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Re: (meteorobs) Angular Speed Equations
Apologies to readers who are heartily bored by this thread.
> me>> I forgot to add....a good angular speed measurement can be used to
> roughly
> > estimate the shower radiant position for one meteor, when combined with the
> > path length, IF you have good alignment as well. Without good alignment and
> > any intersects, it's anybodies guess. How do you know if a visual
> observers
> > alignment isn't off for any one meteor? You don't. This is the reason for
> > visual observing purposes, a known radiant location is enlargened. I
> > personally prefer multiple intersects to direct me into estimating a new
> > radiant position. I still haven't seen any practical use for angular speed
> > estimates other than shower association. Just like the speed scale I use.
> <<
>
> malcolm>>Without the angular-speed estimate you can plot the intersections of
> most pairs of meteors. Yes this will work for strong radiants, but
> for weak minor showers the additional noise of false radiants can
> drown out the real ones. <<
>
> 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.
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. The *ensemble* of intersections defines the
major-shower radiant more accurately, but it's not normal practice
to plot visual meteors when major activity is ongoing. Remember I'm
just discussing the analysis of plotted meteors for identifying
meteor radiants, some of which may not be known or suspected a priori.
> Do you declare a
> new radiant with just one or two meteors by working with angular speeds?
No of course not from visual observations. If you extend the meteor
path back, the orientation and displacement errors lead to a tranverse
spread; errors in the speed and to a lesser extent the path length (or
more precisely start and end points along path) results in spread
normal to that. 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.
> malcolm>>Apart from the path-length constraint the
> radiant can be anywhere along half of the great circle of its
> prolongated path (for simplicity I'm ignoring the observation errors
> in the position). Using an estimate of angular speed, be it from a
> scale converted to deg/s or deg/s directly, gives a much reduced arc
> where the radiant can be. Thus the number of false radiants is
> reduced and the real ones become more evident. <<
>
> In theory this is true for non fireball meteors. You are basically saying
I've seen it in practice for ordinary meteors.
> that, "The apparent path length of a shower meteor amounts at most to half
> the distance from the radiant to the start point".
No I wasn't. I was saying in reference to the path length, that there
is some constraint, as to what is the correct factor was immaterial to my
argument. It's not the only constraint to the longitudinal errors.
> If the data was
> photographic, this would be easy to note. However in practice, meteors
> plotted will often not be this accurate. Rainer a few years back has told me
> that most meteor lengths are over plotted and that twice this distance would
> still be acceptable for shower association.
This assumes the previous incorrect assumption. It's not just path
length. There's still a probability distribution based on the angular
speed. The spread of this distribution takes account statistically of
the observer's error George.
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. 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?
That's why we combine lots of observations to average out the
uncertainties.
Back to telescopic. I have to apply a different path-length cutoff as
Rainer says for the 11-type meteors because the path length is a lower
limit. See Kresakova 1977 BAC, 28, 340 for more details.
> This would introduce inaccuracies
> to any radiant determined from estimated angular velocities. The fewer
> meteors involved, the greater uncertainty for a true radiant location.
No argument there.
> The
> same as if one went just by meteor intersects. So a greater number of meteors
> in the sample would help radiant determinations both by intersects and
> angular velocity estimates. Personally, I would rather narrow down a radiant
> location by the intersects than by angular velocity. I would then just have
> the inaccuracies of intersects due to misalignments to deal with and not the
> added path length inaccuracies to compound the problem. I haven't heard of
> anyone determining radiant locations with angular velocities alone? From all
> the reports I've seen, it appears that intersects is what is being used.
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.
You're kidding yourself using intersections alone. The total error
budget is larger than it appears.
> Angular velocities is just used to weed out unrelated meteors.
Yes it does that too, but you only know that probabilistically.
Malcolm
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