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

re: (meteorobs) Those pesky path lengths



Hello gang,

I tend to think that makeing an on-the-spot shower determination requires
mentally weighing ALL of the meteor's characteristics; angular speed, path
length, distance from the radiant, color, and train properties.  There is
also a lot to be said for observing experience, especially with regard to
minor showers, as Norman, Bob, and other long-timers could aptly demonstrate.

The general thumbrule about the meteor path length being <1/2 its distance
from the radiant should be treated as a loose guideline only.  The apparent
path lengthof a meteor can vary significantly with magnitude and radiant
altitude for the same starting distance from the radiant.  Brighter meteors
will penetrate farther and have longer path lengths than fainter ones.  The
higher in the sky is the radiant point, the shorter will be all of the
meteor path lengths for the shower.  A meteor 30 deg from the radiant might
typically have a path length as short as 5 deg with the radiant at the
zenith, 10 deg or so with a radiant altitude of 45 deg, and more than 15
deg with the radiant at the horizon.  The Eta Aquarids were one of my
favorite visual showers because of the beautiful 40-60 deg trained
"streamers" one would occasionally see prior to radiant rise.  Visual
shower classification is more a matter of experienced judgement than
anything else, especially when you are not plotting.  And even the best
observers make occasional classification errors.

This discussion brings to mind a plate I once found of the 1866 Leonid
outburst which was in a 1920 Astronomy textbook.  It showed the paths of
about 75 or so meteors plotted against the constellations, and even showed
four sporadic meteors as well.  Was this an actual attempt at plotting, or
simply an illustration of meteor shower perspective?  I got out my ruler
and began to measure.  The average ratio between the total distance from
the radiant to the end-point of each meteor as compared to the meteor's
path length was 2.1 to 1.  That is, they were all too long!  Ah well, it
was a nice try.  This was probably just somebody's impression of the storm,
done after the fact.

BTW, this textbook also has a nice discussion of the old view toward those
interesting spiral "nebulae."  The idea that they might actually bedistant
iland communities of stars is not treated very seriously.  However, the
brand new Mt. Wilson telescope is also pictured in all of its steel and
rivet glory, so you know the big change is just around the corner...

Take care,

     Jim


James Richardson
Graceville, Florida
richardson@digitalexp.com

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


References: