Saturday, August 11, 2007


Tomorrow night (August 12/13) the peak of the annual Perseid meteor shower will be upon us. In actual fact the Perseids build up slowly to their peak. Meteors from this shower may be seen as early as July 23rd and as late as August 22nd, though the rate declines more rapidly than it builds up. This means that if the skies are cloudy on the night of the 12th/13th that you still have a chance of catching the shower tonight (11th/12th) especially in the predawn hours or on Monday/Tuesday night (13th/14th). Not all meteor showers are so accommodating (see the Aurigids later in this post).
A "meteor shower", also known as a "meteor storm" or a "meteor outburst" is when there is an unusually high number of meteors in the sky and when these events can be traced back via an imaginary line to a point of common origin in the sky called the radiant.
Meteors happen when the Earth encounters bits of cosmic debris in its orbit around the Sun. Like litter in a campground such debris is ubiquitous in space, and there is always a background of sporadic meteors that occur randomly. Some neighbourhoods, however, are filthier than others. Throughout the year the Earth periodically passes through particularly concentrated trails of material. These have usually been traced to the residue of passing comets, and their position in the orbital plane means that they regularly appear in the same place in the sky at the same time of year.
When the cosmic debris is just hanging around orbiting the Sun (maybe playing cards, smoking cigarettes and, like a Spanish policeman, doing as little as is possible) it is known as a meteoroid . It is only when the Earth catches up to this non-event that things begin to get interesting. As the Earth and the space particle collide the debris becomes a visible meteor . These pass into the Earth's atmosphere at anywhere from 14 to 71 kilometers per second. Most meteors become visible at about 60 kilometers up. They are heated to over 1,700 degrees centigrade, begin to glow from the heat and are usually consumed by the heat as they enter the inner atmosphere. This heating is not caused by "friction" but rather by something called "ram pressure". As the meteor plunges into the atmosphere it compresses the air in front of it, thus heating it up. The heat of this compressed air simple diffuses to the meteor itself. In other words the heat comes from a push rather than a rub.
Should the meteor actually make it through the atmosphere and strike the Earth it is called a meteorite. There are two basic kinds of meteorites. Iron meteorites are made up of about 91% iron. Stony meteorites contain a higher proportion of other elements such as magnesium and especially silicon and oxygen in the form of silicon dioxide- good old sand.
(Hey, Molly could make a song out of this...
"Let's build sand castles in the sky
Let them loose to see if they can fly")
Some meteors explode before they evaporate or strike the Earth. The result is commonly called a fireball, and its sound can be heard dozens of miles away. Iron meteors are more stable than rocky ones. Slow meteors are less likely to explode than fast ones. Meteors arriving at an acute angle to the Earth are subjected to less stress than those that come in at an oblique angle. Yet...even a slow travelling iron meteor that falls from near the zenith has a chance of exploding and creating a fireball.
There is reportedly another sound produced by meteors that is poorly understood. These events are referred to as electrophonic meteors which seem to produce a sound even though it is physically impossible for any sound to travel from the height of the meteor. See 'Electrophonic Meteors' and 'Listening to the Leonids'. The theory is that this sound, variously described as "sizzling" or a "snap,crackle,pop" is produced when electromagnetic radiation in the VLF range is converted into sound energy near the ground.
When a meteor strikes the ground it produces a crater anywhere from 12 to 20 times its own size. Smaller impacts produce simple bowls. When larger objects strike terrestrial rebound creates a central peak along with a rim that is often "terraced" as the ground subsides after the initial impact. The largest impacts of all create multiple inner peaks due to the creation of several rebounds.
The largest impact in recorded history occurred on June 30th, 1908 when an object struck in Siberia. This has become known as the Tunguska event. This object flattened more than 800 square miles of trees. The cause of this explosion has been disputed, but the present consensus is that it was an exploding meteor. An Italian research team has recently obtained results that are suggestive of a remnant of part of this meteorite at the bottom of Lake Cheko about 5 miles northwest of the epicentre of Tunguska. See for the article 'A Possible Impact Crater for the 1908 Tunguska Event'.
Every year many meteorite strikes are reported across the planet. The first structure to be proven as being due to a meteorite impact is the Meteor Crater of Arizona. This crater is 600 feet deep, about a mile across, and its rim rises 150 feet above the surrounding ground. The meteorite that produced this crater impacted sometime between 20,000 and 50,000 years ago. The largest meteorite ever recovered in the USA hit ground in southern Nebraska in 1848. Observers reported that the fireball, which occurred in the afternoon, was "brighter than the Sun" This item, weighing 2,360 pounds was found buried 10 feet deep in a wheat field. Of course the Russians always did it first and better, as anyone who has listened to Chekov in old Star Trek shows can tell you. In 1947 the Russian Sikhote-Alin meteorite created more than 100 small craters some 20 meters across. Eat them apples American imperialists.
There is even a recent controversial theory that the die-off of mega fauna in North America at the end of the last ice age was not due to over hunting by recently arrived paleo-indians-the accepted theory- but rather due to localized climatic change because of an impact event. Maybe yes, maybe no.
For those interested in learning more about meteors in general look to the following sites:


The Perseid meteor shower has been observed since antiquity. The earliest written mentions come from Chinese sources in 36 CE. The annual event was called 'The Tears of St. Lawrence' in medieval Europe because of its proximity to this Saint's feast day on August 10th. The St. Lawrence River in Canada was so named by its "discoverer" Jacques Cartier who came upon it at about this time of year.

