The Lyrid Meteors – 16th to 25th April

The Lyrid meteor shower streams from a point in the constellation Lyra near Vega – a brilliant blue-white star about three times wider than our Sun and 25 light years away. The meteor shower occurs due to the Earth passing through a trail of debris left by comet Thatcher (C/1861 G1). The Lyrids are typically visible between 16th and 25th April with a maximum occuring during April 22nd (Solar Longitude=33°32′), from an average radiant of RA=272°, DECL=+33 deg. Although the maximum ZHR is about 10, there have been instances during the last 200 years when rates were near or over 100 per hour so watch out!

Audrius Dubietis and Rainer Arlt published a detailed investigation of the Lyrids in IMO results from 1988 —2000 in 2001, the most detailed examination of the shower in modern times. Several fresh features were found, the most important of which was to redefine the maximum time as variable from year to year between λ = 32°0 —32°45 (equivalent to 2006 April 22, 8h40m —19h00m UT), with an ideal time of λ = 32°32.

Most years in April there are no more than 10 to 20 meteors per hour during the shower’s peak. But sometimes, when the 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. The average magnitude of the meteors is around 2.4 and the speed is described as rapid. About 15% of the meteors leave persistent trains with occasional fireballs a possibility. The radiant is shown below. Best viewing time will be after 2am on the 22nd April with a predicted peak at 1500hrs UT. 

The early hours of the morning on the night’s leading up to the 22nd will be best but any time during the hours of darkness could possibly yield some fireballs.

Interest in this meteor shower was very slow to develop due to the relative infancy of meteor astronomy. A storm of about 700 meteors per hour had been observed by numerous people in the eastern part of the United States during April 19-20, 1803, but no further attention was given to this shower until 1835. That year closely followed the discovery that the Leonids of November were an annual shower, and as astronomers struggled to identify other annual meteor showers, Dominique Francois Jean Arago (1786-1853) conjectured that April 22 might be a date of frequent meteor activity.

Much of the leg work for confirming Arago’s remark should be credited to Edward C. Herrick (New Haven, Connecticut), who, during 1839, not only carried out coordinated observations of this meteor shower, but also collected accounts of the activity in 1803. Herrick also uncovered further appearances on April 9.6, 1095, April 10, 1096, and April 10.6, 1122. His visual observations (made in conjunction with Francis Bradley) proved that weak, but definite, activity was present during 1839, with the radiant for April 19 being RA=273 deg, DECL=+45 deg. Despite this apparent confirmation, the Lyrids were again ignored until April 19-20, 1864, when Professor Alexander Stewart Herschel observed 16 meteors from a radiant of RA=277 deg, DECL=+35 deg. This observation preceded a new wave of interest in meteor showers in general—an interest that again encouraged observations of the Lyrids.

During 1866, the annual Perseid shower had been linked to periodic comet Swift-Tuttle (1862 III) and the Leonids were linked to the newly discovered periodic comet Tempel-Tuttle (1866 I). As 1867 began, astronomers were still busy seeking further evidence linking meteor showers to comets. In Vienna, Professor Edmond Weiss 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 he successfully traced the history of the shower back to March 16, 687 BC.

Observations of the Lyrids increased during the late 1860s and early 1870s, and, as has been the case with so many meteor showers, William F. Denning played an important role in the understanding of this shower. By 1885, he had obtained evidence that the radiant of this shower moved about one degree eastward each day. By 1923, the evidence had become so convincing, that Denning published a radiant ephemeris indicating the radiant began at RA=259 deg, DECL=+34 deg on April 10, and eventually arrived at RA=284 deg, DECL=+34 deg on April 30.
Denning admitted to only having seen Lyrids between April 14 and 26, but was convinced that further, very weak traces of activity might be present outside of that range‹hence, the extended radiant ephemeris. Visual studies have thus far failed to provide convincing evidence of this extended activity; however, during the period of 1961-1965, the Radio Meteor Project, under the directorship of Zdenek Sekanina, detected probable members of this stream up through May 3.

Aside from the abnormal activity of 1803, the maximum hourly rates have remained relatively consistent from year to year, though there have been other unexpected outbursts. Denning 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.

Several observers have attempted to estimate the orbital period of this meteor stream from the visual observations above. Herrick concluded from his historical study of Lyrid activity that the stream possessed an orbital period of 27 years. Based on the activity observed in 1803 and 1850, Denning concluded that the Lyrids possessed 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 Lyrids are known to possess a sharp peak of maximum activity—a feature generally exhibited by meteor streams which are young or not prone to serious planetary perturbations. Since the inclination of the comet’s orbit is 79.8 deg and since evidence exists showing activity as long ago as 687 BC, then the latter scenario seems most appropriate. Typically, the time of maximum occurs around solar longitude 31.6 degrees, with other well-documented visual observations falling within the range of 31.4 degrees to 31.7 degrees. The earlier mentioned study of photographic orbits by Lindblad gave a value of 31.6 degrees, while Sekanina’s radar study gave 32.0 degrees. All of these tend to indicate a much more pronounced peak of maximum activity than is generally present for other meteor streams.

In 1969, Keith B. Hindley pointed out that the close agreement of the maximums of both visual and photographic meteors “indicates that there is no evidence which could be interpreted as the result of the action of dispersive forces of the Poynting-Robertson type.” As support for this statement, Hindley added that occasional observations extending back to 687 BC indicate there has been little or no motion in this stream’s orbital nodes for at least 2600 years!

The photographic data indicates a period over 100 years, while the radar data indicates a period of about 10 years. Such a discrepancy can only be explained by either some minor presence of the Poynting-Robertson effect or a serious error in the determination of the radar orbital data.

Recent studies of the characteristics of the Lyrids, have revealed many interesting features. In 1972, observations by members of the Moscow planetarium during April 16-20, revealed an average magnitude of 3.3. Hindley’s analysis revealed an average magnitude of 2.09 in 1969. Norman McLeod III (Florida) revealed an average magnitude of 2.65 for the period of 1960 to 1976, and 2.90 for the period of 1971-1984. The Dutch Meteor Society revealed an average magnitude of 2.77 for 1985. In addition, the percentage of meteors with trains was 15.9, according to several California observers in 1974, 16.4, according to Felix Martinez (Florida) in 1977, and 10.4, according to the Dutch Meteor Society in 1985.

Although some of the variation in average magnitude can be attributed to observing conditions, several of the observers were under skies with limiting zenithal magnitudes of 6.5, so that the difference could also be due to a mass variation within the stream. Such was suggested by V. Porubcan and J. Stohl in 1983, after analyzing visual observations obtained by observers at Skalnate Pleso Observatory in 1945, 1946, 1947 and 1952. They noted that an abnormally strong maximum of 40 meteors per hour in 1946, was characterized by an increase in the number of fainter stream members. Similarly, the very strong return of 1982 was also characterized by an increase in faint stream members, with the average brightness dropping by almost one full magnitude from what other years typically possess.

Recent estimates of the shower’s normal hourly rate seem to show no noticeable change from what observers were reporting 80-90 years ago, with ZHRs typically between 8 and 15. Recent estimates have been 13.5 in 1969, 17 in 1974, and 13.1 in 1985.

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.