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 Lyrid Meteor Shower : April 20-22, 2010

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PostSubject: Lyrid Meteor Shower : April 20-22, 2010   Tue Apr 20, 2010 10:26 pm

Quote :
Every year in late April Earth passes through the dusty tail of Comet Thatcher (C/1861 G1), and the encounter causes a meteor shower--the Lyrids. This year the shower peaks on Wednesday morning, April 22nd. The best time to look, no matter where you live, is during the dark hours before dawn. Forecasters expect 10 to 20 meteors per hour visible from dark-sky sites.

Lyrid meteors appear to stream from the bright star Vega in the constellation Lyra:

In fact, Lyrids have nothing to do with Vega. The true source of the shower is Comet Thatcher. Every year in April, Earth plows through Thatcher's drawn-out dusty tail. Flakes of comet dust, most no bigger than grains of sand, strike Earth's atmosphere traveling 49 km/s (110,000 mph) and disintegrate as streaks of light--meteors!

Lyrid meteors are typically as bright as the stars in the Big Dipper, which is to say of middling brightness. But some are more intense, even brighter than Venus. These "Lyrid fireballs" cast shadows for a split second and leave behind smokey debris trails that linger for minutes.

Occasionally, the shower intensifies. Most years in April there are no more than 5 to 20 meteors per hour during the shower's peak. But sometimes, when Earth glides through an unusually dense clump of comet debris, the rate increases. Sky watchers in 1982, for instance, counted 90 Lyrids per hour. An even more impressive outburst was documented in 1803 by a journalist in Richmond, Virginia, who wrote:

"Shooting stars. This electrical [sic] phenomenon was observed on Wednesday morning last at Richmond and its vicinity, in a manner that alarmed many, and astonished every person that beheld it. From one until three in the morning, those starry meteors seemed to fall from every point in the heavens, in such numbers as to resemble a shower of sky rockets..." [ref]

What will the Lyrids do this year? The only way to know for sure is to go outside and look.

Experienced meteor watchers suggest the following viewing strategy: Dress warmly. Bring a reclining chair, or spread a thick blanket over a flat spot of ground. Lie down and look up somewhat toward the east. Meteors can appear in any part of the sky, although their trails will tend to point back toward the radiant--i.e., toward Vega.

Vega is a brilliant blue-white star about three times wider than our Sun and 25 light years away. About 14,000 years ago Vega was the North Star. Earth's spin axis wanders: Now it points at Polaris, then it pointed at Vega. You might have seen Vega in Carl Sagan's movie Contact. It was the source of alien radio transmissions to Earth.

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PostSubject: Re: Lyrid Meteor Shower : April 20-22, 2010   Tue Apr 20, 2010 10:30 pm

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The Lyrid meteor shower is not one of the strongest of the annual meteor showers, but it can be enjoyable to those meteor observers thirsting for something after over three and a half months of weak meteor activity.

The Lyrids generally begin on April 16 and end on April 26, with maximum generally occurring during the night of April 21/22. At maximum, hourly rates can reach about 10 meteors per hour. The Lyrids are particularly interesting for two reason. First, observations have been identified back to at least 2600 years, which is longer than any other meteor shower. Second, the meteor shower occasionally experiences an outburst of about 100 meteors per hour and the reason is basically unknown.

There are other, weaker meteor showers going on around the same time as the Lyrids. The Lyrids move rather fast. When you see a meteor, mentally trace it backwards. If you end up at the constellation Lyra then you have probably seen a Lyrid meteor!

Quote :

During the infancy of meteor astronomy, a number of interesting meteor showers were generally overlooked. One of these was the Lyrids. Following the discovery that the Leonid meteor shower was an annual display, Dominique Francois Jean Arago did some research in 1835 and found enough evidence to support the existence of a probable annual meteor shower around April 22. Responding to this suggestion was Edward C. Herrick (New Haven, Connecticut, USA), who carried out coordinated observations of this meteor shower with Francis Bailey in 1839. These observations revealed weak, but definite activity, which seemed to peak on April 19. Herrick then began scouring the literature and quickly uncovered a large display of meteors that was seen by numerous people in the eastern part of the United States on April 19-20, 1803. He also identified probable observations of this meteor shower from the years 1095, 1096, and 1122. Despite Herrick's observations and historical evidence supporting this stream's existence, the next coordinated observations were not carried out until 1864, when Alexander Stewart Herschel observed several meteors from the region of the constellation Lyra on the night of April 19/20.

