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Re: Fireball Reporting (was Re: (meteorobs) Pre-Moon Observations)
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Subject: Re: Fireball Reporting (was Re: (meteorobs) Pre-Moon Observations)
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From: GeoZay
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Date: Fri, 9 Jul 1999 22:07:47 EDT
In a message dated 99-07-09 21:33:14 EDT, you write:
lew<<
George has pointed out some confusion here: Note that I said "what
LOOKS LIKE a -1 or -2 meteor". Obviously, if you determine that a
fireball near the horizon really *is* only mag -1 or -2, i.e., by
using comparison objects which are at nearly the same altitude as
the meteor, then it is naturally not considered a fireball by IMO...<<
Confusion as to what degree of magnitude extinction occurs is quite dramatic
within about 10 degrees of the horizon. Easy to error....particularly if you
are looking over uneven terrain that could easily put you unknowingly over
the 10 degree range. But if the meteor is somewhat above 10 degrees above the
horizon, there isn't a whole heck of a lot to correct for that would make a
big difference to a -1 or -2 meteor in that area. A -3 meteor in this area
should suffice as the cutoff point for fireball reporting.
lew>>However, contrary to what George says, the FIDAC Website defines
what IMO does consider a fireball as follows. This is somewhat
confusingly worded, which may be the cause of George's post,
but is nonetheless quite specific:
"The definition of a fireball is somewhat arbitrary and in the
literature the required minimum magnitude varies between about -2
mag to -6 mag. In the IMO Fireball Data Center (FIDAC), we
regard meteors of at least apparent magnitude -3 mag (corrected
for zenith position) as fireballs. By zenithal magnitude we mean
the brightness the meteor would have if it had appeared in the
zenith of the observing site. As an example, a meteor appearing
like -1 mag may actually be a fireball if it moves only a few
degrees above the horizon."<<
I interpret their fireball definition as any meteor that appears to be of
magnitude -3 with the understanding that any needed correction would place it
in the acceptable range of being a fireball. The IAU defines the word
"fireball" for a meteor brighter than any of the planets. NASA has pretty
much the same definition. It would seem that IMO would be somewhere near
these guidelines also? The brightest visible planet is Venus reaching around
-4.4. It would seem that a good standard for a meteor fireball after being
corrected would be around -5. So with a -3 meteor within 5 degrees of the
horizon would give it a corrected magnitude near -6. If it was near 10 deg of
the horizon, it would be near -4.4. Anything above 10 degrees of the horizon,
a -3 meteor would be near -3 the higher above the horizon you go. I'd think
making -3 meteors a good cutoff point for fireball classification than
dropping it even lower for extreme special cases. I think IMO's example about
a -1 meteor very low on the horizon was only a case of extremes to get the
point across, but I don't think they are trying to refer that these -1
meteors ought to be reported as fireballs...especially since one's observing
horizon could very easily be above 10 degrees without the individual not
realizing it....especially if not plotted or photographed. Andre knofel is on
this list I believe....perhaps he could clarify what should be reported as
fireballs?
GeoZay
