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The Eta Aquarids
May's New Moon Meteor Shower
By Greg Bryant
 
 Submitted to Universe on 12th April 2000 for publication in May
 



An Opportunity to Observe Debris From Comet Halley

For some ASNSW members, the only time that they are likely to observing in the morning hours from a darkened sky is during the weekend of New Moon, particularly at the Society's Wiruna dark sky site at Ilford.  Consequently, when a meteor shower is scheduled for such a weekend, keeping in mind that most meteor showers tend to be visible in the morning sky rather than in the evening, it's worth highlighting the weekend in question, and such a weekend occurs this year on the nights of 5th and 6th May.

Meteor showers continue to be a topical part of astronomy at the moment.  If asked to name one or two showers, chances are that you could answer with the Leonids or perhaps even the Geminids.

Nature has played an interesting trick on us.  A glance at a star atlas (better still, a glance at the sky itself) will reveal that the majority of the sky's brightest observing delights are best observed from the Southern Hemisphere.  As if to compensate, the major meteor showers of the year (bar one) are best observed from the Northern Hemisphere.

Would you like to swap permanently?

I didn't think so.

May presents us with the opportunity to observe a good meteor shower, the Eta Aquarids; one best observed from the Southern Hemisphere.  If the radiant (originating sky position of the meteors) was located further north and to the west, it would no doubt attract more coverage from the various astronomy magazines, as more of their readers could better see the shower.

History

It was in 1863 that it was first realised that there was an annual meteor shower of note in late April / early May.  Professor Hubert Newton was undertaking a study of records of ancient showers and noticed that there were reports for this period dating back to the year 401.  However, it wasn't until 1870 that the Eta Aquarids were effectively observed and discovered, when observers in Italy and the Mediterranean Sea independently noticed meteors emanating from a radiant in the sky.

In 1876, Professor Alexander Herschel looked at which comets were most likely to produce meteor showers.  He found that the orbit of Comet Halley was closest to Earth on 4th May each year, at which time a radiant was predicted to occur at around Right Ascension 22.5 hours and Declination 0 degrees.  He immediately noticed that the observed meteors of 1870 were very near this prediction.

The shower was rarely observed during the last years of the 19th Century.  Indeed, it wasn't until the 1920's that the Southern Hemisphere saw several good meteor observers in action, and thus the knowledge of southern meteor showers increased significantly.

What To Expect

This year, the Eta Aquarids are expected to reach their maximum at 17:06 UT on Friday, 5th May (3:06am on the morning of Saturday, 6th May).  The zenithal hourly rate (ZHR) for the Eta Aquarids is about 50.  However, you need to remember that ZHRs represent the predicted rate as seen by an observer standing directly under the radiant in skies with a naked-eye limiting magnitude of 6.5.  For the Eta Aquarids, this never happens, as the radiant only rises a few hours before dawn, so don't expect such a rate of meteors.  The shower does have a broad plateau of a few days, so there will be meteor activity for some time.

You may wish to deliberately set out to observe the shower.  In that case, a reclining chair pointed towards the east will enable you to observe the area of sky from which the meteors will be originating.  Chances are, however, you will be at the telescope in the midst of an observing program.  Either way, expect to see heightened meteor activity during those morning hours.

As I mentioned at the start, New Moon weekends are typically the time when members are going to see morning meteor showers under dark skies.  May 2000 is a must-see time to observe the Eta Aquarids : your next chance to see this shower at its peak during a New Moon weekend won't be until the years of 2019, 2030, or 2035, and how many of us seriously expect to be Wiruna regulars then?