Eta Aquarids
The radiant of the Eta Aquarids meteor shower as seen from
mid-northern latitudes at about 4:30 a.m. LOCAL TIME. Much of
Aquarius is still below the horizon and astronomical twilight has
begun. Image produced by the Author using Starry Night Backyard
and Adobe Photoshop 5.5.
This shower is visible during the
period of April 21 to May 12. It reaches maximum on May 5 (Solar
Longitude=45.5 deg), from an average radiant of RA=337 deg,
DECL=-1 deg. During the period of greatest activity hourly rates
usually reach 20 for observers in the northern hemisphere and 50
for observers in the southern hemisphere, but the radiant never
reaches a high altitude before twilight begins and observing time
is very limited. The radiant's daily motion is +0.96 deg in RA
and +0.37 deg in DECL.
Hints that a shower might be active
at the end of April and in early May began in 1863, when
Professor Hubert A. Newton, examined the dates of ancient showers
and suggested a series of periods which deserved the attention of
observers. One of those periods was April 28-30, and included
observed showers in 401 A.D., 839 A.D., 927 A.D., 934 A.D. and
1009 A.D.
The Eta Aquarids were officially
discovered in 1870, by Lieutenant-Colonel G. L. Tupman
(Mediterranean Sea). On April 30, 15 plotted meteors indicated a
radiant of RA=325 deg, DECL=-3 deg, and on May 2-3, 13 plotted
meteors indicated a radiant of RA=325 deg, DECL=-2 deg. At a
later date, William F. Denning examined the records of the
Italian Meteoric Association and identified 45 meteors that were
plotted during April 29 to May 5, 1870, from an average radiant
of RA=335 deg, DECL=-9 deg. Finally, the shower's first
confirmation came on April 29, 1871, when Tupman plotted 8
meteors from RA=329 deg, DECL=-2 deg.
Observations of the Eta Aquarids were
rare, but, during 1876, Professor Alexander Stewart Herschel
discovered something which at least began to generate a greater
interest in the shower. He conducted a mathematical survey to
find which comets were most apt to produce meteor showers. Comet
Halley was found to be closest to Earth on May 4, at which time a
radiant was predicted to occur at RA=337 deg, DECL=0 deg.
Herschel immediately noted that Tupman's observed radiants of
1870 and 1871 were very near these predictions.
The Eta Aquarids remained a poorly
observed shower due to a lack of active meteor observers in the
southern hemisphere. Only occasional hints of an active shower
were reported, since northern observers had to face the
beginnings of twilight shortly after the radiant rose above the
eastern horizon. Nevertheless, H. Corder detected activity on the
morning of May 4, 1878, with 3 plotted meteors revealing a
radiant at RA=334 deg, DECL=-1 deg. During this same year,
Herschel examined all available observations and noted that the
shower's radiant seemed to move further eastward as each day
passed.
Denning finally managed to observe this
shower during April 30 to May 6, 1886. A total of 11 plotted
meteors revealed a radiant at RA=337 deg, DECL=-2.5 deg. From
these observations, he stated that the radiant seemed 5 deg to 7
deg in diameter. He added that the apparent closeness of his
radiant to that predicted by Herschel placed the identity of this
shower to Halley's comet "beyond doubt."
Fortunately, several good meteor
observers appeared in the southern hemisphere during the 1920's,
and the knowledge of primarily southern meteor showers increased
dramatically. One of the most prolific observers was Ronald A.
McIntosh (Auckland, New Zealand) and he published one of the more
significant studies of the Eta Aquarids during 1929. McIntosh
stated that his observations of that year showed activity during
April 22 and May 13, which he said presented "a good
illustration of the dispersive action of the planets during the
centuries that the parent comet has been in existence." His
first radiant was determined on May 3.2 (RA=334.0 deg, DECL=-1.5
deg), while the last came on May 12.19 (RA=342.7 deg, DECL=+2.5
deg). He stated that maximum definitely came in early May, though
bad weather prevented it from being pinpointed; however, hourly
rates remained between 10 and 20 during the period of May 2 to
11. The radiant diameter was consistently about 5 deg across, and
McIntosh's orbital calculations showed excellent agreement with
the orbit of Halley's Comet.
During 1935, McIntosh published his
investigation of the radiant motion of the Eta Aquarids. Using
observations made by Murray Geddes (New Zealand) and himself
during 1928 to 1933, he precisely determined the radiant's daily
motion as +0.96 deg in RA and +0.37 deg in DECL. He also plotted
the observed activity of this stream and developed an activity
curve that revealed the shower to begin with rates of 1 per hour
on April 28, then rapidly rise to a flat maximum of 10 per hour
during May 3 to 6, and finally slowly decline to rates of 1 per
hour by May 16.
Beginning in 1947, the Eta Aquarids
joined the ranks of the first streams to be detected by
radio-echo techniques. During May 1 to 10, an average radiant of
RA=339 deg, DECL=0 deg produced an hourly rate of 12. Little
additional data was gathered about this stream by the Jodrell
Bank observers during the remainder of the 1940's and throughout
the 1950's. In fact, the stream was largely ignored since the
radio equipment was rarely operated during the early half of May.
