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Lining up meteors with radiants (was Re: (meteorobs) assistance please)
Mrs. Webb, thanks for writing again! Sorry to take so long to reply.
>If a meteor started within 10 degrees south of alpha Arietis (Hamal)
>and beta Arietis (Sheratan), then does this meteor "line up" with an
>"Aries radiant?" If not, what can one say about it's starting point?
This is an excellent question, and one which every person struggles
with when they're just getting started recording meteors! I know Bob
and Kim(XY) - both of whom have put in many hours under the sky this
year - offered you some excellent overall advice, but I thought I'd
put in a quick response on this one question. So here goes...
A meteor does NOT need to be near a radiant to be considered a member
of that shower. In fact, I spend most of my time facing 20-60 degrees
(up to one third of the way around the sky!) AWAY from the currently
active radiants... The only requirement for the meteor's path to line
up with a radiant is that a "great circle" drawn through the meteor
eventually traces back through the small region of sky (a region just
2-5 degrees on a side) occupied by that radiant on that night.
Again, radiants for meteor showers are quite small areas of the sky:
seeing a meteor's path trace back to "somewhere" in the constellation
of Leo is not enough to call that meteor a Leonid! After all, Leo is
a pretty big area of the sky (1000 sq-deg in area), while most true
shower radiants are less than 20 total square degrees! :)
The "great circle" I'm talking about above is nothing but an imaginary
arc you draw all the way around the sky, lined up exactly against the
background stars with the path of the meteor you saw! You can get a
fine approximation of this within 90 degrees of the radiant, by just
holding a long piece of rope or cord up straight to the sky, and then
lining it up with the starting and ending points of your meteor: if
this cord travels back through the little area where you know a shower
radiant lies, you may have just seen a member of that shower!
(To be SURE, you also need to be sure of two other things: (1) that
the meteor's path wasn't too LONG to be from that radiant; and (2)
that the meteor didn't appear too fast or too slow to be from that
shower! These are "finer points" which you can pick up once you're
a little practiced at lining up meteors with possible radiants!)
All of this is explained in detail in the NAMN Meteor Guide which
Kim recommended - and we're always here for questions. So don't be
overwhelmed at the start, Mrs. Webb! This all gets clear with time.
Take care,
Lew Gramer
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