This is the strongest daylight meteor shower of the year. The duration
extends from May 22 to July 2, with maximum activity occurring on June 8 (Solar
Longitude=76.8 deg) from RA=44.5 deg, DECL=+23.6 deg. The hourly rate is near 60
at maximum. The radiant's mean daily motion is about +0.5 deg in RA and +0.3 deg
in DECL, while the radiant diameter remains a fairly consistent 3 deg in
diameter.
History
This meteor shower was discovered during 1947, by operators of the radio
equipment at Jodrell Bank (England). Activity was first noted from this stream
on May 30 and continued until June 17. The radiant was not accurately
determined, but was noted to fall within the range of RA=45 deg to 55 deg,
DECL=+25 deg to +35 deg.
The Arietids have been detected in every major radar study since 1947. The
orbit of this stream was first determined by Mary Almond during 1951. Although
the average orbital inclination was found to be 18 deg, Almond used the
"smoothed radiant positions for the first and last dates of observations" and
found the stream's orbit to begin with an inclination of 3 deg and end with a
value of 34 deg, thus "the line of apsides swings gradually forward, and the
main centre of the stream rises farther away from the ecliptic day by day."
The question of the stream's inclination has remained ever since 1951, with
radar studies in Australia, the United States, and the Soviet Union variously
revealing values of 19 deg to 38 deg, however, in 1975 the results of an
Australian radar survey conducted during 1969 were published, which perhaps shed
new light on this problem.
G. Gartrell and W. G. Elford detected six associations during mid-June 1969
that possessed radiants and velocities very close to those accepted for the
Arietids. The problem that existed was that the orbital inclinations of these
streams varied from 2.4 deg to 65.3 deg. They pointed out that the inclination
spread noted by other workers was definitely confirmed, however, "no evidence of
the progressive increase in inclination with passage through the stream...was
found in this survey." Thus, numerous ringlets of material seem to be the reason
leading to the confusion of the Arietid stream's orbital inclination.
Just as confusing as the determination of the stream's inclination has been
the determination of the stream's daily motion of the radiant. In 1951, A.
Aspinall and Gerald S. Hawkins produced a smoothed radiant ephemeris, based on
Jodrell Bank data, that indicated a daily motion of +0.74 deg in RA and +0.92
deg in DECL. Using more precise observations obtained at Jodrell Bank during
1950-1953, K. Bullough determined the radiant's daily motion as +0.48 deg in RA
and +0.30 deg in DECL. Still another determination from Jodrell Bank data, this
time covering the period 1950-1955, was made by T. W. Davidson in 1956. He found
the radiant's daily motion to be +0.47 deg in RA and +0.39 deg in DECL. A radar
survey conducted in the Soviet Union during 1960, by B. L. Kashcheyev and V. N.
Lebedinets, determined the daily motion to be +0.7 deg in RA and +0.1 deg in
DECL. Although no formal explanation seems to have been offered as to why the
radiant's daily motion has been so difficult to determine, it may be possible
that the inclination variances noted previously are directly
responsible---especially since the radiant's motion in declination seems the
hardest to establish. Thus, since Gartrell and Elford's data seem to indicate
several filaments working simultaneously to produce the Arietid activity, the
same filaments might be contributing to the confusion of the determination of
the shower's daily motion.
Other details concerning this stream have not been easy to obtain. From the
numerous Jodrell Bank studies (many of which have been cited in this section)
the diameter of the radiant appears to be about 3 deg, while the maximum hourly
rate ranges from 54 to 76. From observations made in the United States and
Australia during 1971, it appears that meteors from this shower can be visually
detected coming up from the horizon during the hours immediately after sunset
and immediately before sunrise. During June 6/7, Karl Simmons estimated combined
rates of the Arietids and Zeta Perseids (later in chapter) reached 1 to 2
meteors per hour. During one hour on the morning of June 2, 1973, John West
(Bryan, Texas) observed four Arietids.
Various researchers have arrived at some interesting conclusions concerning
links between this stream and other solar system bodies. In 1951, while
obtaining the first determination of this stream's orbit, Almond concluded that
another shower should be encountered as Earth crossed the stream's orbital plane
on July 28. The estimated radiant position was RA=336 deg, DECL=-11 deg, which
falls within 15 deg of the position of the Southern Delta Aquarid meteor stream.
After examining both stream orbits, Almond concluded that, although the orbits
"are now different, it seems probable that they may have had a common origin in
the past."
In articles published during 1973 and 1976, Zdenek Sekanina suggested
several possible associations of meteor streams with comets and asteroids. For
the Arietids, he noted that the Apollo asteroid Icarus (1566) possessed an orbit
with similar characteristics. A D-criterion calculation by Sekanina, comparing
the meteor stream orbit to the orbit of Icarus, revealed a value of 0.245 for
the earlier data and 0.286 for the latter data.