Fortunately, observers using the radar equipment at Springhill
Meteor Observatory (Ottawa, Canada) and, later, at Ondrejov
Observatory in Czechoslovakia, were able to supply some of the
most extensive series of data ever accumulated on this stream.
The sensitive radar equipment at
Springhill observed the Eta Aquarids during 1958 to 1967. Hourly
rates were typically between 350 and 500 at maximum. An analysis
of this data, as well as visual data accumulated during the years
1911 to 1971, was published in 1973 by A. Hajduk. He noted that
there was an "instability of meteor frequencies in
individual returns," which he attributed to "variations
of the stream density along the orbit." Hajduk noted that
"no regular periodicity in the shower activity can be
identified."
Overall, the Springhill data covered the
period of May 1 to 10, and a fact revealed by Hajduk was the
complexity of the activity rates. Using an average compiled for
the period 1958-1967, it was noted that two apparent radar maxima
occurred: one on May 4 and the other on May 7. These figures
represented all radio echoes, but a further study of only the
long-duration echoes (lasting about 1 second) revealed the same
two dates of maxima, except the decline between the two dates was
not as pronounced. Also present was a further rise to maximum
that came on May 10.
The above figures represent a 10-year
average and, although they show some interesting characteristics
for the activity levels of the Eta Aquarids, the annual activity
levels given in the same paper are even more
interesting‹especially when they are compared to the unusual
peaks and valleys noted in the activity curves of the Orionids
(see Chapter 10). Hajduk's study of the Orionids led him to
conclude that the abnormal activity levels were due to Earth's
encounter with filaments within the stream. The same explanation
was also given as the reason the Orionids occasionally possess
secondary maxima or primary maxima on a date other than that
usually accepted as the date of maximum. The same is also true
for the Eta Aquarids. In fact, of the 10 years examined by
Springhill Observatory, only 3 years represented what might be
considered a normal activity curve. Some examples of unevenly
distributed matter within the Eta Aquarid stream are as follows:
Hajduk's study not only revealed
interesting details about this stream, but also about the
Orionids of October--long known as the Eta Aquarids' sister
stream. Although there is a distinct similarity between the
characteristics of the meteors and activity levels of both
streams, an interesting feature displayed in the Springhill data
seems directly due to the distances each stream lies from Earth's
orbit. Using the orbit of Halley's comet as representing the
center of the associated meteor stream, Hajduk noted that the Eta
Aquarids occur when Earth is 0.065 AU from the stream's core,
while the Orionids occur when Earth is 0.15 AU away. According to
the Springhill data, there is a smaller variation between the
annual activity rates for the Eta Aquarids than exists for the
Orionids.
The evolution of this stream was
discussed during 1983, by B. A. McIntosh (Herzberg Institute of
Astrophysics, Ottawa, Canada) and Hajduk (Astronomical Institute
of the Slovak Academy of Sciences, Bratislava, Czechoslovakia).
They published the details of a proposed model of the meteor
stream produced by Halley's comet. Using a 1981 study published
by Donald K. Yeomans and Tao Kiang, which examined the orbit of
Halley's comet back to 1404 BC, McIntosh and Hajduk theorized
that "the meteoroids simply exist in orbits where the comet
was many revolutions ago." Further perturbations have acted
to mold the stream into a shell-like shape containing numerous
debris belts. These belts are considered as the explanation as to
why both the Orionids and Eta Aquarids experience activity
variations from one year to the next.
A good example of how observing
conditions vary between the northern and southern hemispheres
occurred during 1971. Observations in the United States and Japan
revealed the shower to have peaked on May 5/6 with a ZHR of 13.
During the same period, Australian observers noted a peak ZHR of
85. Although this example may be somewhat extreme, average rates
tend to be about 20 per hour in the northern hemisphere, and 50
per hour in the southern hemisphere, according to organizations
in the United States, England, Japan, Australia and New Zealand.
The characteristics of the Eta Aquarid
meteors has been well studied by amateur astronomers. During the
period of 1971-1984, Norman McLeod III determined the average
magnitude of the meteors as 3.04. The Western Australia Meteor
Section determined it as 3.07 in 1978, 3.04 in 1985, and 2.46 in
1986. Robert Lunsford (California) determined it as 3.05 in 1984,
2.68 in 1986, and 2.40 in 1987.
The trains of the meteors have also been
studied by amateur astronomers. The meteors exhibiting persistent
trains was determined by the Western Australia Meteor Section as
23.9% in 1978, 29.4% in 1985, and 32.0% in 1986. David Swann
(Texas) determined this figure as 32% in 1984 and 1986.
Curiously, Lunsford found higher values of 67.3%, 67.9%, and
54.3% in 1984, 1986, and 1987, respectively. Lunsford figures are
not absolutely wrong, but may be a factor of his better observing
conditions.
During the 1985-1986 apparition of
Halley's Comet, several meteor organizations around the world put
their members on alert to check for possible increased activity
in the Eta Aquarids (and the Orionids). Reports from groups in
Australia, New Zealand, Bolivia, North America, and Japan
generally indicate that no enhanced activity from this stream was
present. It is interesting, however, that the data given in the
previous table does seem to indicate an increase in the average
magnitude of the Eta Aquarids. Thus, this stream's meteors may
have been slightly larger than normal during 1986.
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