Iota Aquarids

Observing the Iota Aquarids

For a short summary of this meteor shower and others visible in late July and early August of 2011, click here

This stream consists of two fairly diffuse branches. The Southern Iota Aquarids possess a duration extending from July 1-September 18. The August 6 maximum (λ=133°) produces an hourly rate of 7-8 from a radiant position of RA=337°, DECL=-12°. The Northern Iota Aquarids occur during August 11- September 10. Maximum occurs on August 25 (λ=152°), at which time 5-10 meteors per hour can be seen from RA=350°, DECL=0°. Both streams produce meteors with an average magnitude slightly fainter than 3. The daily motion of the southern stream's radiant is +1.07° in RA and +0.18° in DECL, while the northern radiant moves +1.03° in RA and +0.13° in DECL.


This may represent one of most confused of the well-known annual meteor streams due to a pair of very diffuse radiants. To make matters worse, visual observations of this stream are complicated by the addition of the two Delta Aquarid streams and several streams coming out of Capricornus during July and August.

The first apparent observation of Iota Aquarid activity was made by W. F. Denning (Bristol, England), who, during 1877-1888, plotted several meteors from RA=338°, DECL=-12° during the period of August 3-5. This radiant was falsely identified as belonging to the Delta Aquarid stream, which had been found just a few years earlier. The Author believes this was actually the first observation of the Southern Iota Aquarid stream. Although the number of plotted meteors from this radiant was low, it was capped by the observation of a stationary meteor on August 5, 1888, the radiant of which was at RA=336°, DECL=-11°. Denning was also the first apparent observer of the Northern Iota Aquarids, as he plotted 10 meteors from RA=350°, DECL=0° during August 21-23, 1879. In this instance, Denning mistakenly grouped this radiant in with other radiants forming his stationary "Beta Piscid" stream of June-October.

Observations of both streams continued to be made into the 20th Century, though the radiants tended to be grouped with meteors of other streams. The first person to actually recognize the Iota Aquarid stream was Alphonso King (England), who plotted five meteors from RA=331°, DECL=-12° during July 27-30, 1911. The meteors were described as possessing "Longish paths" with no trains. King's identification referred to the Southern Iota Aquarids, but he gave no indication of a possible northern branch.

Ronald A. McIntosh (Auckland, New Zealand) appears to have isolated both branches of the Iota Aquarid stream from the observations made by New Zealand observers during the 1920's and 1930's. In his classic paper, "An Index to Southern Meteor Showers," two showers called the "Iota Aquarids", were listed which most likely referred to the southern branch. Shower number 272 was based on 7 visual radiants and indicated a radiant movement from RA=330°, DECL=-14° to RA=339°, DECL=-10° during July 31-August 11. Shower 280 was based on 13 visual radiants and indicated the radiant moved from RA=332°, DECL=-15° to RA=338°, DECL=-12° during July 25-August 5. For the Northern Iota Aquarid stream there were also two possibly associated showers. Shower number 301, was based on 4 visual radiants. It had been called the "5 Piscids" and during August 13-21 its radiant averaged RA=346°, DECL=0°. Shower 315 was called the "14 Piscids" and was also based on 4 visual radiants. With a duration of August 13-25, it possessed an average radiant of RA=352°, DECL=-3°.

Although neither of the Iota Aquarid showers were officially recognized by Cuno Hoffmeister, the Author has found that both Iota Aquarid streams were detected by Hoffmeister and his German colleagues. The southern branch was seen on five occasions during 1932-1937. These observations indicated a duration of August 3-10, with the average radiant being RA=338.2°, DECL=-12.2°. Three radiants of the Northern Iota Aquarids were detected during 1930-1937. The indicated duration was August 26-30, while the average radiant was RA=354.0°, DECL=-2.7°.

The first official recognition that the Iota Aquarid stream was composed of two branches should be credited to a 1957 paper by Frances W. Wright, Luigi G. Jacchia and Fred L. Whipple. While examining "all i-Aquarid meteors found on Harvard plates during the interval June 28 to September 1, for all years of observation up through 1955," they noted a meteor found on August 18, 1952, which "seemed to indicate a possible northern stream...." They identified two additional photographic meteors from 1952, one on August 21 and the other on September 1. Using only the two earliest meteor orbits, the authors used a least-squares solution to determine the radiant's daily motion as 1.04° in RA and -0.08° in DECL (the use of only two meteors makes these values highly uncertain‹Author). For the Southern Iota Aquarids, they used five double-station and five single-station meteors to determine the radiant's daily movement as +1.04° in RA and +0.02° in DECL.

Unfortunately, despite the photographic meteors indicating only late August activity for the Northern Iota Aquarids, the authors operated under the assumption that this stream possessed a date of maximum similar to that of the southern branch. Subsequently they identified a visual radiant listed by McIntosh (not discussed earlier) as representing this stream. The radiant was designated shower number 275 and was based on 11 visual radiants. During its duration of July 28-August 2, the radiant was said to have moved from RA=331°, DECL=-8° to RA=334°, DECL=-8°. From these photographic and visual details the authors concluded that the mid-point of the shower's activity occurred at a solar longitude of 132.5° (August 4), with the radiant at RA=330.87°, DECL=-4.95°. Based on the data already given, the maximum of the Northern Iota Aquarids definitely seems to occur in late August (this fact is also supported by the radio-echo data soon to be discussed). Subsequently, the adoption of August 4 as the maximum of the Northern Iota Aquarids (despite the indications of the photographic meteors) caused confusion for several years to come.

