Sextantids

Observing

This daytime meteor shower possesses a duration extending from September 24 to October 9. Maximum occurs during September 29 to October 4 (λ=185-190°), from an average radiant of α=153°, δ=-2°. The shower's maximum activity seems weak, but reached nearly 30 meteors per hour in 1957. It may be periodic, with an encounter with Earth occurring every 4-5 years.

History

This daylight meteor shower was discovered by A. A. Weiss in 1957, during a radio survey conducted at Adelaide (South Australia). The radiant position was not firmly established at that time, due to only one aerial being in use, but a theoretical estimate of RA=155°+/-8°, DEC=0°+/-10° was made. Due to the uncertainty in the radiant position, the shower was referred to as the "Sextantids-Leonids." What was firmly established was the shower's duration of September 26 to October 4. In addition, a peak activity of nearly 30 meteor echoes per hour was noted during September 29 to October 3. Weiss further noted that no trace of activity had been detected during previous radio-echo surveys.

The next detection of the Sextantid stream was made in 1961. C. S. Nilsson operated the radio equipment at Adelaide during September 21-29, and detected 9 members of this stream during September 24-29. The average radiant was given as RA=151.7°, DEC=-0.1°. Interestingly, Nilsson noted a similarity between the Sextantid orbit and the orbit of the Geminids of December. He claimed that "statistically, the difference between the Sextantid and Geminid orbits is not significant, and the former could well represent the daytime return of a branch of the latter stream after perihelion passage, if the stream is wide enough." Indeed, as Nilsson pointed out, the closest approach of the Sextantids to Earth in December is 0.34 AU, while the indicated width of the Geminid stream is 0.11 AU.

The width of the Sextantid and Geminid streams presented the largest problem in their being directly related, but other problems also existed. Nilsson noted that the Geminids occurred annually with consistent rates "indicating that the meteoric matter is extended uniformly along the orbit." On the other hand, the Sextantids had only been detected in 1957 and 1961. Surveys in other years should have detected the stream, but since they did not, Nilsson suggested the shower might be periodic.

The next sighting of the Sextantids came in 1969, during the second session of the Radio Meteor Project. The radar equipment was not in operation during September 27 to October 5, but 9 members of this stream were detected during October 7-9. Zdenek Sekanina gave the average radiant as RA=156.5 deg, DEC=-8.3 deg. No possible relationship to the Geminid stream was discussed.

Orbit

The orbit of this stream has been computed by both Nilsson and Sekanina.

ωΩiqea
N1964213.23.621.80.1460.871.124
S1976212.315.131.10.1720.8160.936

The orbits are very similar, though, as is evident by the ascending node, the respective studies did not cover the period of late September and early October adequately. For Nilsson's study the radio equipment was shut down for three weeks following September 29, 1961. For Sekanina's survey the equipment was not in use during September 27 to October 5, 1969.

Using the orbit from N1964, Ken Fox (Queen Mary College, England) projected the orbit of this stream backward and forward for 1000 years. No shower was probably detectable in 950 AD, as Fox showed the present stream orbit would have been too far from Earth's orbit. However, the following orbit was obtained for 2950 AD:

ωΩiqea
2950227.3349.516.90.150.881.25

The orbit of the Sextantids thus seems to be changing fairly rapidly. In 2950, maximum will occur about 15 days earlier than at present and the radiant will be at α=150.7°, δ=+0.2°.