Sigma Hydrids

Observer's Synopsis

This shower is visible during the period of December 4 to 15. It reaches maximum on December 11 from an average radiant of RA=127 deg, DEC=+2 deg. Its typical ZHR at maximum is usually around 3 to 5.

History

The discovery of this stream should be attributed to Richard E. McCrosky and Annette Posen. During 1961 they published a list of 2529 photographic meteor orbits that had been computed from double-station photographs obtained during the Harvard Meteor Project of 1952-1954. They identified 7 of these orbits as indicating a stream which produced a radiant near Sigma Hydri. Later that year, Luigi G. Jacchia and Fred L. Whipple published an analysis of 413 precise photographic orbits also obtained during 1952-1954 and identified 3 meteors as belonging to this stream. The average radiant indicated by the precise orbits was RA=126.7 deg, DEC=+1.8 deg.
Confirmation of a December shower from this radiant was made by observers at Waltair, India, during a survey for minor meteor showers conducted by M. Srirama Rao, P. V. S. Rama Rao and P. Ramesh during 1961-1967. During December 12-15, 1963, 5 meteors were detected from RA=127 deg, DEC=+4 deg. It was calculated that the radiant had an hourly rate of 6.6. During December 11-14, 1964, the radiant was again detected. On this occasion 31 meteors were detected from RA=123 deg, DEC=+7 deg. The hourly rate was given as 6.0. Together, these two radiants formed what was designated as Minor Shower No. 5.
A search through previously published accounts listing visual radiants reveals occasional indications of this shower's presence in the latter half of the 19th century and well into the 20th century. But the most striking series of observations extracted from these older records, are the 1937 observations made by Cuno Hoffmeister in Southwest Africa (these are radiants 4582, 4612, 4621, 4642 and 4668 in H1948). Between December 6 and 13, five radiants were noted on various days which possess D-criterions that indicate certain membership with the Sigma Hydrid stream. The average position of these radiants is RA=129 deg, DEC=+3 deg.
Recent observations of this meteor shower have revealed some interesting data. Members of the National Association of Planetary Observers Meteor Section in Australia have revealed activity to occur during the period December 3 to 19, with a maximum ZHR of 3-5 meteors radiating from RA=128 deg, DEC=+4 deg on December 11. During 1978, the same group monitored this shower during December 2 to 10. They found the hourly rate to be highest on the latter date at 5. Based on the 49 meteors observed, the average magnitude was determined as 3.06, while 6.1% of the meteors left persistent trains. The meteors were said to have a "Perseid type velocity" and 21.4% were yellow.
On the whole, northern hemisphere observers have rarely observed rates greater than 1 meteor per hour during 1978 to 1984. However, during 1983, Norman W. McLeod, III noted rates rose to 6 per hour on December 8/9, and was still at 4 per hour on December 10/11. During December 10/11-13/14, 1982, Robert Lunsford (California) determined the average magnitude of the Sigma Hydrids as 3.6.

Orbit

Based on 8 photographic meteors obtained from W1954, HS1958, HS1961, JW1961, MP1961 and M1976, the following orbit was obtained:

AOP AN i q e a
120.6 78.9 127.5 0.248 0.991 26.053

Using an orbit similar to the one above, Ken Fox (Queen Mary College, England) projected the orbit of this stream backward and forward for 1000 years. The following two orbits were obtained:

AOP AN i q e a
950 120.5 78.1 125.6 0.25 0.99 29.28
2950 121.5 81.0 124.0 0.25 0.99 30.13

The orbit of the Sigma Hydrids thus seems fairly stable. In 950, the date of maximum was unchanged from that of the present, while the radiant position was RA=125.8 deg, DEC=+1.1 deg. In 2950, maximum will occur about one day later than at present and the radiant will be at RA=127.0 deg, DEC=+1.6 deg.

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