The duration of the southern Delta Aquarid meteor shower covers the period of July 14 to August 18. Maximum currently occurs on July 29/30 (λ=125°), from an average radiant of RA=339°, DEC=-17°. The maximum hourly rate typically reaches 15-20. The duration of the northern Delta Aquarid meteor shower covers the period of July 16 to September 10. Maximum currently occurs on August 13/14 (λ=139°), from an average radiant of RA=340°, DEC=-2°. The maximum hourly rate typically reaches 10.
This represents the view from mid-northern latitudes at about 2:00 a.m. local time around July 31. The graphic does not represent the view at the time of maximum, but is simply meant to help prospective observers to find the radiant location. The red line across the bottom of the image represents the horizon. (Image produced by the Author using SkyChart III 3.5 and Adobe Photoshop 5.5.)
The radiant does not attain a high altitude for Northern Hemisphere observers, but it is enough to produce a consistent show each year. Since the radiant is above the southern horizon, meteors will primarily be fanning out toward the east, north, west horizons. Few meteors will be seen heading southward, unless they are fairly short and near the radiant.
This represents the view from mid-southern latitudes at about 11:00 p.m. local time around July 31. The graphic does not represent the view at the time of maximum, but is simply meant to help prospective observers to find the radiant location. The red line across the bottom of the image represents the horizon. (Image produced by the Author using SkyChart III 3.5 and Adobe Photoshop 5.5.)
The Delta Aquarids are better seen by observers in the Southern Hemisphere, both because of the radiant's higher altitude and because it is winter, with more transparent skies.
During July and August the Aquarid-Capricornid complex becomes active---a region which contains the northern and southern branches of the Delta Aquarids and Iota Aquarids, as well as several distinct radiants in Capricornus. The strongest activity emanates from the two Delta Aquarid radiants.
Activity was first noticed from the region of the Delta Aquarids in 1870, when G. L. Tupman (Mediterranean Ocean) plotted 65 meteors during July 27-August 6. Tupman found the radiant to have steadily moved during the period of observation, with the position beginning at RA=340°, DEC=-14°, and ending at RA=333°, DEC=-16°. Although the motion seems backward with respect to the normal eastward movement of meteor radiants, the Author believes the former radiant represents the true Southern Delta Aquarids, while the latter radiant is either a combination of both the southern and northern streams or a conglomeration of several streams within the Aquarid-Capricornid complex. Following Tupman's discovery, the region became well studied by others, with W. F. Denning listing no less than 20 additional observations by experienced observers during the remainder of the 19th Century.
The above observations clearly refer to the Southern Delta Aquarids, and, although the Northern Delta Aquarids were not officially discovered until the 1950's, it should be pointed out that some 19th Century observations do seem present. Denning made three apparent observations between 1879 and 1893, which he erroneously grouped with several additional radiants occurring between May and November to form a stationary shower called the "Beta Piscids." The first observation involved 10 meteors seen during August 21-23, 1879, from an average radiant of RA=350°, DEC=0°. Another observation was made during August 16-20, 1885, when 7 meteors were seen from RA=345°, DEC=0°. His final 19th Century observation came during August 13-16, 1893, when 6 meteors were seen from RA=347°, DEC=0°. Since the observed radiants involved small numbers of meteors, there was no inspiration for other observers to continue observations and there was no hint as to an association with the well-known Delta Aquarid shower.
Observations of the Delta Aquarids continued into the 20th Century, but they continued to primarily refer only to the southern branch. As an example, English observers made quite extensive observations during 1922. A. G. Cook plotted 13 meteors during July 25, 28 and 31, which revealed an average radiant of RA=338°, DEC=-12° and, from Ashby, A. King plotted 4 meteors from RA=340°, DEC=-16° during July 30 and 31. J. P. M. Prentice also detected the radiant, when he plotted 12 meteors from RA=341°, DEC=-15° during July 30- August 1. Among the 75 additional radiants recognized by these three observers during July and August of that year, only one observation of the Northern Delta Aquarids seems present. That observation was made by Cook during August 17, 19, and 20, when 4 meteors were plotted from RA=338 deg, DEC=0 deg.
The first significant study of the Delta Aquarid stream was published in 1934. Ronald A. McIntosh used the observations made by the New Zealand Astronomical Society during 1926-1933 to determine the daily motion of the radiant. In all, 44 radiants were utilized, with their original observers being McIntosh (Auckland), M. Geddes (Otekura), F. M. Bateson (Wellington) and A. Bryce (Hamilton).
