Gamma Aquarids

Observing

This shower is active during September 1-14, with maximum occurring on September 7 (λ=164°) from α=333°, δ=-5°. Its highest rates of activity only reach 1-4 meteors per hour.

History

This meteor shower was first observed in England during 1921, when A. Grace Cook and J. P. M. Prentice independently detected activity in early September. The former observer plotted five meteors from α=334.5°, δ=-2° during September 1, 3, and 8, while Prentice plotted four meteors from α=335°, δ=-2° during September 6-8. During September 7-8, 1923, Prentice reobserved the shower and managed to plot five meteors from a radiant of α=334°, δ=-3°.

Independent observations are always important in the quick confirmation of any scientific discovery, thus, the 1921 observations seem to offer strong evidence for the existence of an active radiant near Gamma Aquarii. But while the 1921 observations were both made by observers in the same country, independent observations during the period of 1927-1934 were made by groups of observers located in New Zealand and Germany.

The New Zealand observations were analyzed by Ronald A. McIntosh (Auckland, New Zealand) in his 1935 paper "An Index to Southern Meteor Showers." The indicated duration of the Gamma Aquarids was September 1-14 while the average radiant was α=335°, δ=-2°. The German observations were included among the 5406 visual radiants listed in Cuno Hoffmeister's 1948 book Meteorströme. Despite the observations covering the period of 1908 to 1938, Gamma Aquarid radiants were only detected during 1929, 1932, and 1934. The 1929 radiant was detected on September 8 from α=333°, δ=+5°. The 1932 radiant was detected on September 5 from α=333°, δ=-10°, and the 1934 radiant was detected on September 4 from α=330°, δ=-11°.

Although the German observations of September 5, 1932 and September 4, 1934, appear to be slightly further south than indicated by the earlier English observations, calculations by the Author reveal the Gamma Aquarid radiant to exhibit a northeasterly motion, thus, it must be expected that the radiant of September 4-5 would be southwest of the September 8 position. A possible confirmation of the early position of this radiant might be present in a 1964 paper by Charles P. Olivier entitled "Catalogue of Fireball Radiants," in which a radiant with a mean date of September 5 was given an average position of RA=333°, DECL=-8°. Based on five observed fireballs, the indicated duration extended for three days before and after the mean date.

Only one radio-echo survey has ever revealed the Gamma Aquarid stream. During the 1961-1965 session of the Radio Meteor Project, activity was noted during September 10-11. Dr. Zdenek Sekanina gave the date of the nodal passage as September 10.8 (λ=167.7°), at which time the radiant was at RA=335.6°, DECL=-1.7°. The stream's absence from other radio-echo surveys can usually be explained as due to the equipment not being in operation during early September, however, the second session of the Radio Meteor Project did not reveal this radiant, despite its being in operation during September 9-12, 1969. Sekanina describes this session as being more sensitive than that of 1961-1965, yet the Author finds absolutely no evidence of activity. Thus, the possibility may exist that this stream produces a periodic display rather than an annual one.

Although this stream was not noted in any of the analyses of photographic meteor orbits during the 1950's, 1960's and 1970's, the Author has identified two photographic meteors from a list of 2529 orbits computed by Richard E. McCrosky and Annette Posen that appear to definitely belong to the Gamma Aquarid stream. They were both detected on September 11, 1952, by the Harvard Meteor Project and indicate a radiant of RA=335°, DECL=-3°. The magnitudes of the meteors were given as -0.4 and +1.6, while their beginning heights were 98.7 and 87.0 kilometers, respectively.

During the 1970's two positive detections of this stream came from observers in the Southern Hemisphere. The meteor section of the Royal Astronomical Society of New Zealand succeeded in observing this shower on several occasions during the decade. Section director John E. Morgan compiled a list of 213 active radiants which included the Gamma Aquarids. Meteor rates of four per hour were noted on September 10, from RA=335°, DECL=-3°. During 1980, the OrganizationsWestern Australia Meteor Section observed meteors from this shower during September 10-13. A maximum ZHR of 1.12±0.24 was observed on September 10 from RA=335°, DECL=-3°.