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Updated March 14, 2011
Eta Aquarids: April 21-May 12; Peak 2003 May 5 23h
There are two annual meteor showers which have been associated with that most famous of comets, Halley. These are the eta Aquarids in early May and the Orionids in late October. Halley has a retrograde orbit inclined at 163 degrees, meaning it is relatively flat to the ecliptic plane (effectively 180 minus 163, or 17 degrees inclination) and therefore passes fairly near Earth's orbit twice. In the current case Earth is within 10 million km of Halley's orbit, in October 22 million km. The related meteoroid streams have spread out sufficiently that in both instances Earth experiences a fairly decent meteor shower.
In early May, Earth is near the outbound, post-perihelion portion of Halley's orbit, meaning the meteoroids are coming somewhat from the sunward side. The meteors appear to radiate from an area situated near eta Aquarii in the Y-shaped asterism known as the Water Jar of Aquarius.
Because of the retrograde nature of Halley's orbit, the eta Aquarid meteors strike the atmosphere almost head-on at a very rapid 65 km/s, very similar to their cousins the Orionids (66 km/s) -- the 1 km/s difference is possibly due to Earth's slower orbital speed in May as it nears the aphelion of its orbit, and a commensurate slower speed of the meteoroids that are also slightly further from the Sun. Only the Leonids (71 km/s) have faster meteors among the significant showers. The eta Aquarids are typically fairly bright, and often leave persistent trains.
Unlike the Leonids, which feature sharp increases of activity every 33 years when the parent comet passes nearby, there was no evidence of enhanced activity in either eta Aquarids or Orionids when Comet Halley passed in the mid-80s. Both are well-established and fairly consistent showers of older debris which has spread out in the cometís broad wake. The comet itself passes much too far from Earth for freshly released material to interact with the planet.
The radiant is virtually on the celestial equator, meaning it is in the sky for about 12 hours, rising locally at ~3:20 a.m. and setting ~3:20 p.m. The Sun rises a few minutes before six, meaning it is above the horizon about 80% of the time that the radiant is; this is primarily a daylight shower. Add in a useless hour or so before sunrise, and there remains a very narrow window to visually observe the shower. The situation is quite a bit better in the southern hemisphere, where the Sun rises much later in their mid-autumn. Indeed, the eta Aquarids are the richest shower of the year for southern hemisphere observers with a Zenith Hourly Rate of ~50. In the northern hemisphere the ZHR is around 20; at northern latitudes such as Edmontonís, with the radiant located at less than 15 degrees altitude an hour before sunrise, observed rates will be very low. From this position on the globe, this shower is not a big show, and visual observations of eta Aquarids are rare.
The peak of the eta Aquarids is relatively flat, enough so that the RASC Observerís Handbook 2003 predicts that maximum will occur on May 5 at 23h Universal Time (late afternoon in western Canada), while meteor expert Gary Kronk pegs the peak almost 12 hours later, at 10:25 UT on May 6. The morning of May 6 should therefore be the best, however rates should remain fairly flat from about May 4-7. The waxing crescent moon will have long since set by the time the radiant rises and will therefore cause no interference whatsoever.
The eta Aquarids were one of the first streams to be detected by radio-echo techniques, dating back to 1947. Indeed, some of the most extensive observations of the shower were conducted at the Springhill Meteor Observatory in Ottawa from 1958 to 1967, with hourly rates typically between 350 and 500, presumably using more sensitive equipment than we have!
Because radio observatories are not blinded by daylight, the eta Aquarids can be observed much more extensively on radio wavelengths than visual. In theory they can be detected any time the radiant is above the horizon.
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Article by Bruce McCurdy.