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Re: (meteorobs) Radius of shower Radiant points
George,
> Lew<<
> Mark, the annual "IMO Meteor Shower Calendar" has radiant sizes,
> but only for *featured* showers of that year. However, both the
> GIAs and ORIs happen to be featured in this year's! See:
> http://www.imodot net/calendar/cal98.html
>
> (The GIAs are listed as having a 5o radius, the ORIs as 10o! But
> note that the appropriate radiant size to assume depends on your
> meteor's distance from that radiant, due to alignment error...)
> >>
>
> 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.
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.
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. 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).
Sirko
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