METEOR WATCH - AUGUST 2009

Sky chart for August 2009 at 22:00 (10 o'clock) viewed from Southern England

The chart above shows the night sky as it appears on 11 th August at 22:00 British Summer Time (BST).

The centre of the chart will be the position in the sky directly overhead. First we need to find some familiar objects so we can get our bearings. The Pole Star Polaris can be easily found by first finding the familiar shape of the Great Bear ‘Ursa Major' that is also sometimes called the Plough or even the Big Dipper by the Americans. Ursa Major is visible throughout the year from Britain and is always quite easy to find. This month it is in the north west. Look for the distinctive saucepan shape, four stars forming the bowl and three stars forming the handle. Follow an imaginary line, up from the two stars in the bowl furthest from the handle. These will point the way to Polaris which will be to the north of overhead at about 50º above the northern horizon. Polaris is the only moderately bright star in a fairly empty patch of sky. When you have found Polaris you will be facing north. Look for the distinctive 'W' shape of Cassiopeia in the north east. between Cassiopeia and the northern horizon is the constellation of Perseus this is the position of the Perseid meteor shower.

Throughout the month of August there will be a meteor shower known as the Perseid Shower. The best nights to watch for the meteors will be the night of 12th and morning of 13th August when the shower should produce its expected peak of activity.

Some bright meteors originate from collisions between lumps of rock and metal called asteroids; these are generally bright and usually appear singly. There is another type of meteor that occurs in showers, these originate from comets.

The shower this month will appear to radiate from the constellation of Perseus that is why it is called the Perseid Shower. These meteors are associated with Comet 109P Swift / Tuttle. The Radiant point of the shower is located in the middle of the line of stars that form the constellation og Perseus.

The shower is not spectacular every year but some years it produces short intense showers lasting a few hours. This is because the dust from a comet tail tends to form filaments as can be seen in the picture of Comet Swan shown. The last heavy Perseid shower occurred in 1991 when Earth passed through one of the filaments left by the Comet 109P Swift /Tuttle. Calculations have shown that it is possible that Earth will pass through another filament between 18:30 and 23:00 Wednesday 12th August this year when rates of up to 100 meteors per hour may be possible, subject to the clarity of the sky.

The meteors tend to be very fast and leave a long trail across the sky. The shower this year occurs at a fairly favourable time when the Moon is half and low in the sky. With a reasonably dark and clear sky the prospects for a good shower of meteors are hopeful.

No equipment is needed to observe meteors and it can be great fun, especially in a group. All an observer has to do is sit in a reclining chair or lie on the ground and look up. To be a little more scientific a sky chart can be used to mark the tracks of any meteors seen. See the example chart above. By following the tracks back, many will appear to have originated from the same position, this is the radiant point of the shower.

On any night there will be a few stray meteors, these are called sporadic meteors. During showers there may be one or more every minute that can be attributed to the shower. A shower usually lasts from a few days to a couple of weeks, often with an extra high rate peak in the middle of the period.

Observing meteors is very easy and does not require any equipment. However, there are a few things that will help to enjoy the show. It is essential to be comfortable so the first thing to consider is to keep warm. Even on summer evenings it can get chilly after dark, especially when just sitting still for a long time. Needless to say it gets even colder in November when looking out for the Leonids. So warm clothes must be worn from the start because once the cold has set in it is very difficult to get warm again. A hat should be worn and gloves will give extra comfort.

A reclining garden chair will provide a comfortable observing position and will allow observers to support their heads so as to avoid neck ache. If it is likely to be cold a blanket or an old duvet over the legs will be appreciated.

Make sure there are no lights shining in your eyes, turn lights off if you can or set up a screen to block the light from your view. Look to the East at about 45° above the horizon. If you are joined by a friend or family member, allocate each other an area of sky that can be covered by your field of view without moving your head. Make sure your neighbouring observer is allocated an adjacent area of sky that just overlaps your area to make sure the whole area of interest is covered with no gaps.

Observing with friends can make the session a social occasion and will increase the chances of catching most of the visible meteors. If there are three or more it is a good idea to have one person with a note book or a chart to record details of the meteor tracks. The other people can keep their eyes on their allotted part of the sky and call out the position of any meteors seen. They can also estimate the brightness and any interesting characteristic. By doing this the meteors can be identified by virtue of the trail originating from the radiant point in Perseus. Sporadic meteors will appear randomly from other positions in the sky. After a period everyone can change position. An advantage of this team work is the recorder can use a torch and not worry about loosing dark adaptation while on note taking duty.

