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(meteorobs) Re: ZHR
Hi folks,
it has been a long and busy day for me. Now in the evening I find the
time to read my mail and stumble over a discussion about the aims of
AMS, NAMN and IMO. It has been quite interesting to read what different
amateurs see as their main aim in astronomy. However, some of the things
Jim and Lew say about IMO need some commentation.
Jim writes:
> to compute it. The IMO is fond of quoting big ZHR numbers, and I worry a
> little about the validity of some of it.
This sounds really strange to me. What do you want to say with it?
Do you think that the shower rates calculated within the IMO are manipulated
to look better? :-))
> I noted on the IMO site one
> observer's numbers from the Geminids, with a ZHR computed of 119... with an
> error of plus or minus 45! That would come from a sample size of 7 meteors
> or less, and I wonder about such extreme extrapolation from a relatively
> tiny sample.
I don't know which particular investigation you mean. However, there are two
things I want to remark: I observed the Geminids myself, and my personal
ZHR rates where well above 150. That is due to my good perception (or due
to the fact, that I don't see as faint stars) and it should give you a clue
that rates simply where that high during the '96 maximum. If you see more
than 100 meteors an hours with lm near 6.5, what else do you expect?
The second thing is: Have you ever investigated how short intervall for
visual observers can be still containing meaningful data? Juergen Rendtel
presented his investigation about this at the last IMC. Basically he found
out, that you can calculate meaningful ZHRs for single observers down to ten
minute intervalls, if the activity is very high.
I don't want to comment on the particular number mentioned above, but one
thing is to claim that something looks odd, the other is to really check
what is possible an what isn't. Believe me, we really care about
significance and similar questions.
By the way, a little earlier you complained that IMO gives only ZHRs
without error bars. So what is the +/- 45? :-))
> an observation I made in 1972. A group of us here in Hawaii went out to
> look for possible Giacobinids. I saw what I believe was ONE Giacobinid.
> Going through the calculations, the ONE meteor yields a ZHR of 29, with an
> error limit of 29. Statisticly valid? You be the judge.
I'm sure, according to the method as applied by IMO such a result would
be impossible. What was the effective observing time? Far less than one
hour? What about the limiting magnitude? Was the radiant near the horizon?
In any case you need to introduce large correction factors making a 29 out
of one. However, all observations exceeding a certain overall correction
factor are not considered, since they are - statistically speaking - nonsense,
as you correctly remarked.
Lew writes:
> nothing to do with our discussion re: education). That issue is that the
> huge mass of AMS observer data is not merely "contaminated" with
> non-standard observing methods (which can be worked around), but is
I don't know the situation with the AMS data, but I want to raise the
rhetorical question, anyway: Can it really be worked around?
IMO was founded in the 80ies when people started to realize, that it was
virtually impossible to combine the data of different observer groups.
There were simply so many different types of observations involved, that
you could not mix them. You cannot compare apples with bananas!
There were one team doing group observations. The next team observed
meteors individually but didn't care about the limiting magnitude. The
third group was determining lm regularly but did not deal with
brightness distributions, etc.
There is simply no way to correct such observations since most important
data is missing! If you do not know the limiting magnitude, you cannot
even compare observations of two observers. If you do know it but have
not recorded the brightness distribution, you don't know the population
index. That is, you cannot compare your observations of two different
meteor showers. Finally, if you observe as a group all these problems
come together...
One of the major goals of IMO was the standardization of observing methods.
Fortunately, by now many observer groups in the world do follow the regime
proposed by IMO, which makes their observations comparable to others.
Even with such standard methods is it extremely difficult to put all the
observations together and come up with a consistent picture of meteor
activity 24h a day. I guess Rainer could write a book of it. :-)
Just in these days we are able to check whether our formulae are correct
by comparing parallel European and American observations. This is something
which would have been unthinkable without the previous effords to
introduce standards...
We don't say that we know the ultimate truth, but at least we try hard
to obtain results of scientific value. One of the basics for that is
to think in advance what you can do with your observations and what are
the preconditions for that. By that you avoid 'work around' and other
compromises.
> Last but not least, I do agree that IMO's practice of quoting ONLY ZHRs
> without mentioning standard deviation, sample size, or any raw rates
> can lead to some confusion
Lew, if you look at the new handbook, you see nice graphs with error bars
and figures about the sample size used. :-)
Questions is, whether we should mention all these numbers in lists
like the shower calendar. The ZHRs you find there are to our best
knowledge, i.e. are based on the latest investigations. That is, they
normally incorporate all the data collected by IMO so far.
Why should we put in error bars there? The Perseids, for example, have
a pretty stable maximum near ZHR 100. That's what you expect when you
read the calendar, and that's what you will see when you hit the maximum
under the excellent observing conditions. Of course, there are showers
with variable maximum rates, but those are also explicitely mentioned.
Finally a word about IMO and the education task.
Is there really somebody thinking that IMO is a club of semi-professional
amateurs having lost contact to the ground and don't care anymore about
public education purpouses or astronomy beginners?
In fact, one of the major goals of IMO is to promote science and it's
results in different ways. However, you should not forget that IMO is
an organization consisting of amateurs like you and me. There are less
than 10 people involved in the IMO council. How can they possibly be in
all TV shows around Earth, provide information for all astronomy magazines,
keep contacts to all local clubs and, of course, do all the other stuff
like observations and investigations? We work hard to reach the public, but
it is first of all the job of national and regional organizations to
establish contacts to news media, for example. What IMO tries is to
keep communication between these national groups alive. IMO is nothing
special, no collection of want-to-be-supermans. It is just a group of
people from all over the world, who try to make more of the national
amateur activities by bringing them together, by combining their results.
Nobody has to cooperate with IMO. However, fortunately more and more
meteor observers realize, that there can be done more with their data
if they stick to some simple standards.
It's just a fact that you cannot completely monitor the meteor activity
from a single site or country. In fact, you are already quite limited
if you want to investigate a single meteor shower only, since the
activity may vary from year to year. Here is the point where you
definitely need international coorporation. IMO tries to provide a basis
for that, no more and no less.
Sirko Molau
IMO-Webmaster
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