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Re: (meteorobs) Radius of shower Radiant points

In a message dated 98-10-10 05:30:31 EDT, you write:

GeoZay<< 
 > Several months ago I was talking to Rainer Arlt about radiant sizes. The
 > radiant sizes listed in IMO's meteor shower calendar is not correct. They
were
 > suppose to be removed. The radiant diameter of the Orionids are 0.84
degrees
 > based on photographic data. I have some listed in my book Lew. I don't know
if
 > that update is in your copy or not? But look on Table 8.6 IMO's Additional
 > Working List Info. It has listed the one's that I received from Rainer or
 > Rendtel some time ago? It appears that for most showers, the radiant
diameter
 > is roughly 2 degrees or less. <<
 
 sirko>>maybe you mix up two different things here.
 The one thing is the _true_ radiant size. This one is in the order of 1
 deg for 'young' showers with compact radiants. The true radiant is
 obtained from precise position measurements, i.e. from photographs or
 video recordings.<<

I don't think I'm mixing two different things although two different things
are closely related here? I have the radiant size for the S. Taurids at 2.20
degrees. Of what I have this is the largest radiant and it is an Old shower
that is relatively compact. In the meteor shower calendar it use to list it as
5X15 degrees for plotting. For all practical purposes, why list a radiant
diameter that is actually around 2 degrees as a target of 5X15 deg? And the
other shower radiants whose actual diameter is around 1 degree or less as
something like 5 degrees when plotting? Then these diameters vary depending
upon how far away the meteor was in relation to the radiant area to get
something like 5, 10, 15, even 20 degrees in diameter to be accepted for
plotting purposes. Why not make all radiant targets for plotting purposes the
same size instead of varying the diameters? It appears that all the radiants
are around 2 degrees or less...a small target. I have no objections about
accepting a plot if it aligns up a certain distance from the center of a
radiant that is reasonable  to the distance it is from the radiant. But saying
that a radiant is 5, 10 or 5X15 degrees in size no matter how far a meteor is
away from the plot doesn't make sense. Make them all essentially the same size
if you are going to have exaggerated radiant diameters. Make them all 5
degrees for example.
 
 Sirko>>Visual plots contain large position errors. Applying such small
radiant
 sizes would make no sense here, as most shower meteors you plot would not
 hit the radiant point and had to be counted as sporadic. This is why we
 use larger radiant sizes (usally 5 to 10 deg) for the shower association 
 of visual meteor plottings.<<

I understand this...but why the large variation in diameter of the larger
radiant sizes such as 5 to 10 degrees When apparently all showers have
radiants that are essentially very small targets in the 2 degrees or less
range?

 sirko>>The actual number (5, 10, 20, ...) for
 each shower results from earlier investigation by Ralf Koschak. He
 examined, how large we should presume a radiant to be, that the number
 false positive (i.e. a sporadic is counted as a shower meteor because of
 the large radiant) is about equal to the number of false negatives (i.e.
 the plotting error of a shower meteor was that large, that it missed the
 radiant).<<

This I agree with and have no objections to.
 
George Zay
  >>