The first truly scientific astronomical description of this event was penned in 1835 by the Belgian astronomer Adolphe Quetelet. The earliest recorded meteor count was in 1839 when the German E. Heis found a maximum of 160 meteors per hour. Comet Swift-Tuttle , the parent of this storm, was co-discovered in 1862 by the American astronomers Lewis Swift and Horace Parnell Tuttle. In 1992 the comet returned, and it was first seen by the Japonese astronomer Tsuruhiko Kiuchi. The reappearance created quite a stir as computations of its orbit at first predicted a "near miss" or even a "hit" on the Earth or Moon when it returns again on August 14th, 2126. (see )The predicted day of return in 1992 was off by 17 days. If the next passage was off by 15 days the comet would strike either the Moon or the Earth.

Later refinements in the calculations using data from as far back as 62 BCE improved the predictions and gave credence to the idea that the 2126 return would miss our planet by a wide margin. Further work, however, by the astronomer Brian Marsden predicted that a return in 3044 would bring the comet to within a million miles of the Earth/Moon system, making the chance of an impact quite likely. This is important because the size of Comet Swift-Tuttle puts it in the very scientific category of a BFC- a "big fucking comet". At about 10 kms across Swift Tuttle is about equal to the comet that wiped out the dinosaurs 65 million years ago. Well... no more sitcoms on the boob tube after this visitor drops by.

For more on Swift-Tuttle and its cosmic game of pool/billiards see: .

Between 1864 and 1866 the Italian astronomer G.V. Schiaparelli calculated the orbit of the Perseid meteor stream and found that it strongly resembled that of Comet Swift-Tuttle. This was the first time that a meteor storm had been attributed to a comet. Earlier in the 19th century it was supposed that they were due to the asteroid bodies that had been discovered then. Today most meteor showers have been linked to one or another comet (see ). Around the time of the return of Comet Swift-Tuttle the intensity of the Perseids is particularly strong. In 1993 (the comet returned in 1992) observers in central Europe recorded rates of 200 to 500 meteors per hour. In 1994 the peak was over North America and rates were also quite high compared to other years.

The radiant (the apparent point of origin) of the Perseids is actually quite complex. The main origin is near to the star Eta-Per, but as long ago as 1879 the British amateur astronomer William Frederick Denning claimed that he had found two other points of origin near Chi-Per and Gamma-Per. from observations between 1969 and 1971 researchers in Crimea confirmed these two radiants and also discovered other points of apparent origin near Alpha-Per and Beta-Per.

The radiant of a meteor shower is actually an illusion of perspective. Like two parallel train tracks that seem to converge in the distance meteors travelling in parallel lines seem to be coming from the same point of origin. Meteor showers such as the Perseids can exhibit variations in their intensity and apparent point of origin (hence the various radiants) because the meteoroid particles that make up the comet debris are not evenly distributed. The regular appearance of a comet leaves extra material in certain years. The material left behind gradually drifts, more in the direction of the orbit than laterally. The gravitational influence of major planets such as Jupiter will also pull the meteoroids into resonant orbits called filaments.

Encounters with planets such as the Earth will also accelerate or decelerate the meteoroids, leaving gaps in the dust trail. The end result is a "braiding" of the cometary debris that produces meteor showers. A final effect is radiation pressure from the Sun. This pushes smaller particles away from the main stream. Eventually these lose coherence with the meteor shower and become part of the background of sporadic meteors.

If you want to see the Perseid Meteor Shower to best effect you have to get away from city lights. This year the Moon, now in Gemini, will be in its new moon phase and thus there will be no moonlight to interfere with meteor spotting. So 2007 should be a good year for the Perseids. Take note that binoculars and telescopes are beside the point as meteors travel too fast to be properly seen in such devises. Your naked eye is the only instrument that you will need. See 'The Constellation Perseus' (August 7) on this blog to orient yourself.

Perseus begins to rise between 9 PM and 10PM . This is not the best time to see the largest numbers of meteors for two reasons. One is that there is still lingering twilight from the setting Sun, obscuring the fainter members of the Perseids. The other reason is that the hours before dawn are the best for observing any meteor shower. This is because the speed of the Earth is added to the speed of the particles as the Earth comes into the debris stream. This doesn't just mean that the meteors are "faster". It also means that the Earth "sees" the meteors that it may "outrun" on the dusk side of night. During the early evening the meteors are slower in general because the speed of the Earth is subtracted from their speed at such a time as it moves away from the striking objects. The early evening,however, when Perseus is just rising is the best time to spot the long slow trails of meteors known as Earth grazers. These are meteors that come in to the Earth at an oblique angle. Recall that an oblique angle is the one that puts the greatest stress on an incoming meteor, and so the Earth grazers are the meteors most likely to explode and produce fireballs. You may be lucky enough to see one if you begin your observations early enough.