To help with observing the meteors the chart of the night sky above can be used to mark the trail of any meteors seen. During quiet periods, when there are few meteors, the trails can be drawn as they are reported but during busy periods this may not be possible. Another option is to record the events in a note book and transfer the data on to the chart later. The latter may be adopted as the preferred method anyway. The time, track and estimated brightness should be recorded along with any special features like bursts in the trail. A note of the seeing conditions should also be recorded.

ABOUT 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 but 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 becomes very hot as it re-enters 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 fluorescent light.

A typical bright meteor

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, these are known as ASTEROIDS. Most asteroids orbit the Sun in a belt between Mars and Jupiter and may be what is left of a planet that did not form properly. 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 may even cause craters.

A large asteroid

When the asteroids formed (around the same time as the planets) they were hot due to collisions and internal heat generated by the decay of radioactive materials. If an asteroid was big enough, it could become so hot that any metal would melt and sink to the centre. The only way this metal could get out from the centre of an asteroid is if the whole asteroid was to be blown apart in a massive collision with another asteroid. Some METEORITES (a lump from a meteor found on the ground) are almost pure Iron or Nickel. The outer parts of a broken up asteroid have very little metal because most will have drained away into the centre. Meteoroids formed from the outer parts of a large asteroid are rocky with no trace of metal. So we can tell that these types of meteor were part of a large asteroid. Those containing both rock and metal probably came from smaller asteroids that were not large enough to melt the metal in them or were at the edge of a metal core.

If a lump of rock or metal from an asteroid does enter the atmosphere it creates a very bright trail known as a fireball. If the lump is larger than two or three kilogram's and depending on its make up it may survive the journey through the atmosphere and hit the ground. The smaller ones are slowed down by the friction of the air and hit the ground at a few hundred kilometres per hour. Larger objects hit the ground much faster and harder and may form craters. Very large meteors weighing many hundreds or even thousands of tonnes vaporise on impact and produce a huge crater like the one at Flagstaff , Arizona in the USA shown in the picture below.

Large meteors are rare and very large ones are very rare. The last one to hit Earth was the one that caused the crater above, this occurred 50,000 years ago. The Meteor was about the size of half a football pitch and was almost solid Iron. It impacted almost vertically and most of the Iron and the ground in the area of impact was vaporised. The largest fragment of meteorite found was the piece shown below.

The second type of meteor originates from a COMET and they are much more common. Comets are large lumps of ice, typically a few kilometres across, that reside beyond the orbit of the planets. There are millions of these objects just sitting there quietly orbiting around the Sun at enormous distances. 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, that can be thought of as a giant dirty snowball, approaches the Sun, the water and frozen gases begin to boil off and are blown away by the radiation from the Sun. This gas and dust forms the familiar twin tails we associate with comets.

Comet Hale-Bopp

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 or twice a year Earth may pass through this stream of particles that then collide with 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.

The path of a comet as it loops around the Sun

Different particle streams may be inclined at different angles to the Earth's orbit therefore meteors can enter the atmosphere at almost any angle. Each stream always appears to radiate from the same area of the sky each year. The shower on the 15th - 19th November will appear to radiate from the constellation of Leo, that is why it is called the Leonid Shower. Leonid meteors are associated with the periodic comet named 55P / Tempel – Tuttle, discovered in 1865.

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. The following table lists the main meteor showers and the date of the peak of activity.

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 Leonids. 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. Although spectacular and sometimes a bit scary if there is a particularly heavy storm, these meteors originating from comets are completely harmless and almost never come closer than 80 kilometres above the surface of Earth.

Like most meteor showers, the Leonids are best observed after midnight . The reason for this is the radiant point in Leo does not appear over the eastern horizon until after midnight . After midnight 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 68,000 mph 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 120,000 mph. At this speed, the particles burn up violently as they hit the top of the atmosphere, about 100 kilometres above us. We see the trail of the Meteoroid as a streak of light that we call a Meteor.

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 and the snowflakes seem to radiate from a point directly in front of the windscreen. When the radiant point is below the horizon we will only see the meteors that head upwards and they appear to shoot up from below the horizon.

The Leonid meteor shower 2001

If you have a camera with a shutter that can be held open for a period of time the meteor radiant can be photographed. The camera can be mounted on a tripod or simply placed on a table or wall with the shutter open. If left long enough the rotation of the Earth will cause the stars to form streaks or arcs around the pole star. The longer the shutter is left open the more meteors may be caught on camera. The picture above shows what can be achieved with simple equipment.

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