(Image produced by the Author using Starry Night 2.0 and Adobe Photoshop 5.0. It represents the view from mid-northern latitudes at about 4:30 a.m. local time around April 22.)
Lyrids
Best Night: April 21-22, with about 10
meteors per hour (with occasional outbursts of 100)
Total Duration of Activity: April 16-22
The point from where the Lyrid meteors appear to radiate is located east of the constellation Lyra and is referred to as the radiant. The exact location of the radiant in astronomical terms is RA=18.1 hours, DEC=+33 degrees, but the following chart will also help you find it:
(Image produced by the Author using Starry Night 2.0 and Adobe
Photoshop 5.0. It represents the view from mid-northern latitudes
at about 4:30 a.m. local time around April 22.)
The Lyrids are visible through
most of the night, with the radiant rising around 9 p.m. (local
time). It is located high in the morning sky around 4:00 a.m.,
with an altitude of about 75 degrees. The chart above shows
Cygnus, also known as the Northern Cross, and Lyra. Although
Cygnus is the more prominent and well-known of the two
constellations, Lyra contains the bright star Vega, which
outshines all other stars in that area of the sky. To best
observe the Lyrids wear appropriate clothing for the weather and
lay outside in a reclining lawn chair for best, comfortable
viewing. Late in the evening it would be best to lay with your
feet pointing towards the east and look straight up. During the
morning hours, as the radiant gets higher, you could point your
feet towards the north, west, or south and adjust your line of
sight to about 50 to 60 degrees above the horizon. It is
generally not advised to look directly at the radiant, because
meteors will not move much and fainter ones might be missed. When
you see a meteor mentally trace it backwards and if you arrive
near Lyra it is probably a Lyrid.
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 Author generally supports
Hindley's view of the great age of the Lyrid stream after noting
that, despite a lack of serious planetary perturbations, Lyrid
activity is visible every year. Thus, particles within the stream
have spread completely around the orbit---though, admittedly,
somewhat unevenly. It is, however, curious that such a large
difference exists between the determined orbital period of the
Lyrids when photographic and radar data are considered. 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.
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