Observing the Lyrids
Location of the Lyrids
Location of the Lyrids
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.