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Re: (meteorobs) Late Ursid?


A meteoroid stream is a set of very similar orbits in the
solar system. We see a meteor shower when the Earth crosses
this set of orbits. We see the maximum of the shower when 
we pass the densest part of the stream. The scatter of
ejection velocities from the cometary nucleus plus 
perturbations while travelling through the solar system
cause the set of orbits to disperse gradually. 

- Many orbits remain very close to the average orbit. 
- A fair number of orbits have slight deviations from the 
  maximum-density orbit. 
- Several orbits have considerable deviations from the 
  average. 
- A few orbits have changed so drastically, that they will 
  have nothing in common with the maximum-density orbit.

When is the beginning of a shower activity? When is its end?
It seems we need somebody to put stickers on the meteoroids
when they leave the comet, before we argue about late/early
meteors of a shower. The listed activity periods usually
refer to the period when shower activity is above a visually
meaningful threshold, say ZHR=1. Physically, there is no
end or beginning.

Robert, if the meteor you saw was a lousy +5, short and
ordinary -- would you have let us know ;-) ?

Now, for those who like intriguing experiments: The Ursids
have a radiant close to the celestial north pole. That point
in the sky is above the horizon at any time for sufficiently
northern latitudes. The point is also close to the pole of the
ecliptic (in Dra). Hence, also the distance of that point in 
the sky from the apex is fairly constant. The atmospheric entry 
velocity of meteors having this point as their radiant will 
thus be similar ALL YEAR ROUND. Now, it may be illuminating to 
check plots plus speeds for possible "Ursids" all year round!

Best wishes, Rainer


-- 
Rainer Arlt  --  Astrophysikalisches Institut Potsdam -- www.aipdot de
Visual Commission - International Meteor Organization -- www.imodot net
rarlt@aipdot de --  phone: +49-331-7499-354  --  fax: +49-331-7499-526

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