The Eta Aquarids are one of two annual showers caused by Halley's Comet. (The other one is the Orionids, in October.) They are named after their apparent "radiant" point in the constellation Aquarius, near one of its brightest stars, Eta Aquarii.
The shower is active throughout April and November. When it peaks, according to NASA, observers can expect about 30-60 meteors per hour. Generally the peak is spread out over about a week centered on May 7, according to the American Meteor Society.
Eta Aquarids are best visible in the hours before dawn and, due to the southern area of the sky in which the radiant appears, are more readily visible from the tropical and southern hemisphere regions.
A hard-to-spot shower
While the meteor shower has been going on for centuries, it wasn't officially classified by astronomers until the late 1800s. American astronomer Hubert Anson Newton read into older showers, and among his findings discovered a series that took place around April 28-30. His research indicated accounts taking place in the years 401, 839, 927, 934 and 1009, according to astronomer Gary Kronk.
The discovery of the shower took place in 1870, based on a report by G.L. Tupman, who was reportedly a member of the Italian Meteoric Association, and was confirmed the following year. In 1876, British astronomer Alexander Stewart Herschel then noticed a link between the shower and a comet, Kronk added.
"He conducted a mathematical survey to find which comets were most apt to produce meteor showers," Kronk wrote. "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."
Early observations of the shower were hampered because it was in the Southern Hemisphere, where few meteor observers were based, but by the 1920s more frequent accounts occurred. In more recent years, Kronk added, northern and southern observers have seen a significant difference in meteors around that radiant. The Southern Hemisphere sees more because Aquarius is much higher above the horizon. [Halley's Comet Legacy: Photos of Eta Aquarid Meteor Shower]
Observing the show
Skywatchers can expect the best show possible if they go outside around 2 a.m. or 3 a.m. and gaze somewhat away from the radiant in the constellation Aquarius. Looking away from where the shower is centered shows longer trails of meteors. A typical Aquarid moves at about 44 miles (66 kilometers) per second, according to NASA. Aquarius can be observed from these coordinates:
- Right ascension: 23 hours
- Declination: -15 degrees
- Latitudes: Between 65 and -90 degrees
"For most observers, the Eta Aquarids are only visible during the last couple hours before the start of morning twilight," stated the American Meteor Society. "The reason for this is that the radiant is situated approximately 60 degrees west of the sun. Therefore, it rises before the sun in the morning hours." [Infographic: How Meteor Showers Work]
While Northern Hemisphere observers may get mixed results with the shower, those south of the equator report a much better show. Australians, for example, consider the shower one of the best shows of the year, said SPACE.com skywatching columnist Joe Rao.
The meteors do not actually originate near Aquarius; the placement of the radiant there is an orbital coincidence. In reality, Earth runs into a stream of litter left behind by Comet Halley, which is the most famous periodic comet.
Reports of Halley's Comet date back to at least 273 B.C., according to NASA's Jet Propulsion Laboratory. It also arrived in 43 B.C., one year after Julius Caesar was assassinated in Rome. The comet is featured in the Bayeux Tapestry, a chronicle of the Norman conquest of England in 1066.
The comet was first pegged as a periodic one – a comet that visits the inner solar system again and again – in 1705. Astronomer Edmond Halley's calculations showed that three comets sighted in 1531, 1607 and 1682 followed very similar trajectories. He predicted the comet would come again in 1759. Halley did not live to see the comet's reappearance, but it came on schedule as he had predicted.