During 1866, the annual Perseid shower had been linked to periodic comet Swift-Tuttle and the Leonids were linked to the newly discovered periodic comet Tempel-Tuttle. As 1867 began, astronomers were still busy seeking further evidence linking meteor showers to comets. Edmond Weiss (Vienna, Austria) was busy calculating probable close encounters between Earth and comet orbits. One comet orbit, that of Thatcher (1861 I), was found to come within 0.002 AU of Earth's orbit on April 20. As Weiss searched through various publications for evidence of this shower's presence, he came across several references to observed showers around April 20. Later that same year, Johann Gottfried Galle mathematically confirmed the link between comet Thatcher and the Lyrids and successfully traced the history of the shower back to March 16, 687 BC.

The peak rates of activity have remained relatively consistent from year to year with values generally between 5 and 10 per hour, although there have been unexpected outbursts. As noted earlier, a very strong outburst was noted in 1803. William F. Denning (England) pointed out that in 1849 and 1850, observers in New Haven and India, respectively, noticed "unusual numbers" of meteors on April 20. Denning himself observed a maximum hourly rate of 22 during his observations of 1884, H. N. Russell (Greece) found a rate of 96 on April 21, 1922, Koziro Komaki (Nippon Meteor Society, Japan) saw 112 meteors (most were Lyrids) in 67 minutes on April 22, 1945, and several observers in Florida and Colorado noted rates of 90-100 on April 22, 1982.

The duration of this shower is fairly short. Four amateur astronomers from southern California (Alan Devault, Terry Heil, Greg Wetter and Bob Fischer) observed the Lyrids during April 20 to 24, 1974, and concluded that the shower remained above 1/4 its maximum rate for 3.6 days. Denning reported that his extensive observations of this meteor shower revealed that weak traces of activity were present as early as April 14 and as late as April 26. Interestingly, Zdenek Sekanina reported that the Radio Meteor Project, which spanned the period of 1961-1965, detected probable members of this stream as late as May 3.

Several observers have attempted to estimate the orbital period of this meteor stream from the observations above. Herrick concluded from his historical study of Lyrid activity that the stream exhibited an orbital period of 27 years. Based on the activity observed in 1803 and 1850, Denning concluded that the Lyrids had an orbital period of 47 years, but his prediction of possible enhanced activity in 1897 was met by rates not exceeding 6 per hour. After the outburst in 1982, many researchers remarked that the period was about 60 years, based on the showers of 1803, 1922 and 1982. Unfortunately none of these suggested orbital periods fit the observations perfectly, and it might be possible that the Lyrid orbit contains several irregularly spaced knots of material that could make it impossible to arrive at an accurate period based on visual observations.

Using the more precise methods of radar and photographic techniques, several attempts have been made to determine the period of the Lyrid stream. A collection of photographic orbits published by Fred L. Whipple in 1952, revealed two "reliable" Lyrid meteors with periods differing by 300 years! In 1971, Bertil-Anders Lindblad published a Lyrid stream orbit, which had a period of 131 years, that was based on 5 meteors photographed during 1952 and 1953, and, in 1970, Sekanina published a Lyrid stream orbit based on radio meteors which had an average period of 9.58 years.

The discrepancy in the orbital period of the Lyrids is primarily due to a lack of data. The number of meteors obtained from the major lists of photographic meteors totals 12, with only 6 being considered reliable (and, incidentally, giving a period of 139 years---close to Lindblad's despite sharing only 2 meteors). Comet Thatcher's period of 415 years is probably much more reliable today than the computed orbital period of the Lyrids.

The relatively sharp peak of the Lyrids seems to be due to the overall lack of serious planetary perturbations. Observational evidence indicates the stream is at least 2600 years old.

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Lyrid Meteor Shower : April 20-22, 2010
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