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(meteorobs) Re: ZHRs etc.



Hi all,

Currently there is a dicussion about several aspects of the shower 
activity and other characterizations in meteorobs. Parts of this debate
occurred in the past at various places (also in the pre-internet era)
and will re-occur from time to time. These items are of general interest 
since every observer will find some apparent or obvious disagreements 
with his or her own observations. Sirko has already made some essential 
points which I don't want to repeat.

The annual IMO Shower Calendar is meant as a reference for observers
to choose their observing period and the appropriate observation method.
Hence the ZHR information is only a rough number. The observer shall
go out and report what he can see, and then we can find out the ZHR
of the particular shower return. Of course, it may deviate from the
table value - but we can only find this *after* we have combined and 
analysed the data. 

Contradictionary impressions on shower maxima are not a surprise. The
most prominent example are the Quadrantids. It is not sufficient to be 
at the best longitude for the maximum - if it takes place with the radiant
close to the horizon. If we read the reports stating that the Perseids 
of 19** were disappointing, this may be correct for a given observer or 
longitude range, but you have to check which part of the whole activity
curve you actually saw. That is also the problem with old data often
restricted to a relatively small region or even a single country. The
observers may well have missed the maximum period. Sure, the ZHR is 
already an `abstract' measure, and in fact, even this is not a `real
measure' for the shower particle makeup and number density. We also
know that the maximum ZHRs of a given shower may vary from one return 
to the next - and here I do not refer to the outbursts.

Observers at mid-northern latitudes (say, 40-50 deg N) can hardly contribute
to the eta Aquarid data. So all results derived from such observations
are based on small samples and suffer from large corrections. In the IMO's
visual meteor database (VMDB) we have many data from southern hemisphere
observers. Moreover, the eta Aquarid ZHR graph shown in the IMO's Visual 
Handbook is very reliable (small error bars, indicating little annual 
fluctuations). Based on this profile, we also found that the maximum
position is later (May 6) than quoted in many other compilations (often
mentioned May 3 or 4).

Finally a few words about the role of the IMO. IMO has not the aim to
compete with national or regional groups or organizations. Please keep
in mind that you have the advantage that all publications are written in
your language. So the problems may be less obvious for you. But what about
amateurs and new enthusiasts in other countries? They need information
and personal contact in their language. All explanation of the basic
astronomical facts and further instructions for observations etc. need
to be available in the mother tongue of the newcomer. Furthermore, he/she
wishes to meet and contact people in their (relative) neighbourhood. So
there should be local meetings etc. where this is possible. All this can
only work in a local or national framework. 

Juergen Rendtel
(jrendtel@aipdot de)