Observing the Eta Aquarids

Location of the Eta Aquarids
For Northern Hemisphere Observers

This represents the view from mid-northern latitudes at about 4:00 a.m. local time around May 6. 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 and Adobe Photoshop.)

Location of the Eta Aquarids
For Southern Hemisphere Observers

This represents the view from mid-southern latitudes at about 4:00 a.m. local time around May 6. 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 and Adobe Photoshop.)

History

Hints that a shower might be active at the end of April and in early May began in 1863, when H. A. Newton examined the dates of ancient showers and suggested a series of periods which deserved the attention of observers. One of those periods was April 28-30, and included observed showers in 401 AD, 839 AD, 927 AD, 934 AD, and 1009 AD.

The Eta Aquarids were officially discovered in 1870, by Lieutenant-Colonel G. L. Tupman (while sailing in the Mediterranean Sea). He plotted 15 meteors on April 30 and 13 meteors on the night of May 2 and 3. At a later date, W. F. Denning examined the records of the Italian Meteoric Association and identified 45 meteors that were plotted during April 29 to May 5, 1870. Finally, the shower's first confirmation came on April 29, 1871, when Tupman plotted 8 meteors.

Observations of the Eta Aquarids were rare, but, during 1876, A. S. Herschel discovered something which at least began to generate a greater interest in the shower. He conducted a mathematical survey to find which comets were most apt to produce meteor showers. Comet Halley was found to be closest to Earth on May 4, at which time the radiant was in Aquarius. Herschel immediately noted that Tupman's observed radiants of 1870 and 1871 were very near these predictions.

The Eta Aquarids remained a poorly observed shower due to a lack of active meteor observers in the southern hemisphere. Only occasional hints of an active shower were reported, since northern observers had to face the beginnings of twilight shortly after the radiant rose above the eastern horizon. Nevertheless, H. Corder detected activity on the morning of May 4, 1878, with 3 plotted meteors revealing a radiant near the star Eta Aquarii. During this same year, Herschel examined all available observations and noted that the shower's radiant seemed to move further eastward as each day passed.

W. F. Denning finally managed to observe this shower during April 30 to May 6, 1886. A total of 11 meteors were plotted to reveal a radiant near the star Eta Aquarii. From these observations, he stated that the radiant seemed 5 degrees to 7 degrees in diameter. He added that the apparent closeness of his radiant to that predicted by Herschel placed the identity of this shower to Halley's comet "beyond doubt."

Fortunately, several good meteor observers appeared in the southern hemisphere during the 1920's, and the knowledge of primarily southern meteor showers increased dramatically. One of the most prolific observers was R. A. McIntosh (Auckland, New Zealand) and he published one of the more significant studies of the Eta Aquarids during 1929. McIntosh stated that his observations of that year showed activity during April 22 and May 13, which he said presented "a good illustration of the dispersive action of the planets during the centuries that the parent comet has been in existence." He stated that maximum definitely came in early May, though bad weather prevented it from being pinpointed; however, hourly rates remained between 10 and 20 during the period of May 2 to 11. The radiant diameter was consistently about 5 degrees across, and McIntosh's orbital calculations showed excellent agreement with the orbit of Halley's Comet.

During 1935, McIntosh published another investigation of the Eta Aquarids. Using observations made by Murray Geddes (New Zealand) and himself during 1928 to 1933, he plotted the observed activity of this shower and developed an activity curve that revealed the shower began with rates of 1 per hour on April 28, then rapidly rose to a flat maximum of 10 per hour during May 3 to 6, and finally slowly declined to rates of 1 per hour by May 16.

Beginning in 1947, the Eta Aquarids joined the ranks of the first streams to be detected by radio-echo techniques. During May 1 to 10, a radio telescope at Jodrell Bank saw hourly rates peak at 12. Little additional data was gathered about this stream by the Jodrell Bank observers during the remainder of the 1940's and throughout the 1950's. In fact, the stream was largely ignored since the radio equipment was rarely operated during the early half of May. Fortunately, observers using the radar equipment at Springhill Meteor Observatory (Ottawa, Canada) and, later, at Ondrejov Observatory (Czechoslovakia) were able to supply some of the most extensive series of data ever accumulated on this stream.

The Springhill data covered the period of May 1 to 10, and a fact revealed by A. Hajduk (Astronomical Institute of the Slovak Academy of Sciences, Bratislava, Czechoslovakia) was the complexity of the activity rates. Using an average compiled for the period 1958-1967, it was noted that two apparent radar maxima occurred: one on May 4 and the other on May 7. These figures represented all radio echoes, but a further study of only the long-duration echoes (lasting about 1 second) revealed the same two dates of maxima, except the decline between the two dates was not as pronounced. Also present was a further rise to maximum that came on May 10.

The above figures represent a 10-year average and, although they show some interesting characteristics for the activity levels of the Eta Aquarids, the annual activity levels given in the same paper are even more interesting‹especially when they are compared to the unusual peaks and valleys noted in the activity curves of the Orionids. Hajduk's study of the Orionids led him to conclude that the abnormal activity levels were due to Earth's encounter with filaments within the stream. The same explanation was also given as the reason the Orionids occasionally possess secondary maxima or primary maxima on a date other than that usually accepted as the date of maximum. The same is also true for the Eta Aquarids. In fact, of the 10 years examined by Springhill Observatory, only 3 years represented what might be considered a normal activity curve. Some examples of unevenly distributed matter within the Eta Aquarid stream are as follows:

Hajduk's study not only revealed interesting details about this stream, but also about the Orionids of October--long known as the Eta Aquarids' sister stream. Although there is a distinct similarity between the characteristics of the meteors and activity levels of both streams, an interesting feature displayed in the Springhill data seems directly due to the distances each stream lies from Earth's orbit. Using the orbit of Halley's comet as representing the center of the associated meteor stream, Hajduk noted that the Eta Aquarids occur when Earth is 0.065 AU from the stream's core, while the Orionids occur when Earth is 0.15 AU away. According to the Springhill data, there is a smaller variation between the annual activity rates for the Eta Aquarids than exists for the Orionids.

The evolution of this stream was discussed during 1983, by B. A. McIntosh (Herzberg Institute of Astrophysics, Ottawa, Canada) and Hajduk. They published the details of a proposed model of the meteor stream produced by Halley's comet. Using a 1981 study published by D. K. Yeomans and Tao Kiang, which examined the orbit of Halley's comet back to 1404 BC, McIntosh and Hajduk theorized that "the meteoroids simply exist in orbits where the comet was many revolutions ago." Further perturbations have acted to mold the stream into a shell-like shape containing numerous debris belts. These belts are considered as the explanation as to why both the Orionids and Eta Aquarids experience activity variations from one year to the next.

Amateur astronomers have made significant observations of this meteor shower during the last 30 years. Based on information from amateur organizations in the United States, England, Japan, Australia and New Zealand, it is known that there is a dramatic difference in the activity rates of this shower between the northern and southern hemispheres. Where hourly rates can reach 20 per hour for observers in the United States, Europe, and Japan, these rates jump to 30-40 per hour for observers in Australia and New Zealand. The reason is simple, as Aquarius is so much higher in the sky for observers south of the equator. These organizations have also revealed that about one-third of the Eta Aquarids produce persistent trains, which are trails of luminosity left by the meteors which last at least a second.

During the 1985-1986 apparition of Halley's Comet, several meteor organizations around the world put their members on alert to check for possible increased activity in the Eta Aquarids (and the Orionids). Reports from groups in Australia, New Zealand, Bolivia, North America, and Japan generally indicate that no enhanced activity from this stream was present.