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Re: (meteorobs) Geminids from China (Dec. 14/15 ZHUJI)
Jim>>>As you already pointed out, magnitude extinction will also begin to affect
>>>>observed magnitudes significantly below about 45 deg of altitude, such that
>>>>only the brightest shower members will be visible in the area just above
>>>>the horizon (although they won't appear bright).
I saw three or four Geminids this year that were -2 at the horizon, with
extremely short
paths. I felt certain when seeing them that given the height of the radiant and
the
low elevation of the meteors, they would have been fireballs had they actually
passed overhead. Yet tracing them back to the radiant was never as precise as I
would
have preferred, due in large part to their short paths and long distance from
the radiant. Certainly for me, had the night been well off the peak, I would
have been
tempted to label the most peripheral of these as sporadics.
Sincerely, Kim Youmans
Jim Richardson wrote:
> At 09:33 PM 12/25/99 -0500, Kim Y wrote:
> > Let me guess, Jin, that many of these meteors you are describing were
> at or
> >near the horizon...? My (limited) understanding
> >is that (a) a meteor can never have too short a trail to be associated
> with its
> >shower (but can be too long) and (b) a meteor moving from overhead down
> to the
> >horizon will appear to slow (and dim) somewhat near the horizon. I had
> exactly
> >the same experience during the Geminids. I saw MANY meteors with short path
> >lengths near the horizon moving at, say, a speed of two (slow) but I knew
> that
> >most likely they were Geminids because they were traceable back to the
> radiant
> >and the Geminids were at near-peak intensity. So given your other
> descriptions, I
> >would say it's safe to assume that many of your sporadics were Geminids.
> It would
> >be good to have an experienced observer give us both his/her slant on this.
>
> To give you the math for this, Kim, (and skipping the derivation) the
> angular speed of a meteor is given by the following equations:
>
> v_a = (180/Pi) * [(v * sin(theta)) / d]
> d = sqrt[(r^2 * cos^2(z)) + (2 * r * h) + h^2] - (r * cos(z))
>
> where;
>
> v_a = angular speed in deg/sec
> v = meteor linear speed (km/sec)
> theta = meteor angular distance from radiant
> z = meteor zenith angle (90 - meteor angular altitude)
> d = meteor distance from observer (km)
> r = Earth radius (km, 6369 km on average)
> h = meteor altitude above ground (about 70-120 km)
>
> The most commonly encountered form of this equation uses a first order
> approximation for the meteor distance, given by d = (h / cos(z)). This
> will give:
>
> v_a = (180/Pi) * [(v * sin(theta) * cos(z)) / h]
>
> Thus, the meteor angular speed will be
>
> * directly proportional to the linear speed,
> * directly proportional to the meteor distance from the radiant
> * inversely proportional to the meteor zenith angle,
> * inversely proportional to the meteor altitude
>
> As you already pointed out, magnitude extinction will also begin to affect
> observed magnitudes significantly below about 45 deg of altitude, such that
> only the brightest shower members will be visible in the area just above
> the horizon (although they won't appear bright).
>
> You have also caught me working on an article about this topic for the next
> issue of "Meteor Trails" which will go into a bit more detail, and have
> graphical plots / pictures to accompany the equations. In a meteor
> simulation routine, the shortening of paths as meteors appear near the
> horizon (for a radiant at high altitude) is also quite apparent.
>
> Best regards,
>
> Jim
>
> James Richardson
> Department of Physics
> Florida State University (FSU)
>
> Operations Manager
> American Meteor Society (AMS)
> http://www.amsmeteors.org
>
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