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Re: (meteorobs) ZHR question follow-up

On Tue, 27 November 2001, Ed Cannon wrote:

> 
> Following up on (e)ZHR questions -- the NAMN Observing Guide, 
> Chapter 8 -- http://www.namnmeteors.org/guidechap8.html -- begins 
> with the statement, "The zenithal hourly rate is a means by which 
> different observers can convey their results to each other."  So 
> I am a bit disappointed to find out that, at least with the 
> (simplified?) ZHR formula that's in that article, my eZHR 
> calculations based on my own counts are apparently of no such 
> use.  

I wouldn't be too disappointed.  While eZHR calculations by one person are probably invalid, the high number of meteors in a true ZHR (for a period of over an hour) makes for a more statistically reliable sample.  I searched through archives to find your raw data and found that you had a period from 9:43-11:00 UT where you had 950 Leonids and an LM of 5.6.  If you do the ZHR calculation for this period, the results are much more reasonable.  

I'll hazard a guess that if you broke this down into 30 minute or even 15 minute periods, you would still get decent results, especially if you avoid periods where the LM<5.0. If there is enough data, analysts will often exclude data beyond a certain limiting magnitude (usually 5.0 or 5.5), F, K, etc. These correction factors are based on estimates, and add uncertainty and unreliability.  That's why, besides the fact we want to see a lot of meteors, we try to observe under conditions where the correction factors will be small.

I presume that the formula is too simplified -- no estimate 
> of size of field of view (e.g. wide FOV versus tunnel vision, no 
> glasses versus wearing glasses), no estimate of individual level 
> of perception, etc.  Is there a somewhat more accurate formula to 
> use with individual observations?

Probably, but then you'd have to calculate all of these for yourself (no mean feat).  As Norman McLeod has often noted, at least some people's perception coefficients fluctuate from hour to hour.  Rainer Arlt's approach of "let's get all the data in, and a lot of these factors will cancel each other out" is quite prudent, especially when there is an abundance of data.  Besides, these are just more uncertain correction factors that take importance away from the actual data and put it onto a formula.

Or do we just wait for the 
> most accurate ZHR that is calculated for a given hour and compare 
> our individual results with that?  If the latter, how should I 
> interpret the statement above from the NAMN Observing Guide?

I guess the biggest point to be made is that the raw data, not the ZHRs, are paramount.  Your data tell you what you saw; a too-aggressive ZHR calculation is mostly based on what you didn't see.  From a minute's calculation, you may derive an hourly rate of 1320, but the sample size is still just 22. The error is already huge before you start adding to it with correction factors.

A more prudent statement can be condensed out of the 1989 IMO Visual Handbook, Chapter 8.

"Tolerances get considerably smaller for a large number of ZHRs than for too few... This is one of the principal reasons why the IMO needs a lot of results.  We can never have too much data."

"Care has to be taken with too optimistic calculations which often ignore the statistical meaning of a ZHR.  An hourly rate obtained under perfect circumstances always stands for a small statistical sample out of a very large quantity: the density of mass in a meteor stream.

"An hourly rate obtained under perfect circumstances will show random statistical fluctuations. Introducing correction figues will make it worse by magnifying the uncertainty, and even the corrections themselves bear uncertainties because they were based on estimates and not on measurements.  All this is why you cannot find a ZHR for poor observational circumstances or short observing periods."

> Sort of related to that, full Moon is bad and full city glow is 
> bad.  Are the two together the worst of all?  Should one still 
> get out of the city even when the Moon is full?

A few messages back, someone pointed out that Bob Lunsford had obtained a LM of 6.0 under a full Moon. Yes, that's Bob, and I don't know what role pupil dilation plays, but I think in general one should seek the following as an ideal:

1. High altitude (thin air).

2. Dry air (no fog).

3. Clean air (no air pollution).

4. Good transparency (no cirrus).

5. No reflective ground surface (ie, snow)

6. An obstruction to block the Moon behind you (I've used my car).

--
Wes Stone
http://skytour.homestead.com


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