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Re: (meteorobs) eta Aquarids (2)
Tim wrote:
>
>>Bob
>>
>>You mention some debate as to whether the eta Aquarids ever reach ZHR=50.
>>Are you talking northern hemisphere only?
>>
>>In my WGN analysis I showed that this shower reaches ZHR=60 or over
>>consistently for southern hemisphere observers. If you disagree, I'd like
>>to know why.
>>
>>Tim
>>
>
>The controversy over the Eta Aquarid ZHR is an old one. It flared up
strongly in the early 70's when some reports of very high rates came from
Australia, Buhagiar included. Keith Hindley (BAAMS) didn't understand it
any more than I at the time. S&T published 3 separate letters (I think
during 1972) with the argument going back and forth, then decided not to
publish any more until it settled. Hindley found the Eta ZHR to be 42,
which is similar to what I find. I note that Buhagiar's rates from
Australia indicate his perception is 3 times higher than mine, and that
explains our differences.
Another flareup came in the early 80's with the advent of Jeff Wood and his
observers around Perth. It turns out Jeff has twice my perception and
several of the others there were seeing even more. One individual I
determined saw an incredible six times my rates ! He had rates to 100/hr
from the Etas as late as 1981 May 15 ; that same night here I had a fabulous
rate of 5/hr. Talk about bewildering!
To investigate the "Eta Aquarids are far better from the Southern
Hemisphere" notion, Mark Adams produced a graph of Eta radiant altitude at
the beginning of dawn vs latitude. This gave some very enlightening
results. I don't have the graph in front of me so will go from memory. The
optimum latitude for Eta observing is 10oS, and even there the radiant
elevation is only 48o at dawn. From Perth (lat 32oS) the radiant is 42o up,
but from South Florida (lat 26oN) it still comes up to 30o.
The ZHR for Perth is x / sin 42o where x is the observed rate and assuming
the other variables to be unity. This expression then becomes x / 0.669 ,
or 1.49x.
For Florida it is x / sin 30o , or x / 0.500 = 2.00x
Thus, the ratio of Perth vs Florida is 1.49x : 2.00x , which means that
Florida will see 1.49 / 2.00 , or 74% of the Perth rate.
Observed Eta rates are higher from Perth, but the numbers clearly show that
the advantage of being so far to the south are not overwhelming. I should
see 3/4 as many Etas from Florida.
A normal Eta Aquarid rate to me is 15/hr, which yields a ZHR of just 30.
That compares favorably with the Orionid ZHR of 25 ; if the Etas come from
the same stream, why should it be several times stronger? In neither case
do we get anywhere near the stream core. Cutting it a little slack, in a
good year when I get a rate of 20, even there the ZHR is only 40. My
all-time record is 31 (will have to look up the year -- the same year that
Bob Lunsford nosed me out of the U.S. record with a 32) giving a ZHR of 62.
Compare this with the Geminids at elevation 30o -- I have a rate of 40 at
that time, far beyond anything the Eta Aquarids are capable of doing. Once
again, anyone who sees Geminid-like rates from the Etas, or even
Perseid-like rates, is doing so with high perception.
Lyrids were clouded out here. Most of April was pleasantly cool but very
cloudy -- that helped to block the scorching sun. A few clear evenings
allowed me to see the comet. I hauled out the f/4 12.5" to get a look.
Overall I still like the long tail from 1996 far better. We got some
drought-easing rain also; the grass is actually green. "April showers bring
May flowers" doesn't mean much here , usually the driest month of the year
and we have flowers all year anyway. Am hoping another surge of cool air
will come for the Eta max. That has been the morning of May 6 every year
after 1981, from what I have seen. In 1981 it was May 3.
Norman W. McLeod III
Fort Myers, Florida
nmcleod@peganet.com
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