The first problem that emerged involved the radar survey of C. S. Nilsson (Adelaide Observatory, South Australia). During 1960-1961, Nilsson had gathered orbital data on 2200 radio meteors, which led to the determination of 71 meteor stream orbits. Using the work of Wright, Jacchia and Whipple as a guide, two possible Iota Aquarid streams were noted which came to maximum at the end of July. Designated as streams 61.7.3 and 61.7.11, they were said to respectively represent the southern and northern branches. Nilsson noted that differences did exist between his work and the 1957 study, and he considered both of his identifications as uncertain.

From what is now known of the Iota Aquarid streams, the Author believes that stream 61.7.3 was more likely associated with the Southern Delta Aquarids. Stream 61.7.11 (observed during July 25-August 3 from RA=326.2°, DECL=-12.3°) actually represented the southern branch of the Iota Aquarids, instead of the northern branch, while a stream designated 61.8.2 (detected during August 16-24, from RA=343.5°, DECL=+0.8°) was actually the Northern Iota Aquarid stream.

The other problem associated with the statement that the Northern Iota Aquarid maximum occurred in early August, involved the lists of meteor showers published in the years that followed. Amateur groups worldwide subsequently adopted the August 4 date and many had not corrected this error at the time this book was published. A possible negative result involves the complete lack of visual observations of this northern shower during the 1960's, and only a handful during the 1970's.

The first accurate delineation of these two streams came about during the two sessions of the Radio Meteor Project, which had been conducted by Zdenek Sekanina at Havana, Illinois, during the 1960's. During the 1961-1965 session the Southern Iota Aquarids were found to possess a duration of July 14-August 27. The nodal passage was identified as occurring on August 9.2, at which time the radiant was at RA=335.3°, DECL=-9.1°. The Northern Iota Aquarids were described as having a duration extending over the period of August 13-28. The nodal passage came on August 25.1, when the radiant was at RA=351.9°, DECL=-1.1°. During the 1968-1969 session, the Southern Iota Aquarids were found to have a duration of July 1-September 18. The nodal passage came on August 10.4, when the radiant was at RA=343.0°, DECL=-3.2°. The Northern Iota Aquarid activity extended over the period of August 11-September 10. The nodal passage came on August 26.0, at which time the radiant was at RA=349.5°, DECL=+0.3°.

Some of the first significant visual details of these two showers were obtained in 1977 by observers in Florida. Norman W. McLeod III was able to determine the first average magnitude estimates of each stream. The Southern Iota Aquarids generally produced the brighter meteors, with a value of 3.05 being determined from 43 magnitude estimates. The Northern Iota Aquarids were found to have an average magnitude of 3.32, as determined from 34 magnitude estimates. Bill Gates commented that the northern stream reached a peak activity rate of 12 meteors per hour on August 20.

Southern Hemisphere meteor observers have provided some interesting results on the Iota Aquarid streams in recent years. In 1981, Michael Buhagiar (Perth, Western Australia) published details of his 20,974 meteor observations made during 1969-1980. Among the 488 visual radiants determined, there was no convincing evidence supporting the existence of the Southern Iota Aquarids. A possible radiant representing the Northern Iota Aquarids was seen during August 21-24. From a total of 3 observations, it was concluded that a maximum hourly rate of 5 came from RA=354°, DECL=0° on August 22.

Members of the Western Australia Meteor Section (WAMS) have also obtained inconsistent observations of these streams. No convincing observations have been made of the Northern Iota Aquarids, but several interesting details have come forth on the Southern Iota Aquarids. In 1978, 41 meteors of this stream revealed an average magnitude of 3.36, while 15.7% left persistent trains. In 1980, the WAMS detected activity from this shower during August 2-10. A maximum ZHR of 2.02+/-0.45 came on August 2, from a radiant of RA=335°, DECL=-16°. Combining all of the WAMS observations, director Jeff Wood concludes that the duration of the Southern Iota Aquarids extends from July 16 to August 19. A maximum ZHR of 7-8 occurs on August 6 from a radiant of RA=335°, DECL=-15°.

One final note should be added about these meteor streams. As noted, the orbit of the Tau Capricornids (July) bears a striking resemblance to that of the Northern Iota Aquarids. The only significant difference is in the date of the nodal passage. There is also a possibility that the Southern Iota Aquarids could be added to this already striking similarity. What this implies is that the original orbit was perturbed in such a way that its shape and size remained basically unchanged, only the line of nodes and, thus, the longitude of perihelion were affected. During this evolutionary change, three significant events seem to have occurred which are now responsible for the maximums of these three showers. The solar longitudes of these events were 110°, 137° and 152°. Two possible reasons for these three distinct maximums have been considered by the Author. Either they are due to variations in the strength of planetary perturbations, or they appeared when the parent body underwent sudden increases in dust output.