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Re: (meteorobs) Geminids from Down Under




Sounds like you had a wonderful observing session, Robert. And yes, your post is
very appropriate for 'meteorobs'! You mention the GEM radiant being too low to
collect analyzable data and this MAY be true. But your data on other showers you
saw, the "human interest" of your observing session, and the enthusiasm and
dedication you show in your report are all most welcome here. :)


>perhaps you guys can help me out with a query I have. I've looked at
>all the documentation >on the NAMN website but I cannot recall this...

One thing to note, and this really surprised me when I first learned it via this
list: there's no such thing as a meteor that's too SHORT to be a shower member!
A meteor can be three degrees long, 30 degrees from a radiant, and still be
considered a shower member! (That is so long as it's also a candidate based on
path alignment with that radiant, and angular speed.) Rules for pathlength only
restrict the MAXIMUM length of a shower meteor: a meteor 10 degrees long that
starts any less than 20o from the radiant, is NOT a shower member...


In your case, it sounds like you saw several meteors which aligned equally well
with the 5o-wide Geminid radiant, and with the 5o-wide Sigma Hydrid radiant? And
also that none of these meteors were too long to be candidates for both showers?
This is rare, but does happen! In that case, you can use the differing speeds of
the GEMs and HYDs - the GEMs being medium speed (to me slightly slowish) at 35
km/s, while the HYDs appear noticeably speedier at 58 km/s, similar in speed to
the northern Perseids of Aug, or southern Gamma Normids of Mar.


>This is stated in all texts. However, does the meteor length also depend
>on its altitude and the component of motion perpendicular to the horizon?

The maximum pathlength must obey a rather complex equation to be a shower
member. This maximum is (non-linearly) dependent on the meteor's radiant
distance (degrees from its starting point to the radiant); on the radiant
angular altitude at the time of the meteor; and on the angular altitude of
the meteor's start point. This equation is discussed in detail at:

    http://www.imodot net/visual/minor03.html#length


However, a simplifying assumption which *I* use in the field is MUCH easier,
and is also explicitly stated on the above Web page: "For radiant elevations
higher than 30o the apparent path length l of a shower meteor [must be] at
most to half the distance from the radiant to the start point."


>...a 10 deg, medium fast meteor of mag 2 headed south through Octans.

BTW it looks like the Phoenicids are actually an extremely SLOW (18 km/s)
shower, making it unlikely you were seeing anything from them, even apart
from differences in dates (IMO lists PHO activity from 28 Nov to 09 Dec).


Clear skies, Robert, and keep sharing those fine reports!

Lew Gramer from Up Over
Medford, MA, USA


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