McIntosh concluded that activity from the shower is continuous from July 22 to August 9, with the radiant moving northeastward from RA=334.9°, DEC=-19.2° to RA=352.4°, DEC=-11.8° (average motion +0.96° in RA and +0.41° in DEC---Author). McIntosh added that a sharp maximum occurs on July 28 (RA=340.5°, DEC=-17.0°), and a diagram included in his paper revealed the following visual hourly rates: 1 on July 22, 2 on July 25, 3 on July 26, 7 on July 27, 14 on July 28, 9 on July 30, 6 on August 2, and 1 on August 9. As can be seen, McIntosh's interpretation of the New Zealand observations reveals a sharp rise to maximum, followed by a gradual decrease in activity. The radiant ephemeris clearly represents the Southern Delta Aquarids and no mention was made as to a northern branch. During 1935, McIntosh published his classic paper "An Index to Southern Meteor Showers," but among the 320 radiants listed, there are no convincing candidates for the Northern Delta Aquarids.
C. Hoffmeister and his fellow German observers obtained good observations of the southern shower between 1908 and 1938. In evaluating the data, Hoffmeister found seven activity centers. Five of these were based on 2-3 observed radiants each, but it was clear to Hoffmeister that maximum occurred on August 3 (λ=130°). He based this statement on two well-established activity centers: one based on seven visual radiants that occurred on August 2 (λ=128.4°) from RA=342.4°, DEC=-17.7°, and a second was based on ten radiants that occurred on August 6 (λ=132.6°) from RA=341.5°, DEC=-17.2°. Hoffmeister does seem to have isolated a radiant with the characteristics of the northern branch as well. In a table listing 238 radiants observed on at least 4 occasions, is a radiant at RA=349°, DEC=+1°. Based on 5 visual radiants, the average date was given as August 13 (λ=139°).
The first radio-echo observations of the Delta Aquarids were made by equipment at Ottawa, Canada, during 1949, when Canadian astronomer D. W. R. McKinley detected both branches of the stream. Unfortunately, orbits were not determined for the two radiants and the northern radiant was not recognized as being associated with the Delta Aquarid shower. In a 1954 paper that appeared in the Astrophysical Journal, McKinley revealed how his velocity and radiant determinations on July 26-29, 1949, revealed two distinct radiants: a very strong one at RA=339°+/-2°, DEC=-17°+/-2° and a very weak one at RA=340°+/-5°, DEC=0°+/-5°. The velocities of the two radiants were 40.20+/-0.1 km/sec and 41.0+/-0.5 km/sec, respectively.
Radio-echo observations of this shower were also made by equipment operating at Jodrell Bank (England) during 1949-1951. The most reliable data was accumulated during the last days of July 1950. G. S. Hawkins and M. Almond gave the weighted mean date of activity as July 28 (λ=124.5°), at which time the hourly radio-echo rate peaked at 38. The radiant possessed a diameter of 3° and an average position of RA=339°, DEC=-14°. The 1949 observation occurred on July 29 (λ=125.8°). Although an hourly rate of 24 was obtained, no further details could be established. Radio-echo rates reached 41 on July 27, 1951 (λ=123.4°). The radiant determination was not considered to be of the highest quality, with the diameter being given as 6 deg and the position being estimated as RA=336°, DEC=0°. The only hint that the 1951 shower might have been identical to the Southern Delta Aquarid radiant was its date of maximum activity. The actual position lies closer to the where the radiant of the Northern Delta Aquarids might lie at the end of July.
During 1952, Almond made a specific attempt to determine the velocity of the Delta Aquarid meteors. Using a "more selective beamed aerial," 32 probable members of the stream were detected and a mean velocity of 40.5+/-2.7 km/sec was revealed. In addition, maximum was found to have occurred on July 28 from RA=340°, DEC=-15°. When the radiant and velocity were combined they allowed the first accurate determination of the orbit of the Delta Aquarids. From this orbit, Almond noted a strong similarity between the orbit of this stream and that of the Arietids of June (see Chapter 6). Most notable were the similar values determined for the perihelion distance, eccentricity and longitude of perihelion. The discrepancies in the argument of perihelion and ascending node were explained as due to the diffuse nature of both streams. "As the inclination of the orbit plane of the d Aquarids is 24°," she explained, "the stream would be 0.31 a.u. away from the earth at its second approach on June 9. From the duration of 16 days observed for the daytime Arietids the width of the stream must be at least 0.27 a.u., and there is also evidence that the DEC Aquarid shower lasts 18 days in the southern hemisphere. Hence, the system of orbits is so broad, it seems probable that the two showers are connected and are produced by one extended stream."
Using over 2000 photographic meteor orbits determined during the Harvard Meteor Project of 1952-1954, F. W. Wright, L. G. Jacchia and F. L. Whipple pointed out the mounting evidence supporting the existence of a northern branch of the Delta Aquarids. This marked the first time the northernmost radiant had been recognized as being associated with the Delta Aquarid shower and the authors offered the first hint as to the complex evolution of the stream. They noted that the northern and southern branches were "symmetrical with respect to the ecliptic, or to Jupiter's orbit...." Concerning the suggested link between the Southern Delta Aquarids and the Arietids of June, it was noted that the spread of about 134° between the nodes of the two streams "could have been caused by continual perturbations by Jupiter."
The photographic data accumulated during 1952-1954 was analyzed by several astronomers during the 1960's, but the most complete analysis was made in 1971 by B.-A. Lindblad (Lund Observatory, Sweden). For the Southern Delta Aquarids he isolated 13 meteors which indicated a duration of July 21-August 8. The stream's date of nodal passage was given as July 31, at which time the radiant was at RA=340°, DEC=-16°. A second study by Lindblad utilized 11 precisely calculated meteor orbits. The study began with a D-criterion of 0.15, but he noted that when a more strict value of 0.10 was used the Southern Delta Aquarids remained intact. He concluded that it "indicates a high degree of orbit similarity...." For the Northern Delta Aquarids, Lindblad isolated 9 photographic orbits. The indicated duration was August 5-25. The nodal passage came on August 14 (λ=140.5°), at which time the radiant was at RA=347°, DEC=+1°.
Although the radio-echo method had actually produced details about the Delta Aquarid streams as early as 1949, radar studies entered a new age of importance during the 1960's. The first major study was conducted in 1960 by researchers at the Kharkov Polytechnical Institute. B. L. Kashcheyev and V. N. Lebedinets obtained 151 radio meteor orbits from the Southern Delta Aquarids during July 14-August 14. When at maximum the average radiant was at RA=341.2°, DEC=-16.4° and the solar longitude was 126.7°. They gave the daily motion of the radiant as +0.85° in RA and +0.35° in DEC, and determined an average orbit which revealed a semimajor axis of 2.04 AU. The Northern Delta Aquarids were also detected. The 50 radio-meteors observed revealed a duration extending from July 7-August 14. The date of the nodal passage was August 1 (solar longitude=127.7 deg), at which time the radiant was at RA=336.8°, DEC=-4.9°. The radiant's daily motion was given as +0.9° in RA and +0.3° in DEC. Although many of the details determined for the Northern Delta Aquarids matched those of other researchers made both prior to and after the Kharkov study, several aspects indicate the data may have been contaminated by other meteor showers active at the time. Most notable are the dates of earliest activity and nodal passage, both of which occur at least 10 days prior to that generally accepted for this shower. In addition, the argument of perihelion is the highest ever revealed for this stream's orbit, with a value nearly 15° above that given in most other orbital determinations.
The next significant radar survey was conducted in 1961 by C. S. Nilsson (Adelaide Observatory, South Australia). During July 23 to August 4, 48 radio meteor orbits were obtained, which revealed a maximum on July 28 (λ=125.8°) from an average radiant of RA=339.4°, DEC=-17.3°. He determined the radiant's daily motion as +0.9° in RA and +0.2° in DEC, while stream's average orbit possessed a semimajor axis of 2.33 AU. Nilsson tried to be as strict as possible in his evaluation of the data. He said, "there are several distinct radiants... in the vicinity of the main Delta Aquarid radiant. It is not unlikely that some of these have contributed to the data used in previous determinations of the Delta Aquarid radiant." Although Nilsson did not specifically recognize the Northern Delta Aquarids, he did isolate 4 meteors during the period of August 20-23. The average radiant was given as RA=352.7°, DEC=+6.3°. It should be noted that the radar equipment at Adelaide did not operate during August 5-15, so that the true maximum of the shower would probably have been missed. As can be seen in the "Orbit" section below, Nilsson's orbit bears a striking resemblance to orbits determined by other astronomers for the northern shower.
The most elaborate radio-echo survey to isolate both branches of the Delta Aquarids was the Radio Meteor Project conducted in two sessions during the 1960's. For the 1961-1965 session, Zdenek Sekanina found the Southern Delta Aquarids to have a duration of July 16-August 14, The nodal passage came on July 30.9, at which time the radiant was at RA=342.2°, DEC=-16.9°. He pointed out that the stream's distribution "could be matched by no single model curve" and he suggested that the stream "might be composed of two constituents: a very compact filament and a more dispersed stream." The Northern Delta Aquarids were given a duration of July 26-August 27. Their nodal passage came on August 13.0, at which time the radiant was RA=344.0°, DEC=+0.3°. Sekanina considered this stream "to be somewhat looser than the southern branch...and definitely less conspicuous and less populated."
Sekanina's 1968-1969 survey gave the duration of the Southern Delta Aquarids as July 14-August 18. The nodal passage was given as July 29.3, while the average radiant was RA=341.8 deg, DEC=-15.9 deg. For the Northern Delta Aquarids, the duration was given as July 28-September 10. The nodal passage came on August 14.9, at which time the radiant was at RA=345.7°, DEC=+4.8°.
Following the earliest computations of orbits for the Southern Delta Aquarids, came the studies of physical and evolutionary developments of the meteor stream. In 1963, A. K. Terent'eva examined the structure of the stream. He noted that the small perihelion distance (given as 0.06 AU) would bring the temperature of the individual meteors up to 1100 deg K, which is the melting point of silicates. Terent'eva suggested this accounted "for the peculiar general appearance of the shower meteors which are sharp, show no wakes, and give off no sparks." Several visual and photographic radiants were studied and the northern and southern radiants were clearly apparent, with their average radiants for July 29 being RA=334.5°, DEC=-5.4° and RA=338.5°, DEC=-16.9°, respectively. The daily motion of the Northern Delta Aquarids was given as +0.85° in RA and +0.35° in DEC, while the Southern Delta Aquarids' motion was +0.88° in RA and +0.36° in DEC. Finally, it was noted that the orbits of the two streams were "symmetrical relative to the plane of Jupiter's orbit." He added that "This may be the effect of perturbations."
Another very interesting study appearing in 1963, was conducted by S. E. Hamid and Fred L. Whipple. It raises the importance of the Southern Delta Aquarid stream to a very high level as being a link to other meteor showers. It has already been noted that a strong link exists between this stream and the daytime Arietid stream of June, but Hamid and Whipple gave evidence to suggest that the Quadrantids of January also formed from this stream. Taking members of both streams and subjecting them to secular perturbations they found that the orbital planes and perihelion distances were very similar 1300-1400 years ago. "The effects of Jupiter perturbations on i and q are quite remarkable," they said, and "it is possible that the two streams were derived from a single comet...." They added that despite the present differences in the duration and activity levels of the two showers, "the physical characteristics of the meteoroids belonging to the two streams appear to be similar, as judged by their light curves."
Several attempts have been made to identify this shower among ancient displays. C. P. Olivier suggested the first possible link in his 1925 book Meteors. He believed the strong displays of July 19, 714, and July 14, 784, were possible early appearances of the Southern Delta Aquarids. In 1976, Sekanina said the shower of 714 has been classified as a possible early appearance of the Perseids. He added that the most promising early appearance of the Southern Delta Aquarids was a shower that occurred in 1007, in which two independent Japanese sources describe the meteors as flying toward the north---a direction quite inappropriate for a description of the Perseids. Sekanina said this radiant would indicate a nodal regression of 0.8-1.3°/century.
One very interesting finding about the Southern Delta Aquarids is the diameter of the radiant. C. Hoffmeister noted in his 1948 book Meteorströme, that "the radiant is at times very diffuse" and he added that activity tended to be strong within an area 20° in diameter centered on the Southern Delta Aquarid radiant (this, of course, must have included the two branches of the Iota Aquarid stream as well---Author). A similar conclusion of a diffuse radiant has been arrived at by several Northern Hemisphere observers during this century, but for observers south of the equator a different conclusion has been formulated. McIntosh was struck by the fact that the Southern Delta Aquarid radiant seemed fairly small, with New Zealand observers independently determining radiants quite close to one another. From South Australia, Nilsson also noted the "particularly small" radiant diameter based on his radio-echo survey. Such a contrast between northern and southern observations reveals what can happen to a radiant when zenithal attraction comes into play. For the Southern Hemisphere observers the radiant is almost directly overhead, while northern observations tend to occur when the radiant is only about 20 deg above the horizon.
From the material discussed above, it is obvious that visual activity mainly comes from the Southern Delta Aquarid radiant. Recent observations of this shower reveal quite strong activity levels. M. Buhagiar (Western Australia) observed 8-12 meteors per hour during 10 hours of observing on July 28/29 and 29/30, 1972, even though a full moon was present. During 1973, visual rates reached 14.6 per hour on July 27/28, according to four observers in the United States, while Buhagiar detected 20 per hour on the same night from Western Australia. During 1974, ZHRs in the United States reached 12.5+/-3.0 on July 29.4, while Buhagiar and Robert Oates noted hourly rates of 37-44 meteors per hour on July 28 in Western Australia.
Members of the Western Australia Meteor Section (WAMS) have had much success in observing the Southern Delta Aquarids. Section director J. Wood said a maximum ZHR of 42.64+/-9.78 was observed on July 29, 1977, with an overall observed duration extending from July 23-August 14. The radiant position at maximum was given as RA=339°, DEC=-15°. Maximum rates during 1979 were significantly lower, with a ZHR of 16.85+/-1.21 occurring on July 28, from a radiant of RA=338°, DEC=-17°. The observed duration was July 20-August 5. The shower's 1980 appearance was hampered by a full moon on July 27. Subsequently, hourly rates were significantly lower, with the shower's maximum actually coming on August 3---the night of the last quarter moon. The ZHR reached 7.35+/-1.19 and the radiant was then RA=343°, DEC=-15°. The duration extended from July 18 to August 10.
Observations of the Northern Delta Aquarid shower by the WAMS have revealed three puzzling facts: very low activity, an uncertain radiant location, and an earlier than usual date of maximum. In 1979 possible meteors from this shower were observed during July 27-August 5. The maximum activity peaked on August 4 when the ZHR reached 3.12+/-1.10, but the average radiant was then given as RA=328°, DEC=-3°---roughly 7° to the west of the expected position for that date. In 1980, meteors were observed from the shower during August 2-16. A maximum ZHR of 5.67+/-1.21 came on August 4 (one day after a first quarter moon), from a radiant of RA=341°, DEC=-2°---roughly 6° east of the expected position for that date. Of course both radiants were given dates of maximum activity which definitely contradicts the results of photographic and radar surveys, as well as the apparent visual observations of the past. It is interesting that an earlier maximum has also been noted in the United States by N. W. McLeod III (Florida). In an article published in Meteor News during April 1984, McLeod stated that his observations since 1971 have revealed that maximum occurred at the same time as generally accepted for the Southern Delta Aquarids. He gave the Northern Delta Aquarid radiant as RA=326°, DEC=-7.6° for July 30, which is about 4 deg west of the expected position for that date.
The Author believes the minor controversy over the likely date of maximum for the Northern Delta Aquarids might have a simple explanation that points to a more complex structure for the Delta Aquarids as a whole. For the photographic meteors, the orbital inclinations tend to be 6-8° higher prior to August 10, than after that date. If this means that two different streams actually produce the overall Northern Delta Aquarid activity, then two different maximums might not be out of the question. On the other hand, if it is assumed that the inclination discrepancy among photographic meteors is due to a weak database, then the earlier, less supported maximum might simply reflect how easy it is to observe a weak meteor shower like the Northern Delta Aquarids during late July and early August, than during the time of the Perseid maximum in mid-August.
The characteristics of the Delta Aquarid meteors have not been well studied. Observers tend to lump the Southern Delta Aquarids and Northern Delta Aquarids into one shower and, to make things even worse, the Southern Iota Aquarids and Alpha Capricornids are occasionally thrown in as well. The few instances of specific Southern Delta Aquarid meteor details being obtained reveal an average magnitude of about 3, while very few meteors exhibiting trains.
The only apparent average magnitude determination for the Northern Delta Aquarids was made in 1977 by McLeod. From 33 observed meteors, he gave the average magnitude as 3.70. Recent studies seem to reveal that both streams' average meteor magnitude fades as each day progresses. This presents a very interesting picture when it is compared to a study published by L. Kresak and V. Porubcan (Czechoslovakia) in 1970. Using all available double-station meteor photographs, they found that the southern stream was essentially compact around the time of maximum, but then proceeded to become more diffuse, especially in right ascension, until the compact northern radiant became active about mid-August.