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Re: (meteorobs) Geminids from China (Dec. 14/15 ZHUJI)
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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