Re: fireball or reentry ... ?

Alan Pickup (alan@wingar.demon.co.uk)
Fri, 20 Nov 1998 00:02:54 +0000

With regard to Mark Hanning-Lee's observation, which I had previously
indicated was not a Leonid since the radiant was below the horizon,
Steven Rogers <srogers@onr.com> writes
>At 06:38 PM 11/16/98 -0800, Mark wrote:
>> . . . I saw a nice
>> fireball. It moved from approximately E elevation 25 degrees to
>> approximately N elevation 45 degrees in ~1-2 s. Silent, blue-white,
>> shedding a lot of reddish particles with 1 larger reddish particle
>> ~mid-track. 
>
>This was probably a Leonid - it is not necessary for the radiant to
>be above the horizon to see shower meteors...

While the latter part of this sweeping statement is strictly true, it
deserves some clarification and I apologise for the fact that what
follows is off-topic for SeeSat.

It is possible for meteors to be seen from radiants which would
otherwise be a short distance below the horizon. Essentially, a meteor's
radiant is elevated in the sky because its path is bent by the Earth's
gravity - the phenomenon is called zenithal attraction. However, its
amplitude is small and depends on the geocentric velocity of the
incoming meteoroid, being least pronounced for fast moving meteoroids -
and Leonids are just about as fast as you can get. Incidentally, it does
not just apply to shower meteors, but to any incoming meteoroid (or
asteroid or comet...).

What has been omitted from the above quotation of Mark Hanning-Lee's
original posting is the time of his observation and his location
(specifically 02:21 UT and lat 33.712 long -117.837 altitude 34 m).
At that moment the Leonids radiant was some 35 degrees below Mark's
northern horizon. I need hardly add that zenithal attraction could not
transport the radiant into the eastern sky from where the fireball
appeared to come. Perhaps Steven has some other effect in mind?

Alan
-- 
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