THE PERSEID METEOR SHOWER - AUGUST 2018

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On any clear night if you sit back and look up into the night sky for a while 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 it is just a small speck of dust known as a METEOROID entering 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 30 ,000 km/h these dust particles (Meteoroids) get even hotter at their re-entry speed of up to 270 ,000 km/h. At this speed the dust is vaporised by the heat and the surrounding air is also heated until it glows in a similar way to a fluorescent light.

WHAT ARE METEORS?

There are two types of Meteor, the first is thought to originate from the large lumps of rock and iron left over when the planets formed , known as ASTEROIDS. Most asteroids orbit the Sun in a belt between Mars and Jupiter. The huge gravitational forces exerted by Jupiter may have pulled the rocks apart before they could accumulate into a planet. Very rarely two asteroids may 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 they may even cause craters.

The Main Asteroid Belt between Mars and Jupiter

METEOROID Is a speck of dust released from the ice in a Comet that travels through space.

METEOR Is the streak of light seen when a Meteoroid enters our atmosphere.

METEORITE Is a piece of a Meteoroid that is large enough to survive entering out atmosphere.

The second type of meteor originates from a comet and is much more common. Comets are large lumps of ice, typically between ten and thirty kilometres across that reside beyond the orbit of the planets called the Kuiper Belt. There are millions of these objects just sitting there quietly orbiting around the Sun at enormous distances.

The Kuiper Belt located beyond the orbit of Neptune and Pluto

Occasionally one of these objects may be nudged out of its orbit by a close encounter with another object and may begin to move in towards the Sun. As a comet which can be thought of as being like a giant dirty snowball, approaches the Sun, the water ice and frozen gases begin to boil off and are blown away by the radiation from the Sun. This gas and dust will form the familiar twin tails associate with comets.

Comet Hale-Bopp imaged by Lee Mcdonald in 2002 from Newbury

Dust particles released by the melt are heavier and therefore continue more or less on the same orbit. These particles spread out along the orbital path and may eventually form a complete ring around the orbit. Once or twice a year Earth may pass through this stream of particles that then collide with the atmosphere as Meteors. Meteoroid dust particles are usually small and very light and generally have the consistency of cigarette ash.

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 cloudy there is of course less chance of seeing any meteors at all. It is never possible to predict exactly 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. 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 shower.

WHAT IS A METEOR SHOWER?

Travelling at between 11 and 76 km per second meteors have a lot of kinetic energy (energy due to velocity) and burn up in the atmosphere at a height of about 100 km.

The path a comet might take as it loops around the Sun

Different particle streams may be inclined at different angles to Earth's orbit therefore meteors can enter the atmosphere at almost any angle. The speed of entry varies enormously depending on the angle of entry. Those entering the atmosphere head on to Earth's orbit have the highest combined speed and appear to streak across the night sky fastest.

Meteor showers occur at the same time each year and appear to radiate from the same point in the sky so each shower is named after the constellation in which the radiant point is located. The following table lists the remaining main showers for the rest of this year and the date of the peak of activity.

The following table lists the main meteor showers and the date of the peak of activity. The summer showers are shown in bold.

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 20 Piscis Aquarids July 31
July 15 - Aug 25 Capricornids Aug 2
July 27 – Aug 17 Perseids Aug 12
Oct 15 - 25 Orionids Oct 21
Oct 26 - Nov 16 Taurids Nov 3
Nov 15 - 19 Leonids Nov 18
Dec 9 - 14 Geminids Dec 13
Dec 17 - 24 Ursids Dec 23

Occasionally there is a particularly heavy shower and this is known as a Meteor Storm. One such storm occurred over North America in 1966 and an even heavier storm occurred in 1833 when hundreds of meteors were seen every minute. Both these storms were attributed to the Leonid s hower . Every 33 years or so the Leonids may produce a heavy shower and sometimes produce a storm. Showers and storms occur when Earth passes through a particularly dense clump of dust deposited by the comet. Heavy meteor storms are spectacular and sometimes a bit scary if there is a very heavy storm. These meteors originating from comets are completely harmless and seldom reach closer than 80 kilometres above the surface of Earth.

Meteor showers are best observed after midnight. The reason for this is the point on Earth where we are sitting starts to face toward the direction Earth is travelling around the Sun. At dawn we face directly forward. Earth is travelling at nearly 100,000 km/h in its orbit around the Sun. As Earth ploughs head on into the stream of dust particles (meteoroids) the combined speed of the collisions can be up to 270,000 km/h.

