October Cetids

History

The first sighting of this meteor shower seems to be linked to observations by W. F. Denning (England) during 1916 and 1917. During the former date, four meteors were seen over the period of October 20 to 25, from a radiant at α=28°, δ=+4°. The meteors were described as rapid. On October 16, 1917, a 1st-magnitude stationary meteor was observed from α=28°, δ=+3°.

Other observations are hard to come by in the records of visual radiants; however, four radio-echo surveys may have detected this stream during the 1960's. Using the radio equipment at Adelaide Observatory (South Australia) in 1961, C. S. Nilsson detected stream members during October 23-30. Although the data revealed a nodal passage on October 27 (λ=213.9°) and an average radiant of α=39.3°, δ=+0.2°, the equipment did not operate during September 30-October 19, so that the early part of the shower's activity was missed. During the 1961-1965 session of the Radio Meteor Project, Z. Sekanina isolated a stream which he called the "Beta Cetids." The stream possessed a duration extending from September 8 to October 1. Its nodal passage was given as September 25.7 (λ=182.2°), while the average radiant was α=16.3°, δ=­11.7°. The second session of the Radio Meteor Project detected a possibly associated stream during September 22-October 22, 1969. The apparent date of the nodal passage was October 5.2 (λ=191.6°), at which time the radiant was at α=12.5°, δ=+4.2°. This stream was called the "Delta Piscids." Finally, G. Gartrell and W. G. Elford operated radio equipment at Adelaide Observatory during October 15-19, 1969. Although the indicated nodal passage came on October 16 (λ=203°) from α=28°, δ=+3°, the equipment had been shut down since mid-June, so that the early part of the activity was definitely missed. The orbits revealed by each of these surveys (in order of their discussion above) are as follows:

ωΩiqea
196194.033.913.90.520.863.70
1961-1965102.42.214.60.4930.7251.792
196993.611.60.70.5660.6931.843
196996238.20.470.904.17

As previously noted, the data published in Nilsson's 1964 paper and Gartrell and Elford's 1975 paper did not cover the early half of this stream's activity, thus giving the indicated nodal passages little meaning. In addition, the orbits from these two surveys were each based on only three meteors, so that the reliability should not be considered very high.

The orbits obtained in each of Sekanina's surveys should be considered more reliable, as they were based on about 10 and 45 meteors, respectively. Most interesting is the inclination change between the 1961-1965 and 1968-1969 surveys, but this may be a result of the different selection methods being used: the early study used a D-criterion of 0.20, while the second survey used a less stringent value of 0.25. After obtaining the 39,145 radio meteor orbits computed by Sekanina during the 1960's, the Author has found 57 meteors that are probable members of this stream. Combined with 13 photographic meteors found in McCrosky and A. Posen's 1961 paper and Ceplecha's 1977 paper, the subsequent orbit was obtained:

ωΩiqea
95.415.27.30.5290.7452.075

By utilizing a D-criterion value of about 0.15, the Author finds that this stream splits into northern and southern branches. The fairly compact northern branch was composed of 46 radio and photographic meteors, while the more diffuse southern stream was composed of 16 meteors.

ωΩiqea
Northern95.416.24.20.5370.7402.065
Southern96.013.613.60.5280.7442.063

The northern branch is definitely the strongest portion of this stream, but neither branch is especially strong visually and the Author believes telescopic aid will probably be necessary for future observations. What makes this stream especially interesting is its orbital similarity to the minor planet 69230 Hermes.

ωΩiqea
Hermes90.735.36.20.6170.6241.639

This asteroid's ascending node occurs around October 29, which coincides with the end of the October Cetid shower.