Eta Aquariids: Halley’s Comet Crumbs

Comet Halley
Carnegie Institution of Washington
After the spectacular display of Leonid meteors in November 2001, skywatchers might have difficulty readjusting to "normal" showers that traditionally display only a few dozen meteors per hour. One of these, the Eta Aquariids, peaks on the night of May 4–5, 2006, and while it may not dazzle you with large numbers, this shower has one significant claim to fame: its particles come from the reigning king of comets, Halley.

Earth passes near the long, looping orbit of Comet Halley twice per year, and each approach peppers our atmosphere with dust strewn along Halley's orbit. Yet, despite their impressive lineage, the Eta Aquariids in May and the Orionids in October are not particularly well known. In fact, the Eta Aquariid shower remained unrecognized as such until the late 1800s.

Hints of a pulse of meteor activity at the end of April and in early May first arose in 1863, when Hubert A. Newton noticed a coincidence in the dates of a series of showers in historical records dating back to A.D. 401. But official credit for discovering the Eta Aquarids goes to English astronomer George Lyon Tupman. On the nights of April 30 and May 2, 1870, he observed more than two dozen meteors streaming from a radiant in north-central Aquarius. A year later Tupman again recorded a weak shower radiating from this location.

The Eta Aquariids remained poorly observed 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. However, in 1876 Alexander Stewart Herschel deduced that on May 4th, when Comet Halley's orbit is closest to Earth, it should create a meteor radiant at right ascension 22h 28m, declination 0°. Herschel immediately noted that the radiants observed by Tupman in 1870 and 1871 were very near these predictions, and within a few years the link between Comet Halley and the Eta Aquariids was secure. Fortunately, several good meteor observers appeared in the Southern Hemisphere during the 1920s, and the knowledge of primarily southern meteor showers increased dramatically.

Beginning in 1947, the Eta Aquariids joined the ranks of the first streams to be detected by radar. From 1958 through 1967, the sensitive radar equipment at Springhill Meteor Observatory near Ottawa, Canada, detected hourly rates typically between 350 and 500 at the shower's peak.

As with most annual showers, this one exhibits year-to-year variations in intensity, and observers have noted unusual peaks and valleys in the activity of the related Orionids as well — undoubtedly due to Earth's encounter with denser filaments within the stream of particles along Halley's orbit. In 1962, for example, the radar-echo rates climbed to 328 per hour on May 2nd, only to drop to 133 the following day. By May 4th they had climbed back to 468.

In 1973 Anton Hajduk (Astronomical Institute of the Slovak Academy of Sciences) noted that the Eta Aquariids occur when Earth is 0.065 astronomical units (10 million kilometers) from Halley's orbit, presumably when the stream is most dense, while the Orionids occur when Earth is 0.15 a.u. (22 million km) away. Moreover, the Eta Aquariid stream has an orbit somewhat different from the one now occupied by Halley. Apparently these comet crumbs made their escape from the nucleus many centuries ago.

Watching for Halley Bits

Eta Aquariids in Northern and Southern Hemispheres.
Sky & Telescope diagram by Steven Simpson.
Because Halley has a large retrograde orbit, its meteoric dust slams into our atmosphere at very high speed — 65 km per second for the Eta Aquariids and 66 km per second for the Orionids. The "typical" Eta Aquariid is rather bright, averaging about 3rd magnitude. Many of these meteors leave persistent trains that linger as glowing fingers of light in the sky after the bright flash ends.

In 2006 the shower's peak is predicted for the night of May 4–5 (about 5h Universal Time on the 5th). The radiant, centered at right ascension 22h 30m, declination –1.3°, is just a few degrees from the intersection of Aquarius, Pegasus, and Pisces. Activity remains high a few days before and after the peak. The first-quarter Moon sets before prime meteor-watching time.

Shower activity will become noticeable once the radiant clears the eastern horizon at about 2:30 a.m. local daylight (summer) time. Make a point to be ready and watching when the radiant is low on the horizon. That’s when a bright Eta Aquariid may skim overhead, creating a long, dramatic "Earth-grazer."

At this time of year, nights are longer in the Southern Hemisphere, so even though the Eta Aquariid radiant straddles the celestial equator, it climbs higher in the sky before dawn for southern observers than it can for those in the Northern Hemisphere. Consequently, from far-southern vantage points the shower's hourly rates can be significantly higher. Indeed, this shower is usually the year's richest for observers south of the equator.

Don't expect a big, showy display: average rates tend to be about 20 per hour in the Northern Hemisphere and 40–50 per hour in the Southern Hemisphere. But studies suggest a secondary maximum may occur two to five days after the main peak. Although these rates are a far cry from the spectacular Leonid displays of 1999, 2000, and 2001, think of the Eta Aquariids as interplanetary postcards from the most famous comet of them all, currently coasting out near the orbit of Neptune.

Where to Look

Observers in the Northern and Southern hemispheres can use our Interactive Sky Chart to see the appearance of the sky at 4:00 a.m. during the peak morning of the Eta Aquariids. The northern chart is set at 40° north latitude on the 5th for central North America; the southern is at 35° south on the 5th for eastern Australia. On the chart, the meteor shower name and symbol is visible in both windows. Click on the "change" button to alter either the date and time or viewing location displayed by the chart. Generally, there will be more meteors than usual visible for a few days on either side of the peak of a meteor shower.