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Re: (meteorobs) Leonids: max ZHR in 1966



Dear Robert and others,

While perhaps Peter Jenniskens's 15,000 figure is a slight underestimate for 
the 1966 rates, it is clear that the 150,000 figure is WRONG even if we take 
the 40 Leonids/second value at face value (which we should not, is my 
opinion. They used a bizarre method of 'sweeping' the head and counting 
(already quite a-typical!), in what they thought was 1 second but might in 
reality have been 1.5, 2 or even 3 seconds..., who knows. Tests by Peter 
Bus suggest that in this kind of exercises the look is actually longer 
than 1 second).

First: the 150,000 figure is NOT a ZHR (!). It is simply the figure of 
40/second multiplied by 3600 seconds to get... 150,000/hour! This 
COMPLETELY IGNORES radiant altitude as well as limiting magnitude (and 
thus, it is not a ZHR).

Milon et al. observed under extreme good conditions. Note that they 
report visibility of the 'Gegenschein' earlier that night. In my personal 
experience, this indicates Lm's of +6.8 or better, and anyway gegenschein 
is only visible under the most perfect of conditions.
Now, assuming an average perception for the observers, a Lm near 
+6.8 to +6.9 which is certainly not unreasonable given what is mentioned 
above, and correcting for radiant altitude (65 degrees at the peak), the 
actual ZHR (or something close to it, given that we estimated LM, which 
is nowhere given) becomes......less than 100,000....! That is, assuming 
r=3.0.
Then consider the problem of whether the 'one second sweeps' were in 
reality 1.5 seconds or whatever, then it is clear that Peters argument 
that the 150,000 figure is grossly overrated is true. With 1.5 seconds in 
stead of 1 second, the 'ZHR' already drops to less than 70 000, 
continuing the above exercise.

Second: for what accounts the photographic data: under reasonably dark 
conditions a good camera is able to catch meteors up to +1 (we regular do 
so in our Dutch photographic survey). Under conditions as mentioned 
above, going even fainter might actually be possible. Moreover, much of 
the conversion of photographic records to actual rates is theory, no 
more. How many meteors you catch (and how faint) for example is highly 
influenced by your equipment (and then, I mean quality and precautions 
taken, not simply 'how fast is the film, how fast the lens') -a good 
heating device and flat film surface can make a large difference: put two 
camera's next to each other, heat one and the other not, and note the 
difference. Or put a Canon and a Practica next to each other, both with 
1.8/50mm lens and same 400 ISO film: note the difference once out of your 
dark room! We photograph considerably more meteors since we started with 
heating not only the lens, but the body as well (while the camera and 
lens stayed the same on our multi-camera units). So much for the theory 
of rates from photographs. And then, I don't even mention that most 
likely only the best of the pictures were published -the ones that 
catched that statistic flurry that happens once in a while.

Please note that this is not to pull the legs under your feet Robert. But 
I think there is much more to the debate than most people realize. It is 
very unfortunate that so much 'rate' figures (often miscalled 'ZHR's, 
which almost invariably they are NOT!) are quoted on several historical 
displays, and just copied in every popular textbook chapter, without 
anyone checking on what they are actually based. The 150,000 figure for 
the ZHR of 1966 would have no credit at all if people realize on what 
WRONG reasoning it is based, wrong in the sense that it does not live up 
to modern methods of ZHR calculation. Same to many other 'ZHR's quoted, 
e.g. the 7000 'ZHR' often quoted for the 1866 Leonid display. This is a 
figure which is much too low: it is based on counts from Greenwhich 
(120/minute. Multiply by 60 minutes to arrive at a figure near 7000, and 
wrongly call that a 'ZHR'!) but completely neglecting that these counts were 
done with the radiant at only 23 degrees altitude. Even assuming deep 
Lm's of +6.9, the 'ZHR' does not come much below 20,000 if we correct for 
the radiant altitude to arrive on a figure more close to what we consider 
a ZHR in a modern sense. Peter Jenniskens gives ZHR 17,000 for that year 
by the way, which certainly seems more realistic than the 7000 figure.

- Marco Langbroek
  Dutch Meteor Society, the Netherlands



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