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Re: (meteorobs) LM, Debby, ZHR

Okay I'm curious why is LM important?

Debby

	Good question Debby (So far they've all been good!).
	The answer has to do with standardizing the data. Most times you will 
hear quoted the ZHR (Zenithal Hourly Rate) for a shower. What is that? Well,
 it is the "Number of meteors that would be seen by an observer with a 
limiting magnitude of +6.5, with no clouds in the sky, and with the radiant 
at the zenith".
	Let's explain some of these terms. Limiting magnitude, as Lew has 
stated, is the faintest star that can be seen, given the clarity of the sky,
 and/ or light pollution (man-made, solar or lunar)present. This is 
important, because if the sky is not as clear, or is brighter, you will 
miss many of the fainter meteors. Since (in most cases) there are about 2.5 
to 3 times as many more meteors for each magnitude fainter, this becomes 
quite significant. For example, you might see 1 magnitude zero meteor (as 
Bright as Vega), then there should be 3 of mag +1, 9 of mag +2, 27 of mag 
+3, 81 of mag +4, 243 of mag +5 and 729 of mag +6. If your skies are 
limited to magnitude + 5, then you will see none of the +6, a very small 
percentage of the +5's, a few of the +4, more of the +3, etc....and most of 
the 0 magnitude meteors.
	So when adjusting the counted meteors to the standard level so they 
can be compared worldwide, a correction is applied to the actual count, 
based on your limiting magnitude. This allows all observations to be used 
together.
	The other correction is for the radiant elevation. If the radiant 
(where the meteors appear to come from) is at the zenith, directly overhead,
 you should theoretically be able to see them all. As the radiant position 
becomes closer to the horizon, more will appear below your horizon, so 
unless you have X-ray eyes and can peek through the earth, you won't see 
them. In addition, down toward the horizon, you are looking through more 
air, making the meteors fainter, and therefore harder to see. In fact, with 
the radiant 30 degrees above the horizon, instead of directly overhead, 
you'll only see about a third as many meteors.
	Of course, the cloud correction factor is pretty obvious...if any of 
your field of view is blocked by clouds, you'll have a wee bit of trouble 
seeing the ones behind them.
	The net effect of this, as well as human perception, is that unless 
you meet these ideal standard conditions, you will almost always be seeing 
less meteors than the standard corrected rate. And, by the way, the 
standard is not just arbitrarily selected..dot it is used in further 
calculations involving the mass density of the shower stream, timing of 
peaks, timing of when the dust was ejected from the cometary source, and a 
whole bunch of other fun stuff that makes our standardized observations 
useful to the scientific community. If you're not interested in going that 
far, it's always fun to watch, and it can be gratifying to understand some 
of what you see. Remember as you gaze on comet Hale-Bopp, that that 
beautiful dust tail you see is a meteor shower in the making! Unfortunately,
 it is not one that we will see, since our orbit does not intersect it's 
path...but it's still fun to imagine plowing through that pile of 
dust....ooooh I've gotta go take a cold shower now!
	There are many subtleties as far as the details of what I've said 
above, but this isn't the official NJAA/NAMN/ALPO/IMO/AMS training course, 
so I've simplified it to get across the basic concepts. Hope this answers 
your question...if not, fire off another one!! As I've always said, I love 
beginners questions!! It makes us clear up in our own minds why we do 
things the way we do.
	And as we all know....METEOR OBSERVERS DO IT ALL NIGHT LONG

Wayne, and PS I'm 44 (physically that is , heh, hehe, hehe)