[Prev][Next][Index][Thread]

Re: (meteorobs) Re: Angular velocities

At the risk of annoying Lew, just a few final comments.

Ralf Koschack, one of the pioneers in using omega (i.e. degrees/second),
says in WGN 1990 18:4, p104 "Estimating duration and calculating omega
by means of path length is difficult.  It is much better to convert
the sensation of the meteor's velocity directly into degrees/s.  While
watching a meteor, the observer stores the phenomenon as a whole in
his memory.  Then he fixes the path and estimates the magnitude.  Now it
is possible to estimate omega also: in thought, the observer makes the
meteor move for one second.  Then its path length is omega in degrees/s.
One becomes able to estimate omega directly without this procedure
after observing a number of meteors if one has the `scale' in one's
head.  Estimates by experienced observers difer by no more than 50%."

The measurement of the distance in one second is the way I'd do it. 
It may well be more accurate as Bob claims.  It certainly *feels* more
accurate.  On my scale a one step error is twice the scatter (i.e. two
standard deviations) for multiple observations, so 68% of the time you
have to be in the right step bin.  Therefore I'd suggest that visual
plotters at least try Ralf's technique.  If it's uncomfortable, then
revert to the scale.  If not, you've not lost anything, and if the
method really is more accurate, you'll be providing better data and
making Rainer Arlt's life easier.

Here some numbers from a seminal paper also by Ralf
Koschack, WGN 1991 19:6, p.225 on plotting accuracy and sporadic
pollution.  All numbers in degrees/s.  These give the angular-velocity
errors for different speed ranges by intercomparing observations of
the same meteor by a team of very experienced observers.  You mileage
may vary.

Velocity range    Standard deviation
  3--8                  1.67
  7--13                 2.3
 12--18                 2.8

Thus it seemed to me you could bracket +/- one-standard-deviation ranges
for 0--3, 4--7, 8--12, 13--18, 19--24, >24.

Bob wrote: 
> I agree with Pierre that methods should be kept simple in the field. I
> would also ask him if he will be recording meteors in exactly the same
> manner 10 years from now?

Don't want people to think we should stagnate.  We should strive to
improve our techniques.  That's another reason for meteorobs.  I'm
open minded re. which method is best.  For telescopic, I didn't have a
choice.

The "keep it simple" is largely directed novice observers, who do
struggle if they are expected to record lots of data, many of which
have little or no practical use.  As you become more practised, it
becomes easier to accept and record accurately more information. It
was a big thing in the early 1970s in the BAA Meteor Section. Plotting
and various other parameters were reducing the accuracy of the main
data---magnitude and shower association---in the majority of reports.
The aim was to allow more data to be compiled into shower anaylses,
including from the once-a-year Perseid-peak crowd.  I think Ralf's
description of how to estimate the angular speed is simple.  It's
really just switching from a 5-point to a 30-point mental scale
afterall.

Malcolm


To UNSUBSCRIBE from the 'meteorobs' email list, use the Web form at:
http://www.tiacdot net/users/lewkaren/meteorobs/subscribe.html
Follow-Ups: References: