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WHAT DO ALL THOSE CODES MEAN??? (was RE: (meteorobs) Five predictions for the Leonids - are those all?)
Just a little "glossary", for those who may be new to
our list, and to the wee wonderful world of meteors.
Hope this is helpful to some. And as always, any of
your QUESTIONS and/or CORRECTIONS are most welcome! ;>
Clear skies,
Lew Gramer
--
"HR": Hourly Rate. Usually an estimate of how many
meteors from a particular shower that an experienced
observer would see in a given hour, under VERY GOOD
conditions: no street lights in his or her face, no
white flashlights shining every five minutes, no moon
light in the sky, no clouds, no obstructions, etc.!
Confusingly, "HR" is also sometimes used to mean the
ACTUAL number of meteors (total, OR from a particular
shower) that an observer really saw in some one hour.
--
"ZHR": *Zenithal* Hourly Rate. This is essentially the
number of meteors you will NEVER see in an hour... :)
(Officially, ZHR is the number of meteors per hour from a
given shower, that could be seen in VERY GOOD conditions,
assuming "normal"(??) perception, AND with the often very
unrealistic assumption that the shower's radiant is right
overhead at the Zenith. This is a very theoretical number,
but is also very useful in comparing observed rates and/or
model predictions for different observers around the globe.)
--
"IMO": International Meteor Organization. World organizing
body for amateur meteor observers, and for most of the pro-
fessional (or amateur) scientists who rely on their data:
http://www.imodot net
IMO collects, stores, and makes available meteor data of
all kinds from regional and national organizations around
the world, such as "North American Meteor Network" (NAMN):
http://www.namnmeteors.org
and the "Dutch Meteor Society" (DMS):
http://www.dmsweb.org
just to give two examples.
--
"Magnitude": Of course most amateurs are familiar with this
term as a measure of the brightness of a star, planet, or
other astronomical object. The lower the number, the brighter
the object, and REALLY bright objects have negative "mags.".,
like Jupiter at about -2.5, or the Full Moon at about -13...
But of course, as anyone who's been on 'meteorobs' for a
while knows, magnitudes are also used as a measure of how
bright a METEOR is - and in fact, meteor magnitudes are
usually estimated by comparing a meteor with nearby stars.
--
"LM": Visual Limiting Magnitude. Essentially, the faintest
star that any one person can see, in any particular hour or
half hour, on a particular night. LM is normally estimated
by counting all the stars you can see in some small area of
a constellation, for example, the body of Leo:
http://www.seds.org/billa/lm/rjm9.html
http://www.seds.org/billa/lm/rjm.html
Once you have your star count, you can come back home and
look it up in IMO's "Limiting Magnitude Conversion Tables":
http://www.imodot net/visual/lm.html
--
"7-rev", "4-rev", etc: When trying to predict Leonid meteor
storms (or other meteor shower outbursts), it is important
to keep track of the "parent body" for the meteor shower -
in other words, the comet (or asteroid) responsible for
spewing out all the dust that we get to see as meteors when
it strikes earth's atmosphere... Each time the parent body
draws close to the Sun in its orbit, it will deposit a new
batch of this dust (aka 'meteoroids'). The very LAST time
a comet passed near the sun (its most recent "perihelion")
is called its '1-rev' - its revolution around the Sun "one
time ago"... In the case of the parent body of the Leonids,
Comet 55P/Tempel-Tuttle, its '1-rev' was in February 1998.
So since Tempel-Tuttle goes around the sun once each 33.25
years or so, you can probably guess when its '2-rev' was -
that's right, it was in 1932! And before that, the '3-rev'
for this comet was in 1899. And the '4-rev' (the one when
comet Tempel-Tuttle dropped off the dust that we hope gets
those in North America, the Western Atlantic, and the NE
Pacific excited on Nov. 19th!) was in 1866. Continuing back
in time 3 revolutions and so about 100 years, T-T's '7-rev'
happened in 1767 - and THAT is the trip round the Sun that
is predicted to cause a "European" storm this year.
Note that this year, Earth encounters the '7-rev' trail at
around 0400 UT on 19 Nov (11pm Eastern US time the *18th*),
or in other words BEFORE the time we will enter the '4-rev'
trail, at around 1045 UT (5:45am in the East US) the 19th.
--
"r-factor": As explained in an earlier post, the 'r' factor
is a number used by IMO, to describe roughly how BRIGHT any
particular meteor shower will be: basically, if 'r' is 3.0,
then there will be THREE TIMES as many mag. 3 meteors from
that shower as there are mag. 2 meteors... And that is also
ROUGHLY how many more mag 3s you will SEE vs. mag 2s... If
on the other hand, a meteor shower's 'r' is 2.0, then we'll
see only TWICE as many mag 3s and mag 2... And that is GOOD,
because it means there should be many more bright meteors!
And by the way, meteors convert kinetic energy into radia-
tion, which is why we see them... So if two meteors strike
our atmosphere with the same VELOCITY, and they're both at
about the same distance from you, and one is brighter than
the other, that also means that the particle the caused that
meteor was more MASSIVE than the other one... This is key!
(Remember in High School/Gymnasium physics, when your teacher
wrote up the formula "Kinetic Energy equals one half of mass,
times the velocity squared"? Well in meteors, you can really
use that formula to understand something interesting! ;>)
In fact, for more technical readers, 'r' is related to another
factor 's', which is used to describe a first-order fit of the
mass distribution function for particles in a meteoroid stream.
And as our theorists made clear for us in some recent posts,
both 'r' and 's' are ONLY good for doing approximations inside
a narrow range of meteor masses/magnitudes where people happen
to be able to observe meteors visually (versus using telescopes,
expensive intensified video, or REALLY expensive radar systems,
all of which can observe meteors fainter than the eye can see.)
>-----Original Message-----
>From: owner-meteorobs@atmob.org [mailto:owner-meteorobs@atmob.org]On
>Behalf Of ataju@emaildot si
>Sent: Thursday, 07 November, 2002 4:04 PM
>To: meteorobs@atmob.org
>Subject: Re: (meteorobs) Five predictions for the Leonids - are those
>all?
>
>
>> What would really be interesting, for someone with the time, would be to
reduce
>> last years predictions vs. actual results then compare that to this year's
set
>> of predictions...
>
>I recall that Esko's final prediction for the 2001 7-rev peak
>was about ZHR 1800, which is very close to ZHR 1620 (or so) by IMO.
>Therefore, personally, I am most confident in Esko's this year's
>prediction for ZHR 3500 for the 7-rev peak. It would, however, be
>nice if dr. Jenniskens was right this year!! :>
>
>11 days to go...
>
>Clear skies!!
>Jure A.
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