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Re: Radiants, was Re: (meteorobs) A blank meteor reporting form

Hello George,

if you will recall, you just went through this same conversation with Sirko
Molau, concerning the discrepancy between the photographic radiant size
(generally less than 1-2 deg), and the visual plotting/shower association
size (of about 5-10 deg, depending upon distance from the radiant).
--------------------

George had written:

> 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. 

To which Sirko replied:

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 


--------------------

To add my own ideas to the above:

When performing visual meteor "counts" (i.e. collecting visual descriptive
data about each meteor without plotting), if the meteor traces back to an
area roughly 5 or so degrees in diameter about the known radiant point, and
IF the meteor also meets the correct angular speed and path length
criteria, then  it is probably a good bet that this meteor was a shower
meteor.  For me, this used to be the angular width of the three middle
fingeres of one hand held at arms length (to get 5 degrees).  Medium to
slow speed showers below 30 degrees of radiant altitude  might be given a
bit more "forgiveness" due to the zenith attraction effect, along with
meteors seen far from the radiant.

In tracing great circles across the sky back towards a particular radiant
point, I doubt most visual observers have the ability to accurately trace
every shower member back to the photographically determined radiant width.
If someone claimed to do so, then they were probably doing a bit of "curve
fitting" on the fly.  As you correctly point out, asking folks to simply
look up and write down the photographic radiant diameter on the form
doesn't accomplish anything.

What the form should be asking for is the approximate radiant diameter used
by the observer to differentiate shower members from sporadic meteors.  For
a beginning observer this might be as large as 20 degrees (two fist widths
at arms length for myself).  if the beginning observer can generally trace
a meteor roughly back to the correct constellation area, it is often called
a shower member, perhaps with some speed and path length also considered.
With a good bit of practice, the observer can slowly tighten these criteria
(all three), as experience is gained at tracing paths, measuring path
lengths, and recognizing speeds.  Thus, my own suggestion is that observers
fil in their own personal criteria used in the field while observing that
particular shower, with the IMO calendar values used as a guideline for
plotters and more experienced observers.

Happy Halloween, all,

     Jim


James Richardson
Tallahassee, Florida

Operations Manager / Radiometeor Project Coordinator
American Meteor Society (AMS)
http://www.serve.com/meteors/
References: