible, as it will be safer and more enjoyable. A remote observing site also has the disadvantage of having to transport any equipment. If it is decided to try a remote site always check the weather forecast first this might save a lot of travelling and anguish when the sky clouds over shortly after all has been set up.

After making yourself warm and comfortable and allowing enough time for your eyes to become adapted to the dark it is time to start observing. The first thing to do is to look around the sky to find a familiar object, the most common thing used is the constellation of Ursa Major also called the Plough, The Great Bear or to the Americans The Big Dipper. This is probably the best known of all the constellations and is always somewhere in the sky from northern latitudes.

When Ursa Major has been found there are two further steps to take. First find Ursa Major on your star chart, remembering to only use a dimmed red lamp. Next look again at Ursa Major in the sky and satisfy yourself that you can identify the 'saucepan' shape. Then locate the two stars forming the side of the pan furthest from the handle. Follow an imaginary line upwards from the two stars, away from the saucepan. About six times the distance between the two stars away from them, there is a lone star in an area with no other bright stars, this is Polaris, the North Star. If you face the direction of Polaris (it is not directly above) this is north. Turn completely around and you will be facing south. When facing south, west is to your right and east to the left. Now position your star chart just above your eyes ensuring that the south position marked on the map is at the bottom. What you see on the chart should be what you see in the sky. Normally the centre of the chart is the position directly above the ZENITH.

THE ORIONIDS OCTOBER 15th - 25th

Orion will be rising over the eastern horizon at about 11 o'clock so will not be fully visible until about 1 o'clock in the morning. The position where the Orionid meteors appear to originate, the radiant, is above and to the left of Orion's left shoulder. You will not need to use binoculars and a telescope will be useless for observing meteors. They are best observed with the naked eye. Position your chair so you can see the sky from the eastern horizon to almost directly above. The Orionids should appear to radiate up from the horizon. You can start before midnight but there will most likely be less meteors at this time. There are two reasons for this, first the radiant is below the horizon so fewer meteors will appear above the horizon and secondly after midnight Earth will be ploughing head on into the main meteor stream. It will be useful to familiarise yourself with the positions of the constellations in the direction you are looking. There will be plenty of time to do this while you are waiting for the meteors. You can then use the constellations to record the track of the meteors as they speed across the sky.

If you feel quite enthusiastic about observing the meteors, you may wish to make a log of every one you see, this can be done in two ways. Notes can be made on a pad, detailing the time, and direction and brightness. If will be necessary to note which constellations the meteor passes through or at least where it ended. These notes can then be plotted on to the chart later. You could alternatively draw the path on your chart and note the time and brightness on the line. The second should be more fun because if the shower is good you will soon see a pattern develop where the lines trace back to a common point in Orion. There may also be some sporadic meteors which do not originate in Orion these are also interesting when marked on the chart. Here is an example of how the chart might look, note the sparodic meteor marked in blue.

THE SKY AS IT APPEARS AT 12:00 ON THE NIGHTS OF OCTOBER 21st AND 22nd

(The paths of Orionid meteors are shown drawn on the chart)

The Orionids tend to be very fast meteors because they hit the atmosphere head on combining the speed of Earth (61000mph) with the meteor (perhaps 50000mph) to make a combined speed of 111,000mph. Some brighter meteors may leave a persistent trail in the sky for up to 5 seconds after the meteor has gone.

The clarity of the sky will make a significant difference to the number of meteors that can be seen. Any mist or hazy cloud will severely reduce the chance of seeing the fainter meteors especially if observing from a light polluted area. If it is clouded there is of course no chance of seeing any meteors at all. It is never possible to predict when the maximum peak might appear and sometimes it may not appear at all. This it because the dust from the comet that produces the meteors moves through space in wisps and filaments so all depends on whether Earth passes through a filament and how thick that filament is. The only thing that is predictable about meteor showers is they will always be unpredictable.

Just hope for clear skies and a good meteor shower.

 

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