The reason the meteors appear to radiate from a point in the sky is because of the effect of perspective. It is rather like when a car is driven in a snow storm (or rain storm) and the snowflakes seem to radiate from a point directly in front of the windscreen. When the radiant is below the horizon we will only see the meteors that head upwards and they appear to shoot up from below the horizon.

A composite image of a meteor shower radiating from the Radiant Point

As can be seen from the chart above there are a number of meteor showers during the summer months. T he Perseids are the only major summer shower they are worth looking for and it is not so cold at this time of the year.

WHAT SHOULD YOU DO TO OBSERVE METEORS?

Before rushing out into the garden to look for meteors, there are a few things to consider for your own comfort, the first and most important is clothing. The nights, even in summer, can be cold so it is essential to dress in warm clothes. A number of layers of clothes are often better than one overcoat. A vest or tee shirt, a long sleeved shirt and a jumper, perhaps a body warmer and then an outer jacket should be considered. Two layers on the legs are also necessary. Long leg thermal pants are excellent for men or women but 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 and jackets 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 a lot of the body heat 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 ‘lounger' chair is an excellent piece of equipment especially the type that can be reclined into a near horizontal position. This will help avoid neck and back ache when looking up. When sitting in a garden chair a blanket or old quilt can also be used for additional warmth. A sky chart, like the one at the end of this magazine , will be useful to locate the part of the sky where the meteor radiant will be located. 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 a piece of card can be used to shutter off some of the light. The card can be secured over the lamp with tape or an elastic band. A 10mm hole cut in the card should provide enough light to read the chart and not ruin the night vision . Turn off any light that you can and set up a screen to hide street lights to stop them dazzling you.

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 prevent missing some of the show and avoid losing dark adaptation by going indoors to make a hot drink.

Most importantly 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 to 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.

WHERE TO LOOK FOR THE PERSEIDS

Now we will consider the best way for us to watch the meteors. For the Perseids a clear view towards the north is recommended so set up your lounger seat with your feet facing north. Although the Perseid meteors radiate from a point in the north the meteors can be seen anywhere from the west through north to the east. They will be at a height of between 30° above the horizon to directly over head.

The chart below shows the radiant of the Perseid Meteor Shower on the night of the maximum of the shower. Some Perseid Meteor can be seen from about a week before the maximum on the night of 12th -13th August. So it is worth looking from about July 27th . There is expected to be a noticeable peak of activity around 1 o'clock to 3 o'clock on the morning of Monday 13th August so this may be the best time to look for them. A lower level of activity may continue until around 17th August.

The Radiant Point in the constellayion of Perseus

Look for the 'saucepan' shape od Ursa Major the Great Bear (the Plough). Find the North Star 'Polaris' by following the two poiner stars. From Polaris look for the distinctive 'W' shape of Cassiopeia. Also look for the bright star Cappella (close to the horizon) to locate the approximate position of the Radiant point. But look away from the Radiant to see the meteor trails as shown below.

A chart showing the Radiant of the Perseid Meteor Shower at about midnight

Use a 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 in the dark.

To start viewing allow about five minutes for your eyes to become adapted 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 a garden umbrella , step ladders, washing poles, string and old sheets, curtains, towels or even newspapers. Even lights that appeared dim when you first start 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 the 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 slight 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 familiar objects. The most common thing used is the constellation of Ursa Major also called the Plough. Use the instructions on the back page to align the chart. Now position your star chart just above your eyes ensuring that the south position on the map is at the bottom. What you see represented on the chart should be what you see in the sky. Once the orientation is complete the chart can be lowered into a convenient reading position.

Observing can start before midnight but there will most likely be fewer meteors at this time. There are two reasons for this, first the radiant of the Leonid shower will be below the eastern horizon until just before midnight so fewer meteors will appear above the horizon . Secondly, after midnight Earth will be ploughing head on into the main meteor stream. It is normally best to look up at an angle of around 45° above the horizon and 30° to 90° right or left from the radiant point. It will also be useful to familiarise yourself with the positions of the constellations in the direction you are looking while you are waiting for the meteors.

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, direction and brightness. It 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 latter should be more fun because if the shower is good you will soon see a pattern developing where the lines trace back to a common point which is the radiant. There may also be some sporadic meteors which are not members of the shower and do not originate from the same place. These are also interesting when marked on the chart as it may be possible to establish if they came from another old sparse shower.

A very bright meteor known as a Fireball

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