The best time for observing the maximum number of meteors, however, will be between 2 AM and dawn. Perseus will gradually ascend throughout the night so that the radiant of the Perseids will appear higher as the night goes on. At the peak you can expect to see about one meteor per minute, but the meteors seem to come in "clumps". A lull will be followed by several meteors in rapid succession. The actual frequency is unpredictable, and only the broadest of estimates can be offered as predictions.

The Earth will enter the densest part of the cometary trail at about 2 AM EDT (about 1 AM here in Winnipeg). Because of the motion of the Earth previously discussed, however, the densest part doesn't necessarily coincide with the highest rate of sightings. This will occur somewhere in the 2 AM to dawn window of viewing.

Don't forget to bring along lots of mosquito repellent. Given the amount of rain that Manitoba has had recently it's a certainty that the little biters will be out in full force. The dead of night is also, unfortunately, happy hour for the species of mosquitoes that are the carriers of West Nile Virus, now in an outbreak here in Manitoba. This is the sort of time to use "nuclear DEET".

A reclining lawn chair may be a "must" as you will soon tire of cranking your head up to the sky, and you really don't want to lay back on the ground and be attacked from both above and below by the insects. Wear as much clothing as possible, once more because of the mosquitoes. Take along a good astronomy guide book and a small penlight to read it with (large lights will set back your eyes' adjustment to darkness). You may want to try and see other celestial items such as the other constellations mentioned in the Perseus myth cycle (see 'The Mythology of Perseus' on August 7th on this blog). Or maybe the nearby Pleiades (the seven sisters) or the constellations of Auriga, Aries, Taurus, Gemini and Orion. The planet mars also puts in an appearance in the wee hours(see the diagram above). Look down from perseus and the Pleiades towards the horizon and the constellation Taurus. Mars and Earth will be making one of their close approaches this December, and the Mars mission carrying the Phoenix Lander was launched earlier this month to take advantage of this window of opportunity. In the early Spring of 2008 the Phoenix will land in Mars' arctic regions and, amongst other things, will look for evidence of life past or present.

If you want to catch a photo of a meteor it is best to use a fast lens (f 2.8 or better) and ultrafast film (ISO 400 to 1600) if you are using a non-digital camera. The camera should be mounted on a tripod for stability. If you are in aparticularil;y dark area you may be able to set exposures for up to 20 minutes, but the greater the light pollution the shorter the exposure time that you can use. Point the camera not at the radiant but a short distance away in a region with several bright stars.

If you are using a digital camera first make sure to set the time exactly -yes "to the second". Choose a "night", "bulb" or similar setting. Set the light sensitivity to ISO 1600. Choose a field of view not larger than the great square of Pegasus. Wider fields of view will mean that the dimmer meteors won't be effectively captured by the camera. If possible take successive 10 second exposures. Don't reorient the camera if you want it to be an accurate record of the rate of meteors. You can also get a continuous record of the Perseids by videotaping them. Once more don't move the camera if you ant the result to be an accurate record of frequency.

The website Space.Com is interested in images of the Perseids. If yours are good enough they will put them up on their site. The American Meteor Society and the International Meteor Organization are also interested in amateur observations of meteor showers, both photographic and otherwise.

If you seem to get a handle on meteor photogarphy and you like the results you get with the Perseids you also have an opportunity to help out in ongoing research on saturday, September 1st when the rare Aurigid meteor shower will peak at about 4:36 AM on the west caoast of North America. This meteor shower, whose parent body is Comet Kiess discovered by Carl Kiess in 1911, is quite different from the Perseids. It is actually a rare rather than an annual event. Comet Kleiss has a period of almost 2,000 years. The duration of the Aurigids is extremely short, only about an ahour and a half. This is because the Earth only move through the thin dust stream from comet Kleiss on rare occasions, unlike the other more dependable annual showers. The meteors seen during this rare storm are, however, often extremely bright.

There is still a lot to be learned about both Comet Kleiss and the Aurigid meteors that it produces. Researchers are interested in knowing the duration and peak time of the event as well as the colour of the meteors. In addition to the International Meteor Organization and the American Meteor Society, the Aurigid Multi-Instrument Aircraft Campaign is interested in integrating ground based observations with their high altitude ones. You can also email your images to them care of .

That's it for now. Happy viewing. To read more about the constellation Perseus go to the post 'The Constellation Perseus' (August 7) on this blog. To read more about the legends behind the name of the constellation go to 'The Mythology of Perseus' (August 8), also on this blog.


1 comment:

Werner said...

Back in the nineteen sixties there was a 'theory' going around for a while that the Tunguska event was caused by the impact of a large piece of anti-matter. wow. That was even better than ancient Georgians. Chekov would have been really pissed. Also at about 4:45 in the morning there are almost no mosquitoes in Regina. (maybe out of town though) That is even more amazing.