Arietids

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 α=45° to 55°, δ=+25° to +35°.

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°, 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° and end with a value of 34°, 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° to 65.3°. 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° in α and +0.92° in δ. 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, K. 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, J. 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°, DECL=-11°, 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, Z. 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.