Sigma Capricornids

History

The first observations of this minor stream were made by C. Hoffmeister during 1937. Although normally observing at Sonneberg, Germany, Hoffmeister conducted an expedition to South West Africa during May 7, 1937 to February 5, 1938. On July 9 (λ=106.5°) a radiant was detected at α=296°, δ=-15°. Another radiant was detected on July 12 (λ=109.3°), the position of which was α=298°, δ=-14°, while a final observation came on July 14 (λ=111.2°), when the radiant was at α=296°, δ=-13°.

The only additional record of visual observations comes from Western Australia, when observations made during July 5-6, 1980, revealed a radiant of α=298°, δ=-13°. A maximum ZHR of 2.23+/-0.18 came on July 6. The shower was referred to as the “Alpha Capricornids.”

If it was not for the meticulous nature of the observations made by Hoffmeister and the Western Australians, no visual detection of this shower would be on record. No doubt the congestion of meteor showers in this region of the sky during July and August have contributed to its anonymity. Fortunately, important details have been gathered about this stream thanks to two important radar surveys conducted during the 1960’s.

C. S. Nilsson (Adelaide Observatory, South Australia) conducted a radio survey during 1960-1961. During July 14-25, 1961, five meteors were detected from an average radiant of α=306.9°, δ=-15.4°. Although the average date of activity was given as July 20 (λ=117.1°), it should be pointed out that the radio equipment was not in operation during June 20-July 13, so it is possible that only part of the shower was observed. The orbit was given as

A second radar detection was made by Z. Sekanina during the 1961-1965 session of the Radio Meteor Project. He found the shower’s duration to be June 18-July 30. The nodal passage came on July 9.6 (λ=106.9°), at which time the radiant was at α=297.6°, δ=-18.7°. Sekanina gave the orbit as

Sekanina also made the suggestion that this stream might be associated with the Apollo asteroid 2101 Adonis. The possibility of the association seemed very good as he computed a D-criterion value of only 0.097. The orbit of this asteroid follows:

Finally, although no photographic survey seems to have isolated this stream, the Author has found three definite meteor orbits among the list of 2,529 photographic orbits obtained during the Harvard Meteor Project of 1952-1954. All of the meteors were detected during July 17-21, 1953. The average radiant was α=307.8°, δ=-15.2° and the orbit is