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Re: (meteorobs) Angular Speed Equations


me>> Yes, any kind of errors from path length, alignment, distance from 
radiant 
> will magnify any inaccuracies. The intersections of pairs of meteors for a 
> minor shower will be less accurate than for a major shower.<<

malcolm>>Why?  The observed rate is lower so there's more chance of plotting
accurately without being distracted by another meteor before you've
recorded the earlier one. <<

Yes you are right. I was thinking more along the lines that the more plots 
available, the more defined a radiant will be. 

malcolm>>By having a speed you reduce the second error.
Unfortunately, I don't have to hand example plots from RADIANT with
and without velocity information to show the improvement in the
definition of a weak radiant.<<

Couldn't using a speed scale show the same improvements?

malcolm>>Interesting that you should mention path length.  For telescopic we
often don't have a path length because the meteor entered and exited
the field of view (type 11).  So am I supposed to rely on crossings?
With a few hundred telescopic meteors you get lots of crossings.  I've
done this in the days before computer analysis (cf BAAMS Newsletters).
Some radiants are strong enough, but others are distinctly marginal.
The velocity data reduces the number of possible radiants so the true
ones will stand out. <<

Yes, as you said previously you use a speed scale to do this. It is used for 
shower associations. The same purpose I use my speed scale for. You obviously 
aren't able to make very many radiant determinations based upon angular 
velocities if you don't have the complete path length to work with. If 
meteors entered and exited the field of view and you are plotting, extending 
their paths into crossings would be a good place to note a possible radiant 
location after eliminating meteors with inappropriate speeds.

malcolm>> I find it suprising that you think velocity has
no role.  Given a precise velocity from a photograph, you can derive a
radiant position for a single meteor, so why can't a visual
observation produce a radiant position too, ableit extremely fuzzy?<<

Because the human observer introduces errors into their plots. Determining a 
radiant location from meteor intersections after eliminating non related 
meteors by speed, path lengths and proposed radiant locations, you primarily 
have only alignment errors to deal with. Determining a radiant location from 
angular velocities, you have the same alignment errors, plus judgement errors 
in path length, distance from radiant, and durations to deal with. 

malcolm>>That's why we combine lots of observations to average out the
uncertainties.<<

Then with lots of observations you refine the uncertainties of plot 
alignments and have better radiant determinations. Angular velocities become 
less valuable in locating a radiant. Angular velocities become merely a tool 
for shower associations which eliminates non related meteors to fuzzy up a 
radiant location determined by plot alignments. 

malcolm>>Please read the RADIANT article in WGN again.  Intersections alone is
one method, but also probability distributions using velocity *and*
position is the main mode.  Several other papers refer to this too
including some of my own.  Without velocity there are many
intersections which aren't due to a radiant.  It's also open to bias
and preconceptions.<<

How is velocity used in the probability distributions? Is angular velocities  
solely being used to significantly determine radiant locations? Is angular 
velocities doing something that a speed scale can't do for the same purposes 
in conjunction with intersections?

malcolm>>You're kidding yourself using intersections alone.  The total error
budget is larger than it appears. <<

I bet it's not that large where angular estimates is gonna do anything more 
than a speed scale estimate would? In use I bet they would be at least equal. 
In practice, a speed scale would be easier and less involved out in the 
field. 

GeoZay
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